Chlorophyll has unique photophysical properties - a first excited singlet state (S1) at the energy embodied by red light, and a second excited singlet state (S2) at the energy embodied by blue light. It is extremely rare for organic molecules to have high rates for useful photochemical processes (at the reaction center Chl pair) and low rates of loss by internal conversion (to heat) or intersystem crossing (to the first excited triplet state, to which direct absorption is first-order forbidden by quantum symmetry rules, and with which there is a danger of creating damaging singlet oxygen from ground-state triplet O2). G. Wilse Robinson at Caltech had a great course on this topic, using a sophisticate analysis of the density of states (rovibronic states, or quantum energy levels). I wrote about this years ago - see V. P. Gutschick. 1978. Concentration quenching in chlorophyll-a and relation to functional charge transfer in vivo. J.Bioenerg. Biomembr.10: 153-170. Note that absorption spectra are broadened by local environmental interactions of the Chls to cover a good fraction of the solar spectrum, and also there are auxiliary pigments (carotenoids) to fill in even more of the spectrum. Absorbing 85% of the solar spectrum is a good deal! Remember, too, that overstory plants spend a lot of time light-saturated, so that not absorbing too much light (and thus, not getting too much photodamage, with the help of xanthophyll dumping of excess absorbed light) is a good thing. Light saturation could only be avoided by having even bigger investments in Rubisco, and that's insupportable of the basis of total energetic cost of making leaves (see also V. P. Gutschick. 1984a. Photosynthesis model for C3 leaves incorporating CO2 transport, radiation propagation, and biochemistry. 1. Kinetics and their parametrization. Photosynthetica. 18: 549-568, and V. P. Gutschick. 1984b.____. 2. Ecological and agricultural utility. Photosynthetica 18: 569-595. Thus, getting good photochemistry). So, getting good photochemistry means taking the best molecules (Chl a and b) that evolved. Interestingly, we've all seen publications about the possibility that there are better Rubiscos, but I haven't seen articles about better Chls, other than having stacked systems, one using red and blue and the other using green....but, again, too much light absorption is a more common problem than too little, in most environments. Using Rubisco more efficiently in C4 plants vs. C3s is a partial solution for warm regions - not a universal solution.

One theory is that the earliest photosynthesisers used a molecule other than chlorophyl, (retinal as used by halobacteria) which absorbed the green very efficiently, leaving the ancestors of plants to evolve to absorb the remainder - hence the dip in chlorophyl's absorption efficiency around the peak of the light intensity. Photosynthesisers also may need to be able to reflect the strongest amounts of energy to prevent absorbing too much energy.

Good question and not an easy one to answer. But some algae have phycobilins. These are open-chain tetrapryrroles some of which can absorb green light for use in photosynthesis. This allows the algae to grow in ecological niches where the red and blue light has been depleted by "normal" photosynthetic organisms.

Another aspect of this is that thicker green leafs actually do use green light quite efficiently. It is true that the green light is absorbed less in single cells, but once absorbed it might be even more efficient.

Early experiments on the efficiency of different wavelengths used unicellular, or filamentous, algae and here it is clear that most of the green light is “lost” to the plant due to poor absorption. However, in thicker tissues (like in a normal land plant) the “green” photons can drive photosynthesis efficiently deeper in the tissue.

Here are some interesting papers that explain this much better than I do:

Sun, J., Nishio, J.N., Vogelmann, T.C., 1998. Green light drives CO2 fixation deep within leaves. Plant and Cell Physiology 39, 1020–1026.

Terashima, I., Fujita, T., Inoue, T., Chow, W.S., Oguchi, R., 2009. Green light drives leaf photosynthesis more efficiently than red light in strong white light: revisiting

the enigmatic question of why leaves are green. Plant and Cell Physiology 50,

684–697.

Nishio, J.N., 2000. Why are higher plants green? Evolution of the higher plant photosynthetic pigment complement. Plant, Cell & Environment 23, 539–548.

The absorption of blue and red light might be a valuable compromise between the needs to absorb light and avoid photodestruction.

Really good question and I think evolution is the only answer for this. I searched and found some information on, why green and not black?

It still is unclear exactly why plants have mostly evolved to be green. Green plants reflect mostly green and near-green light to viewers rather than absorbing it. Other parts of the system of photosynthesis still allow green plants to use the green light spectrum (e.g., through a light-trapping leaf structure, carotenoids, etc.). Green plants do not use a large part of the visible spectrum as efficiently as possible. A black plant can absorb more radiation, and this could be very useful, if extra heat produced is effectively disposed of (e.g., some plants must close their openings, called stomata, on hot days to avoid losing too much water, which leaves only conduction, convection, and radiative heat-loss as solutions). The question becomes why the only light-absorbing molecule used for power in plants is green and not simply black.

The biologist John Berman has offered the opinion that evolution is not an engineering process, and so it is often subject to various limitations that an engineer or other designer is not. Even if black leaves were better, evolution's limitations can prevent species from climbing to the absolute highest peak on the fitness landscape. Berman wrote that achieving pigments that work better than chlorophyll could be very difficult. In fact, all higher plants (embryophytes) are thought to have evolved from a common ancestor that is a sort of green algae – with the idea being that chlorophyll has evolved only once.

Shil DasSarma, a microbial geneticist at the University of Maryland, has pointed out that species of archaea do use another light-absorbing molecule, retinal, to extract power from the green spectrum. He described the view of some scientists that such green-light-absorbing archae once dominated the earth environment. This could have left open a "niche" for green organisms that would absorb the other wavelengths of sunlight. This is just a possibility, and Berman wrote that scientists are still not convinced of any one explanation.

Although I cannot find the citation right now, I remember a paper titled Why aren't plants black? which addressed this question. The main conclusion was that a black plant simply could not dissipate enough solar radiation to prevent overheating. Using chlorophyll to absorb mainly blue and red light was a compromise that maintained high rates of photosynthesis without causing too much overheating. When paleo CO2 levels were higher, stomatal conductances (how open the stomata are) would have been lower, making it more difficult to dissipate heat via transpirational cooling. While green light does penetrate deeper into leaves to the spongy mesophyll, in higher flux densities it can cause photodestruction. Many leaves synthesize anthocyanins (which absorb primarily in the green wavelengths) in the adaxial epidermis or palisade mesophyll. This pigment allows blue and red light through so that photosynthesis can continue while at the same time protecting the spongy mesophyll from too much green light.

Hi, The reason for the above thing is the excitation energy required for the photosystem to work is in that wavelength, we know the lights with blue and red are on extreme in terms of wavelenght so the energy in red is hight and the blue has a low wavelenght. so the enrgy required for the excitation might be one of the cause for the plants to look green. We can do an experiment by growing plants in monochromatic lights and observe for effects. It may also relate to signaling for the insects which do get attracted to blue light.

Dear Dr.Ralf Gregor Niemann,

your question is really a taught provoking one. when I went through literature I came across the following points. As it is I have given it to you for your understanding

Why are plants green?

(Lansing State Journal, August 14, 1996)

Plants are green because they have a substance called chlorophyll in them. Understanding why chlorophyll is green requires a little biology, chemistry and physics.

If we shine white light on chlorophyll, its molecules will absorb certain colors of light. The light that isn’t absorbed is reflected, which is what our eyes see.

A red apple appears red because the molecule of pigment in the apple’s skin absorbs blue light, not red. Thus, we see red. Chlorophyll molecules absorb blue light and some red light. The other colors are reflected resulting in the green color that we associate with plants.

Plants get their energy to grow through a process called photosynthesis. Large numbers of chlorophyll molecules acts as the antenna that actually harvest sunlight and start to convert it in to a useful form. Here’s where the absorbent properties of the chlorophyll molecule come into play.

It turns out that eons of evolutionary design have matched the absorbance of chlorophyll to the actual color of the sunlight that reaches the leaves. Sunlight consists of primarily blue and red light mixed together, which are exactly the colors that chlorophyll molecules like to absorb. Light is a form of energy, so the chlorophyll is able to harvest the sunlight with little waste.

I hope this may certain extent clarify your doubt.

Dr.Chandralekha

AIMST university

In the marine environment, photosynthetic organisms are not only green... In coral reefs the predominang colour is red-brown (peridinine and chlorophyll c), if we talk only about the photosynthetic pigments and forget the host pigments. In terrestrial environments, the predominant colour is green because this environment was colonized by chlorophytes (the green photosynthetic organisms). The optical characteristics are then caused by the type of photosynthetic pigmentation of these taxonomic group, Chlorophyll a and b. Both pigments are very inefficient in the green window. As other people has already said, this inefficiency does not mean than the leaf suffers of the same inefficiency as an efficient structure for absorbing light due to multiple scattering can resolve perfectly the evolutionary constraints of the pigment collectors (biophysical properties of the pigment-protein complexes). So far, scleractinian corals have been described as the most efficient solar energy collectors (see Enríquez S, Méndez ER, Iglesias-Prieto R (2005) Multiple scattering on coral skeletons enhances light absorption by symbiotic algae. Limnology and Oceanography 50 (4) 1025-1032). We know that leaves have not a large variability in their capacity to absorbe light. Often, we used a constant to describe this capacity (0.83-0.85), which means that the leaf absorbs between 83-85% of the incident light. However, what we know about the variation of the efficiency? very little. No body has paid much attention to this important aspect, but absorbing the same constant using 100 mg chlorophyll m-2 or 600 mg chlorophyll m-2 has strong different consequences for the costs of building the photosynthetic structures, to get similar "benefits" in terms of energy imput.

Coming back to your question, why plants are green? Well, the answer is not yet clear, but has to be resolved understanding why only chlorophytes (the green algae) were able to colonize the aerial environment. One of the possible answers is probably related to the higher stability to heat that the green photosynthetic membranes have got. The melting point for these membranes can be above 50ºC whereas for dinoflagelates, for instance, this melting point can be observed already at 33-34ºC, and explains the strong fragility shown by symbiotic dinoflagelates to small increment in the water surface temperatures of the ocean. We need to do more research and talk more terrestrial and marine photobiologist to be able to answer questions like yours, remembering that the largest diversity of photosynthetic organisms and physiological processes occurrs in the marine environment.

What it is also important to take into account, is that the option of being inefficient for absorbing green light, has not reduced any nanogr the huge ecological and evolutionary success of green organisms in the terrestrial environment (aerial and aquatic). Light is absorbed at a much higher rate than the speed of any other biological process that allows incorporating this energy into organic molecules. When the sinks are saturated.... being inefficient is a great evolutionary achievement as it reduces the energy absorbed in excess and potential harmful for the membranes. We need to recognize (and avoid) to extrapolate cultural values of scientists to nature. Being efficient has not been always the best adaptive solution. Natural selection has found important evolutionary benefits in the "inefficiency".

Chlorophyll has unique photophysical properties - a first excited singlet state (S1) at the energy embodied by red light, and a second excited singlet state (S2) at the energy embodied by blue light. It is extremely rare for organic molecules to have high rates for useful photochemical processes (at the reaction center Chl pair) and low rates of loss by internal conversion (to heat) or intersystem crossing (to the first excited triplet state, to which direct absorption is first-order forbidden by quantum symmetry rules, and with which there is a danger of creating damaging singlet oxygen from ground-state triplet O2). G. Wilse Robinson at Caltech had a great course on this topic, using a sophisticate analysis of the density of states (rovibronic states, or quantum energy levels). I wrote about this years ago - see V. P. Gutschick. 1978. Concentration quenching in chlorophyll-a and relation to functional charge transfer in vivo. J.Bioenerg. Biomembr.10: 153-170. Note that absorption spectra are broadened by local environmental interactions of the Chls to cover a good fraction of the solar spectrum, and also there are auxiliary pigments (carotenoids) to fill in even more of the spectrum. Absorbing 85% of the solar spectrum is a good deal! Remember, too, that overstory plants spend a lot of time light-saturated, so that not absorbing too much light (and thus, not getting too much photodamage, with the help of xanthophyll dumping of excess absorbed light) is a good thing. Light saturation could only be avoided by having even bigger investments in Rubisco, and that's insupportable of the basis of total energetic cost of making leaves (see also V. P. Gutschick. 1984a. Photosynthesis model for C3 leaves incorporating CO2 transport, radiation propagation, and biochemistry. 1. Kinetics and their parametrization. Photosynthetica. 18: 549-568, and V. P. Gutschick. 1984b.____. 2. Ecological and agricultural utility. Photosynthetica 18: 569-595. Thus, getting good photochemistry). So, getting good photochemistry means taking the best molecules (Chl a and b) that evolved. Interestingly, we've all seen publications about the possibility that there are better Rubiscos, but I haven't seen articles about better Chls, other than having stacked systems, one using red and blue and the other using green....but, again, too much light absorption is a more common problem than too little, in most environments. Using Rubisco more efficiently in C4 plants vs. C3s is a partial solution for warm regions - not a universal solution.

я думаю, очень подробно рассказывается в этой статье:

http://www.google.by/url?sa=t&rct=j&q=%D0%B8%D0%B7%D0%BC%D0%B5%D0%BD%D0%B5%D0%BD%D0%B8%D0%B5%20%D1%81%D0%BF%D0%B5%D0%BA%D1%82%D1%80%D0%B0%20%D1%81%D0%B2%D0%B5%D1%82%D0%B0%20%D1%81%20%D1%8D%D0%B2%D0%BE%D0%BB%D1%8E%D1%86%D0%B8%D0%B5%D0%B9%20%D1%81%D0%BE%D0%BB%D0%BD%D1%86%D0%B0&source=web&cd=6&cad=rja&ved=0CD4QFjAF&url=http%3A%2F%2Fwww.ras.ru%2FFStorage%2Fdownload.aspx%3FId%3D7ae6443e-3fe9-41bf-bf42-e9fc3485f86c&ei=qygzUIr-GYr0sgbanoH4CA&usg=AFQjCNGFhey0yLCkSMi3KxvdigCPuJrMCg

Here are excerpts from the article, possibly inaccurate translation:

"....During the pre-biological-chemical evolution, when in the atmosphere does not have a lot of free oxygen, and therefore could not form the ozone shield, which protects the surface from UV radiation at wavelengths shorter than 300 nm, the region of the spectrum of solar radiation supplies energy to synthesize variety of biologically important compounds. From simple substances, present in the atmosphere, hydrosphere and lithosphere, and in near space (H20, N2, CO, C02, CH20, HCN, etc.), to form different compounds that formed in the future based biopolymers. The absorption spectra of aromatic amino acids and nucleic acid bases cover the wavelength range 260-280 nm, which corresponds to the "failure" in the solar spectrum. Formation of ozone screen led to the formation and preservation of the photodecomposition of complex polymers of amino acids (proteins) and polymers of nucleic acids, which absorb radiation in the range 260-80 nm.

Let us analyze the involvement of metals in the biochemical processes of origin and evolution of life. Metals such as Mg, Fe, Ca, are basic drives the most important metabolic processes in living organisms. Is now generally recognized essential trace elements (Fe, Zn, Mg, Ca, Mo, etc.) for the functioning of living systems, and in this regard is increasingly developing new branch of science - Inorganic Biochemistry. The importance of trace elements is particularly evident when you consider that several hundred enzymes (more than one third of all known) - metalloenzymes and their operation is not possible without the participation of metals. The role of the metal is not only to stabilize the structure and the creation of the active conformation of the enzyme, but also direct involvement in the process of catalysis. Apparently, during the prebiotic evolution metal ions act as a catalyst, which then operated in combination with organic matter to form protofermenty. Further evolution of the latter led to the emergence of metalloenzymes. According to Egami, those metals that have prevailed in the sea water are included in the metal complexes, and then evolved to metalloenzymes. We now consider certain patterns of evolution of photosynthesis. It is known that the first plants originated in water. As a result of their fotozinteza gradual change of the spectral composition of the incoming radiation due to absorption of solar radiation is formed in the atmosphere of oxygen and its derivatives.

When ozone was so much that he has formed a shell around the Earth, it creates conditions for the growth of plants on land. However, the problem of protection of plants from excessive intensity of sunlight coming into the plant.

Light saturation of photosynthesis in most plants is in the range of 100-300 × 103 erg/cm2 s. A further increase in the intensity of light decreases the rate of photosynthesis. In [7] it is shown that under conditions of excessively high light when the light intensity exceeds 300-400 x 103 erg/s2 to go marked disturbances the biosynthesis of pigments, inhibition of photosynthetic reactions and growth processes, which reduce the overall plant productivity. Intensity coming to the surface to sunlight can exceed this limit several times, especially on cloudless days. Therefore, the plants had to develop a defense mechanism against the destructive changes in the intensity of solar radiation. It can be recalled that the photosensitizing systems of plants include two main groups of pigments: tetrapyrroles (chlorophylls, cytochromes, phycobilins) and isoprenoid natural pigments that are grouped in carotenoids. Structural basis of tetrapyrrole compounds is porphin.

The molecules, the spectral absorption peaks are in the region of sharp changes in the intensity of incoming solar radiation, have the opportunity due to the weak conformational changes in the local environment or rebuild (in the range of a few nanometers) the position of its maximum absorption - shift it to decrease or increase the intensity of light in this the solar spectrum. The use of such a possibility could be a step towards the process of adjusting the amount of light absorption. The presence of two close conformational states of the same molecule, sharply differing probability of absorption of a light quantum, apparently triggered the specialization of photoactive molecules and turn them into precursors of future antenna, light-harvesting complexes and reaction centers for processing of light energy into chemical energy. If we now turn to the chemical to biochemical evolution, and traces the "attachment" of the spectral absorption maxima fully developed pigments to the deepest minimum in the solar spectrum. In this case, the more ancient organisms peculiar pigment, the more broad and deep minimum in the solar spectrum corresponds to the maximum absorption. The basic maxim we bacteriochlorophyll absorption "and" are "tied" to the much more powerful "failures" of the solar spectrum (365-375, 390-400, 760-765) than the absorption maxima of chlorophyll "a" (410, 430, 660 nm). This is for two reasons:

• Photosynthesis in bacteria with only one photosystem, goes without oxygen evolution and is the most primitive process. This suggests that the photosynthetic pigments of bacteria arose earlier in evolution than the chlorophyll "a". It can be assumed that in the evolution of bacteriochlorophyll education "and" went the independent route, starting from the stage of synthesis uroporbilinogena and matching red absorption maximum bacteriochlorophyll "a" with atmospheric oxygen absorption band (760 nm) is evidence in favor of the hypothesis of abiogenic origin of oxygen on Earth.

• In bacteria per molecule bacteriochlorophyll "a", which is in the reaction center, between 20 and 100 molecules of antenna bacteriochlorophyll "b", constituting the light-harvesting system. In higher plants, this value varies from 200 to 400. Hence, the level of specialization in bacteria pigments of the photosynthetic apparatus is much lower than that of higher plants. Way to adapt to the level of intensity of solar radiation in bacteria more primitive than that of higher plants: bacteria use more adjustment mechanisms of the absorption bands of pigments on their lows of the solar spectrum, and their antenna systems are imperfect. The main pigment of higher plants (chlorophyll "a") is less "guided" to the minimum in the solar spectrum: higher plants have high light-harvesting complexes, which include, in addition to chlorophyll "a" highly specialized pigment - chlorophyll "b"..."

To answer this question one must consider biophysical and physico-chemical aspects. Upon light absorption, an energy transition must take place between the ground state and the excited state, the difference of which must be compatible to the energy levels of other reactions of photosynthesis. In PS2, this energy must drive the water splitting complex on the donor side and must initiate the electron transport chain on the acceptor side. In PS1, a similar story must happen to drive the reduction of NADP+ and the cyclic electron transport. Here one must calculate the redox potential differences or the free energy changes. During the electron transport, the production of ATP is bound to a given energy difference, too. Thus, when discussing the energy needs (in other words the photon quality or photon energy) needed for photosynthesis, we must consider the whole process. The delocalized electron system of chlorophylls and the conjugated double bond system of carotenoids can absorb photons, which can drive the whole machinery. (Actually, two photons' energy is needed to the energy difference (redox potential difference) between water and NADPH.) An other interesting consideration is evolutionary: the first porphyrin molecules could absorb only UV or blue light, the red absorption band (Qx transition) was very small - but at that time, the quality of the sunshine was different from the one we have now, it contained more UV. Probably, the first photo-autotrophic organisms were not blue.

The water ecosystems are different, in which additional accessory pigments like phycobilins can absorb green light or the photoautrophic bacteria which can utilize near infrared light (they absorb in the blue and in the near IR region).

The question is of course of interest. I agree with Felix that it is not easy to answer. Many hypotheses are possible. My first answer would be that the leaf absorption in the green part of the electromagnetic spectrum is not that weak (see the answer from Mats). To be convinced of this, just have a look at the action spectrum of photosynthesis. Jonathan gave a 'practical' reason for not using the full energy available in region: it is the relatively weak capacity of the photosynthetic apparatus to treat an actual high photon flux density. To be convinced of that, juts have a look at a the increase of the oxygen emission vs photon flux density. The curve deviates from linearity well below full sun light intensity.

Its like the question why blood is red in colour as it contains haemoglobin and cockroach blood is colourless as it contains haemerythrin ,and as it is plant cells contains chloroplasts cells filled with chlorophyll component by nature and exposed to sunlight for preparation of its own food with help of sunlight , water and co2hence ,no one can reduce this or induce any other components . I mean it . And hence plants are always green and few plants are in many colours and few colourless due to lack of chlorophyll or presence of some other components. Though sunlight is an cosmic rays can exhibit white rays where it falls on the leaf and the absorbant rays transmits only green rays as reflected particles and we are able to recognise that particular green shades of the plant system . Sorry if this is wrong answer .

Very good answers and I agree with most of them. But I humbly say: is there a physiological answer as well? Could the green light have a very important signaling effect for the green plants so that it is necessary to NOt adsorb it? There are in literature some papers about that and it may make perfectly sense. It is known that shaded plants or plants in spring grow darker or spindly rispectively but maybe if all the light is adsorbed plants would not know what is going on above them.

Why are plants green? Very good and subtile question:

They ARE NOT GREEN, You CALL them GREEN.

Yours and Our conclusion that plants are green is based on the observation by our eyes. And what the eyes see is sent to the brain as a signal. By education we learnt that such a signal is called GREEN. This is a social-cultural phenomenon to BELIVE that what we SEE IS in reality what IS and EXISTS. Not by vision. Vision is like reading a newspaper. If you believe dogmatically what is written in a certain Newspaper, then you may be wrong because the reality may not be what you read. The same with the Vision ore any signal which is sent by our boddy to the brain.

In short:

1) We call the plant as being greeen when we observe the plant in Day light

2)Without light you do not see any plant and you will be wrong by concluding that there is no plant around.

3) In the disco you may have for some moment red laser light. You will see your partner, your glas and all your decorating plants around you, but they will be RED.

The same for any other color. So what is wrong with our language and our observations?

4) It's our habit to think that what we see is the reality. If I see that my car has a flatt tyre I'll realize immediately that this is the reality. Because the message is: There is NO AIR in the tyre. Therefore it is flatt.

The same with the plant

We think (wrongly) that if a plant is accummulating inside "green stuff" it becomes GREEN, therefore we should see it GREEN, because we think that the impression we are getting by seeing is the reality. This is true for most situations. e.g. If I see that a cup is broken, then indeed it is broken. However if I see a plant GREEN, then this plant is every thing EXEPT Green because it is REJECTING the green light (like it does reject any colored light in a certain degree). The REJECTED LIGHT is reaching our eyes and the brain "says" "I see GREEN" and our interpretation is in a materialistic way that the plant IS GREEN and there has to be some GREEN STUFF which we never will find chemiclly. What we find is chlorophyll as you mentioned. But Chlorophyll is absorbing mainly blue and red and lets pass or scatters or reflects the "surplus" GREEN light in all directions, so to our eyes, creating there the impression of GREEN, so that we think the plant IS GREEN, but chemically or materialistically it is every thing but NOT GREEN. It REJECTS green like any other colour but in a preferential manner.

Therefore if a plant is illuminated with any "pure" color it reflects a part of that light and creates the sensation in us of seeing that color. If a plant is illuminated by a MIXTURE of color lights then it rejects a part of all components of the mixture and it absorbs a part of all components. As a green plant it absorbs preferentially more red and blue light than green and therefore it reflects more green light, what we see as green, but chemically the plant IS (chemically) of molecules of any color but the least it IS green. In terms of Chemistry (the molecular and materialistic existance of the plant) IS ANY THING, but the LEAST what the Plant is, is GREEN.

Therefore: We SEE the plant GREEN in Day light, because it IS (chemically) NOT green.

In red light we see the Plant red

In blue light we see the plant blue

In Yellow light we see the plant yelloow, but

In "white" or day - or sun- light we see the Plant GREEN.

Funny isn't it?

If the solar light would be composed of "white" photons only, then all plants all forests would be seen as WHITE to GREY to BLACK (how terrible that would be!). Enjoy the green what ever the reason is for that sensation.

Regards: [email protected]

I do beleive that this phenomenon must be closely related to evolution of oxygenic photosynthesis first emerging in oceans when the probably high levels of dangerous UV (and heat) radiations had not allowed any life forms without the beneficial screening effect of water. Probably, the first 1 to 2 meters below the ocean's surface was the most safe and convenient for some light driven life forms. The visible light penetrating in the ocean might have maxima in the red (and blue) spectral region promoting the key role of pigments readily absorbing these photons. Later, heat stability (tolerance) of biological membranes containing those light absorbing pigments could be the key factor when plants started to seize the continents. This is my opinion.

I like to contribute an opinion on why plants are green (in the sense that they have a lower efficiency in absorbing the green light while they do better with blue and red light. The photochemical properties of porphyrins have already been reported above by dr. Gutshnick. For photosynthesis plants need a pigment whose excited S1 state lives long enough in order to efficiently transfer energy to another pigment, tipically ns. This is why carotenoids, although present in protosynthetic systems cannot work in reaction centres (they have short living, few ps, S1 excited states which leads to heat degradation fast). Porphyrins are good photosensitizers. Chlorophyll is better than hemes because Mg++ is much more soluble (available) than iron. Modern plants evolved from pre-existing cyanobacteria, which, in turn, derived from purple bacteria. Both purple bacteria and cyanobacteria have pigments, although different, that absorb in the green light range (bacteriochlorophyll, phycocianin, phycoeritrin, allophycocianin. In order to compete with cyanobacteria green algae had to absorb light in spectral regions that were available in the cyanobacteria-colonized waters. This is why they dropped phycobilisomes absorbing in the green region and evolved a different types of membrane-embedded antenna systems (LHC proteins) with optimized absorbtion in the blue and red regions. This was further favored because membrane-embedded antenna proteins allowed not only for light harvesting but also for photoprotection when green algae climbed out of water and became mosses. From this time on, photoprotection became the major selection factors for colonization of land environment. Therefore life made it right on the first attempt by using for photosynthesis pigments that absorbed light in the spectral range where there was a lot of it (green) but later coming competing organisms took a different way, aborbing light where there was less of it (blue and red). This turned out to be a good strategy when moving from the water environment with low light to land invironment characterized by daily periods of excess light that cannot be avoided because plants cannot swimm away. This is the eternal fight between photosynthesis and photoinhibition.

I recommend to read the article by Terashima et al. (Plant Cell Physiol. 50(4): 684–697 (2009) entitled "Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are Green". This is actually very interesting article to read.

Leaves absorb more than 90% of the violet and blue wavelengths and almost a comparably high percentage of orange and red wavelengths, while absorbing very little of the green wavelengths (Salisbury and Ross 1992). This is why as sunlight passes through the canopy, red (640–740 nm) and blue (425– 490 nm) wavelengths are depleted because of absorption and scattering of light, thereby turning the spectral distribution of light under a forest canopy towards the yellow to green portions of the spectrum (Gates 1980; Kendrick and Frankland 1983).

@RGN: Why are plant green or why they look green? what do you want to discuss?

Whatsoever the question is twisted or reversed , the answer for ur question is because sunlight is the main source of energy to reflect green light on plants, and hence they emit green colour always , hence plants look green as they have chlorophyll content in them as a major technique to prepare its own food using solar energy , and the plastids in them is also the main cells which utilises sunlight and emit green colour always .

It must be a fantastic question; otherwise it would not have drawn the attention of so many scientists. In addition to green plants, there are also several others that are brown, red , blue green and so on. In fact, when higher plants are grown in the dark, they develop etiolation in toto. Chemicals such as amitrole can completely bleach a plant.Such plants do not look green

It only means either in dark or by chemical treatment , the chloroplasts can be knocked off and the plants become pale yellow or white. Therefore, it is obvious that chloroplasts confer green color to the plant. Some plants are pink or even dark red but they do contain chloroplasts but they are masked by other pigments like anthocyanins or flavonoids.

In evolution chloroplasts were engineered in such a way to strongly absorb in the red region followed by absorption in the yellow and blue regions but unfortunately the pigment centres/ photosystems are not designed to capture green light and it is simply frittered away

If the plant absorbs more radiations, must need more energy for cooling the excess of heat. Is it?

This is taken care by the energy of water evaporation. 500 g of waret is evaporated per g of fixed CO2. Largely enough.

There are some mistakes on the question/answers " why plants are green". I have some different ideas on it:

a) chlorophyll molecule and a leaf are different things. The nanometric size of chlorophyll, carotenoids, etc ... within a leaf and the molecules' environement represent a different system. Moreover, there is no scientific proof that Chl does not absorbs and emit the same green photon.

b) The experimental results on transmission and reflection on leaves give bout 15% reflection for the green colour. The absorption is about 55% for the green colour (535 nm). For the entire visible spectra, (400 - 700 nm), the average absorption is about 79%. This will kill leaves, in few minutes, on sunny days. So, to many mistakes in considering the green colour of plants a reflection of green photons. We should start looking for: where it goes this 79% energy absorption and what does leaves with 55% of green colour absorption?

c) If photosynthesis is firstly a photon absorption system, followed by electrical (e-, h+) charges separation. So, we should look for the mechanism of charges separation and for the spent energy to do this (e-, h+) separation. In my opinion, most of the presented theories violate physical laws. Most of the models that try to explain charges mouvements for the photosynthesis processes violates physical laws; e.g.: negatifs charges go into the direction where the authors want... A net flow of negatifs charges to somewhere needs an electric field.

Photosynthesis and charges separation mechanism is a nanoscale process. It should obey physical laws.

There are some mistakes on the question/answers " why plants are green". I have some different ideas on it:

a) chlorophyll molecule and a leaf are different things. The nanometric size of chlorophyll, carotenoids, etc ... within a leaf and the molecules' environement represent a different system. Moreover, there is no scientific proof that Chl does not absorbs and emit the same green photon.

b) The experimental results on transmission and reflection on leaves give bout 15% reflection for the green colour. The absorption is about 55% for the green colour (535 nm). For the entire visible spectra, (400 - 700 nm), the average absorption is about 79%. This will kill leaves, in few minutes, on sunny days. So, to many mistakes in considering the green colour of plants a reflection of green photons. We should start looking for: where it goes this 79% energy absorption and what does leaves with 55% of green colour absorption?

c) If photosynthesis is firstly a photon absorption system, followed by electrical (e-, h+) charges separation. So, we should look for the mechanism of charges separation and for the spent energy to do this (e-, h+) separation. In my opinion, most of the presented theories violate physical laws. Most of the models that try to explain charges mouvements for the photosynthesis processes violates physical laws; e.g.: negatifs charges go into the direction where the authors want... A net flow of negatifs charges to somewhere needs an electric field.

Photosynthesis and charges separation mechanism is a nanoscale process. It should obey physical laws.

Marco

Olle Lind

Please: send me a few articles on black plants. Sorry, I have never seen a black plant.

Are they black because emiting infra-red light?

In a recente paper we have linked the colours of plants (and many others systems) to the mechanism of charges' separation physical effect. The colour could be linked to the spent energy to separate negative from positive charges, avoiding violation of physical laws.

Please see: http://www.scirp.org/journal/wjnse/

Interface Recombination & Emission Applied to Explain

Photosynthetic Mechanisms for (e–, h+) Charges’

Separation

The reasons far warded by Marco & Vincent is more near to answer. The plants are best suited for this light spectrum for maximum phtosynthsis/food, Avoiding heat generation at that wave length( green) and amount of the molecules absorbing green wave light , probably burden on the plant. Tetrapyrole such a big molecule , if it would have adopted to that green, what could have molecule shape?

Not all plants are green in their leaves. So the answer must lie in the chemistry of the pigments themselves, chlorophyll for the green plants and carotenes and anthocyanins for plants with other colors.

All plants have evolved from a chlorphycean member some 350 million years ago and since then the photosynthetic pigment like chlorophyll has passed on to the evolved members of plants. The wavelength of green light is 520–565 nm, Chlorophyll a, the most important light-absorbing pigment in plants, does not absorb light in the green part of the spectrum. Therefore the plants having this pigment appear to be green in colour. Plants use light in the 450 and 700 nm range, which mainly from the visible spectrum of light. Absorption of light by chlorophyll a is at a maximum at two points on the graph 430 and 662 nm. Sunlight contains 4% ultraviolet radiation, 52% infrared radiation and 44% visible light. The photosynthetic efficiency of most of the plants ranges from 1-8% in natural conditions meaning that much of visible wavelength of light has to be dissipated back to the heat sink (36-43%). Now according to Carnot’s heat law states temperature of source and sink cannot be equal and thus determines the maximum efficiency of a heat engine. The dissipated heat escapes back to the crust and atmosphere which in turn may be helpful for biogeochemical cycle, ocean currents and atmospheric thermal balance. The site of photosynthesis is the thylakoid membrane, which contains integral and peripheral membrane protein complexes, including the pigments that absorb light energy, which form the photosystems. This membrane contains the chloroplast DNA which has changed very little in times of evolution due to lesser recombination as compared to that of nuclear DNA, and thereby maintaining the consistency of chlorophyll pigment phenotype and genotype.

This question is more difficult to answer. It questions the very basis of plant adaptation to the environment over millions of years. The energy studded in the green spectrum (520-565)is frittered away. The action spectrum lies at 430 and 662 nm. Absorption in the blue and red regions of the spectrum is just sufficient for induction of light and the so called dark reactions as well as for light activation of certain if not all photosynthetic enzymes. Even by absorption at these regions, quite frequently one observes photo-oxidative damage more so during the summer at high intensity of light. Probably considering that there might be more heat generation and super oxidation of antennae molecules inspite of innate mechanism to overcome the process, plants had nort resorted to green light photosynthesis. In fact, if I remember right, photo activation of enzymes peaks in red region of the spectrum. Of course no one might have studied against green light. Quite sensibly,it may not have any effect at all with the existing conformational structure of the light absorbing molecules of the present day plants

Because plants do not absorb green light. Photosynthesis uses blue and red light from sunlight.

Dears Ahmed Bamouh, Munusamy Vivekanandan and Shivaji Chaudhry.

Experimental results on reflection and transmission gives about 45% for these two effects (R & T) for natural leaves. So, the absorption on plants gives about 55% for the green colour. With these experimental results, it is impossible to say that plants are green because Chl is present and the green colour is not absorbed.

There are, presently, many experiments on plants, showing that they can grow up with green light (please, see Terashima ref. below). If these plants can grow up with the green ligth, the mechanism of charges separation (to perform the photosynthesis next steps) must operate on the leaves. That means: the evolutionary steps of plants gave them the possibility to be feeded with most of the sun available colours (UV to the Visible light).

Concerning the plants evolutionary steps, 2 billions of years ago, the Earth CO2 composition was about 10% (and nothing for O2). So to much food was available to plants and that gives rise to the exuberant florests on Earth. If the Nature's evolution move from 10% of CO2 food to 400 ppm CO2 today, the plants' DNA should have on them the necessary codes for adaptation. May be, consuming UV, blue, green or red for their life (depending on the present H2O, CO2, etc), plants should be able to use the entire solar spectra to keep populating Earth. So we cannot say that plants are green just because Chl does not absorbs grenn photons. Chl and leaves are physically difrerents systems. Plants absorb green photons and produce the photosynthesis processes (biomass and O2 waste). The enigm here is: how the absorption mechanism gives electrical charges separated? It means: how to separete an electron (negative charge) from a hole (positive charge), avoyding violation of physical laws? Note: charges does not get separated just because a photon is absorbed. It needs a further step to separate the negative from the positive charge. This step must spend energy.

Note: if evolutionary steps on Earth change from 10% to 400 ppm of CO2, with an exuberant florest about 600 millions of years ago, why are we discussing about green house effect today? May be we should increase CO2 level, to decrease the desertification effect, and not the contrary...

Please, see: Terashima, T. Fujita, T. Inoue, W. Chow, R. Oguchi, Green light drives leaf photosynthesis more efficiently than red light in strong white light: revisiting tquestion of why leaves are green, Plant and Cell Physiology 50 (2009) 648-607.

Dear Sacilotti. You proposed very interesting view although many points are still fuzzy to my mind. someone should explain 79% absorbtion of solar light by leaf as soon as possible. Then, we ll get rid of many parts of photosysnthesis mechanism.

As far as I understood, you reported that as leaves absorbs higher photons energy than the green photons and emit lower energy photons (green, yellow, red), to the green colour intensity of leaves it should be added the green transformed by their internal machinery...May be the act of this "internal machinery" an answer for why plants are green! .

But the point you noticed for example " The enigm here is: how the absorption mechanism gives electrical charges separated? It means: how to separete an electron (negative charge) from a hole (positive charge), avoyding violation of physical laws? " is still hanging for me. Yes, you added an attachment, but far from my understanding (due to the physical concepts of the issue) could you make it little bit understantable, at least for me... :) thx a lot.

Dear Buhara Yucesan.

Most of the books on biology (photosynthesis) present the absorption of photons by leaves, giving directely electrical charges "walking" freely to the reaction center, where the "mother" molecule is waiting for the negative charge. The following step is to produce sugar (biomass) from these separated negatives charges.

In physics and more, in the nanometer scale world, the absoption of a photon by a molecule can give enough energy to the electron, that jumps from the conduction band (HOMO), to the valence band (LUMO). These HOMO and LUMO are energetic positions to be filled or not with electrons. So, the absorption of a photon by a molecule make the electron jumps from the LUMO to the HOMO. The jumped electron leves a hole at the LUMO. Up to this stage, we have a pair of electron and role under attraction. This attraction is called "excitonic". More, up to this stage, both electron and hole are separated by the energy gap between HOMO and LUMO (this is called forbiden band or forbiden band gap energy). Up to this stage, both electrons and holes can recombine, emiting a photon which energy is linked to the forbiden band gap.

The excitonic energy (attraction between positive and negative charges) on organic molecules is very high and difficult to separate both electron and hole, when both are seated on the same molecule. The charge can flow to the nearby molecules. For it, we need to know the relative positions of the HOMO and LUMO energy levels of the nearby differrent molecules (related to the first one absorbing molecule). If an appropriate energy (for HOMO and LUMO) configuration is present between both molecules, the flow of charges can take place, for both (e- and or h+) or just one of them. The flow of charge from one molecule to the nearby molecule create an electronic desequilibrium on both molecules. This electronic desequilibrium creates electric field. This electric field can pull others non jumped charges away from the initial absorbing molecule. The jumped charge, from a molecule to the nearby molecule can recombine with the hole seated originally at the first abosorbing molecule.

This is called an interface recombination between an electron and a hole seated on different molecules. This electron-hole recombination gives a photon with a lower energy than the first one and the second one forbiden energy band gap. This lower energy photon is related to the spent energy to separate others charges.

I'm proposing that this interface recombination (and emission) is associated to the green colour of plants (and, also, related to others less intense colours of leaves). The evolutionary steps of plants holds many possibilities to separate charges (it is a statistical possibility), giving different colours. Today we have the green one. 600 millions of years ago, this waste could be red shifted. Note that the red colour "waste" can give better efficiency to leaves, compared to the green colour as a waste. Today, plants dont need waste less, they just waste the green, ... it is enough!.

Please, see these charges movements on a hypotetical A/B interface in a ppt slow motion enclosed. A and B represent different molecules.

In pursuance of the green color of the leaf of higher plants, lot of deliberations have been made, rather acrimonious . I think we are nearer to the answer but it is rather not easy to understand the innate adaptability mechanism that the green plants had undergone during the course of evolution. I very well agree with Dr. Bukhara Yucesan that leaves absorb higher photon energy and emit lower energy photon (green, yellow and so on). This might also contribute to the higher intensity of green emission.

Another interesting information comes up from Dr. Marco Saciloth quoting the article of Tearashima et al.(2009) that green light drives photosynthesis more efficiently than red light upon parri passu irradiation with strong white light. Therefore, it is discernible that green light has some role to play in triggering photosynthesis. The role of green light in isolation on green plants might give us some clue but no one might have ventured on this. The bio-physical aspects of photosynthesis in different regions of the actinic light might throw still better picture on this

Dear Munusamy Vivekanandan

Your phrase below call my attention:

"it is discernible that green light has some role to play in triggering photosynthesis"

I should say that it is exactely what do type II interface between two generic A/B molecules. This kind of energetic interface can help on the carges separation mechanism even if it does not excite one or both A and B molecules. It can be an interface absorption and emission mechanism.

Suppose that A and B are Chl and Car and suppose that both separately does not absorbs green light. When they are togheter (and if its interface is type II), it is possible that the interface absorbs and emit quickly the green photon. In this case, the absorption creates energy band bending (potential variation). This potential variation creates the necessary electric field (the driving force) to separate electrons from holes on each one (A and/or B***), performing the photosynthesis process. The ppt enclosed propose this mechanism of charges separation and interface absorption and emission.

*** Note: (e-, h+) pairs should be available on A and/or B to be separated by the interface green absorption.

Nature has created only one mechanism for O2 production in 2 billions of years evolution. I do not believe that Nature created more than one mechanism for charges separation on leaves. The "mechanism" described on books (biology) violates physical laws!

By the way, insects shows some intriguing behaiviour, chaching colors growing under different conditions (see enclosed), showing that Nature's evolutionary steps on plants and insects can have some "ambiguity".

Dear Munusamy Vivekanandan

Enclosed the paper about insect: green and yellow.

Light- induced electron transfer and ATP

synthesis in a carotene synthesizing insect

by: Jean Christophe Valmalette1, Aviv Dombrovsky2,4, Pierre Brat3, Christian Mertz3, Maria Capovilla4, Alain Robichon

SCIENTIFIC REPORTS | 2 : 579 | DOI: 10.1038/srep00579

Dears all

Below an other paper on plants being feeded with green photons.

Effect of green light wavelength and intensity on photomorphogenesis and

photosynthesis in Lactucasativa

by: M. Johkan1,K.Shoji∗,F.Goto,S.Hahida,T.Yoshihara

Environmental and Experimental Botany75 (2012) 128–

133

Dear Ralf Gregos Niemann

May I try to suggest you an idea about your question below:

" I am talking about the methodological reason why plants do not take advantage of the green light, since sunlight emits the highest light intensity in the green spectrum."

Why the green ? Why Nature provoque this choice? Or the choice for green depends on the possibility of making the best for the natural conditions we have today?

For sure: the green colour of plants is not a reflection and it is not due to the non absorption of green colour by the Chl. Experiments proof the contrary. 55% of absorption for the most intense sun light green color. This is to much. As it is to much the 79% absorption for the role sunlight spectra. Leaves need an efficient way to waste most of the 79% absorption. The most intense (green) can be the waste window to get rid of the problem.

If we do accept the fact that the green colour comes from an interface effect, like I did describe before, the green colour being absorbed, followed by geen emission, it can help on the photosynthesis process (charges separation). Wy? Because an interface absorption also creates an electric field. This electric field can help to separate charges. Today, with the available CO2 concentration (400 ppm), the leaves does not need the most intense colour to grow up or to live. So absorbing and emiting the green color can helps on the charges separation mechanism but it does not supply enough energy and charges to make the role process.

May be, if we have more CO2 (*), plants need to change genetically and work more. This means that more photons are needed to separate more charges. In this case, the green window should change to the red side. In this case, plants should look brown to red.

* 0.6 to 2 billions of years ago the atmosphere was composed of 1 to 10 % CO2. In this case and working more, plants should look not as a green color. May be red shifted.

If the sun light most intense color were the red one, may be plants should look red, if the available CO2 concentration were low (as today). If the available CO2 concentration were high, the plants should look blue shifted.

Note: The interface absorption happens when an absorbed photon make electron to jumps from the valence band of one molecule to the conduction band of the nearby different molecule.The interface emission makes the contrary: an electron recombines with a hole sited on the nearby different molecule, emiting a photon. This emitted photon should have lower energy than both band gap molecules playing this game.

A big question: why researches dont' work with interface effect if it does exist on Nature?

Dear Sacilotti, your last summary seems very intersting summing up your previous descriptions and making more sensible (at least for me). In fact, there seems so many parameters to check the availibity your thesis; of those interface effect (very empirical term :) ) was also quite interesting. Incresea in CO2 level might account for leaf color change next time, however todays level as you noted seems very low. Many plant evolved having alternative pathways creating extra CO2 pathways (i.e. C4 and CAM plants). Might many of those green plants be negatively affected by greenhouse effect?...

I have observed that biology has played a major role to form the

earth.infect earth itself is a single living organism like a tree.please

observe the following explanation for this hypothesis. this hypothesis is

like a Gaia hypothesis supported by some more evidences.

1. Amino acid and Biological chemistry in chondrite meteorite.

we have found amino acid and biological chemistry inside the

chondrite meteorite. as i consider them seeds of planets. one planet is a

result of one asteroid as one tree is a result of one seed. these amino

acid and biological chemistry is a main property of any seed.

http://yfrog.com/m9meteoriodj

2. bark as continent; if you will observe the continents can be fit in

small globe like a puzzle game same bark can be fit in small gorth of same

log of tree.

http://img861.imageshack.us/i/treebarkcontinents.png/ --- Bark &

Continents

http://yfrog.com/gh08810treebark1221170loj TREE BARKS

http://yfrog.com/6zpicxaj bark Earth & Tree

http://yfrog.com/h4moo6j Safeda

3. core and crust; same as in log of tree.

http://yfrog.com/5ucorecrustj Core Crust

4. plate tectonic; yes i agreed with PT but at the end biological process

beneath the earth surface is responsible for the motion of plates. same PT

is happening in the log of tree.

http://yfrog.com/0g72697054j Plate Tectonic 4.

http://img705.imageshack.us/i/platetectonics.jpg/ PLATE TECTONIC LINK

5. subduction zone;same in log of tree.

http://yfrog.com/0tplatetectonics2j Subduction Zone

6. HYDROCARBON a scientific ; this is a scientific evidence of my

hypothesis.that earth itself is a single living organism and producing

hydrocarbons like any all living organism. I believe crude oil has both

deep and organic origin . fossil oil theory is not correct. we have

observed hydrocarbons at almost whole universe including Titan. there is no

solid reason that hydrocarbons at earth has been from fossils of past life

and at rest universe with different method where no life has been observe

yet.

More over i have solved the mystery that why sediments are signatures of

presence of oil while has no involvement to produce it. this is the main

logic of fossil oil theory. infect i have solved this mystery of

petroleum. my theory is between the current biogenic and abiogenic

theories. I believe oil has both deep and organic origin like a bark oil.

Making Oil From Birch Bark

http://www.youtube.com/watch?v=CC7i5CY6XNo&NR=1

http://www.youtube.com/watch?v=a3lG3FX9D68

7. presence of same minerals like iron,nickel,moly,crome,V,MN,ZN,,,,,,,,,

etc at earth are also present in all living organism.this is very much

common factor in all living organism inculding hydrocarbons.

I have lot of these type of evidence and putting all together we can

conclude it that earth itself is a living thing that has been grown from

small asteroid.I need some assistance for further examinations.

regs

[email protected]

Dear Rienk van Grondelle

Most of the energetic representation, to explain the photosynthesis mechanism is based on a non existing physical possibility: the ground state energy representation, e.g.: the SI state and so on. In this way, electrical charges cannot see a non existing energetic configuration. Electrical charges see energetic barriers (up or down).

If energy transfer (e.g. FRET mechanism) does not separate electrical charges, how can we explain charges separation in the photosynthesis process? In my opinion, most of the "scientific" explanation on these photosynthesis processes violate physical laws. I'll expose two:

a) to separate an electron from a hole in such a small distance, we need the driving force (an electric field). This is absent on the present theories.

b) to separate an electron from a hole, we need to spend energy. Idem.

Note that the electronic desiquilibrium on the molecules (Car, Chl), caused by the charges' flow, is not taken into consideration. This electronic desiquilibrium creates energy band bending (potential variation = electric field).

As the FRET model is based on a non existing energetic physical configuration, we do propose that the mechanism is simply an interface effect, as described enclosed and on: World Journal of Nano Science and Engineering, 2012, 2, 58-87

Plants are green due to production of chlorophyll a and b in chloroplast. If the pigments are of other types they will not appear green

Dear Professor R.C. Dubey

May be Chl looks like green and seems not absorbs the main green colour, but there is no scientific proof that Chl does not absorbs and emitts quickly the green colour.

But, the problem beween Chl and leaves (in plants) is that they are not the same system.

As they are (in plants) a different system (of that of just an ensemble of Chl molecules), composed by nanometer molecules, separated by nanometer distances, the absorption, reflection and emission properties are completely different.

The experimental results on reflection and absorption on the visible spectra gives only 15% for the green colour relection. Absorption is about 55%. These two % does not explain the green colour intensity on plants

Dear Professor Rienk Grondelle

You talk about "charge transfer states", frequently used to explain the photosynthesis mechanism. I'm trying to use solid state physics to explain organic molecules interactions'.

Using inorganic/inorganic materials, as far as I have seen, they use the concept of charges transfer states (copied from photosynthesis) to explain photovoltaic (charges separation) effects that is nothing more than interface emission. Please, seee the work by Scholes' group, below.

In my opinion, FRET can also be explained by interface properties (as explained in the pdf eclosed). As I said before, the energetic configuration proposed to explain FRET, Charge Transfer States, GFP, etc does not exist in physics. We have, according to 3 Nobel Prize Laureates, only 3 energetic relative positions between two different materials (or molecules). More, energy band bending between organics molecules does exist, according to many experimental results. This is not taken into account on the existing models. The energy band bending (potential variation) gives the necessary electric field to separate (e-, h+). Concerning the flow of electrons to the RC, may be we should be carefull. Electrical charges does not have a net flow somewhere without an electric field.

These differents ideas are published on: http://www.scirp.org/journal/wjnse/

S. Kumar, M. Jones, S. Lo and G. Scholes, “Nanorod

Heterostructures Showing Photoinduced Charge Separation,”

Small, Vol. 3, No. 9, 2007, pp. 1633-1639.

doi:10.1002/smll.200700155

G. Scholes, “Controlling the Optical Properties of Inorganic

Nanoparticles,” Advanced Functional Materials,

Vol. 18, No. 8, 2008, pp. 1157-1172.

doi:10.1002/adfm.200800151

S. Kumar and G. Scholes, “Colloidal Nanocrystal Solar

Cells,” Microchim Acta, Vol. 160, No. 3, 2008, pp. 315-

325. doi:10.1007/s00604-007-0806-z

Note: in one of his publications, Scholes uses the right energetic configuration (type II energetic interface) but fails in using 'charges transfer states' configuratio/explanation. This simply because the ground state energy model does not exist in physics.

Plants are green because natural selecction goes always in the sense of protecting the cell and tissue of any danger and water balance. When we see at the light espectrum comparing with the expectrum of clorophy a and clorophyll b the have the optimun rise pick, early in the morning and late in the everning, respectively during the day light length. In both two moments of the day the evaporative demand of water is low. So they can work happyly . At least during a hot day, leaves have the opportunity to begin and end well , working several day hours. One day long, were the stomata should be closed, perhaps by the high temperatures. Leaves ignore how is going to be the day temperatures, or what the temperatures have done all way along the day. They do there best each the time.

Plants are green because natural selecction goes always in the sense of protecting the cell and tissue of any danger and water balance. When we see at the light espectrum comparing with the expectrum of clorophy a and clorophyll b the have the optimun rise pick, early in the morning and late in the everning, respectively during the day light length. In both two moments of the day the evaporative demand of water is low. So they can work happyly . At least during a hot day, leaves have the opportunity to begin and end well , working several day hours. One day long, were the stomata should be closed, perhaps by the high temperatures. Leaves ignore how is going to be the day temperatures, or what the temperatures have done all way along the day. They do there best each the time.

Sorry .

And cosider how during the firsts day hours light are more blue and the last ones are more red that are coincident with chlorophyll a and chlorophyll b . And are also coincident with there chlorophyll colors.

Dear Alberto

I do agree with you first phrase but this does not explain the question by Ralf. "Why the maximum intensity of sunlight (green) is not the most important for leaves absorption with the present natural conditions?" In my opinion, absorbing higher energy photons (from UV to green), transforming them on lower energy photons (e.g., green, yelow, brown, red) and spending energy to do these transformations, is a way to protect leaves from burning and keeping just enough energy to keep the standard conditions we have today. The quantity of food (CO2 & H2O) available today (very low) impose the evolutionary steps to be the green colour as the most important emission/reflection/transmission "waste" energy.

Another important point: why researches never talk about the spent energy to separate electrons from holes, following the photon absorption by leaves (their proteins)? I do think that this is, may be, the most important point for leaves to survive on hot sunny days. This attraction energy is considered as an exciton. On organic material this energy can be as high as 0.5 eV for each (e-, h+) pair and absorbed photon. To separate this (e-, h+) pair we need to spent energy. This is not taken into account on books and the 'physical" explanation violates physical laws. Note that books on photosynthesis consider that it is enough to have the photon absorption to get the (e-, h+) separated. In physics we need some more steps, to separate both (e-, h+), in such a small distances (nanometer size).

We must look the origin of chlorophyll, chloroplasts, and atmosphere in this time. And why bacteria beginning to generate energy from this system. The consequence was the generate of oxygen from CO2.

Hi,

Chlorophyll gives plants their green colour. There are other pigments in the leaves too, such as xanthophylls (yellows) and carotenoids (yellows, oranges and reds). These pigments are also used in photosynthesis but occur in lesser quantities than the green chlorophyll. The combinations of the different pigments make different shades of green.

Now the reason that plants look green is that they are trying to obtain energy from the sun using a particular part of the light spectrum, mainly the red and infra red wavelengths. If you remember from your physics classes the colour you see is the colour that is reflected from the object, the other colours are absorbed. So in the case of green plants, the green wavelength is reflected and all the other colours, especially reds and blues, are absorbed to drive the energy cycle in the plants.

Chlorophyll does best in the red (around 670 nm) and blue (around 500 nm) areas of the spectrum. That's why many plants have the additional pigments (xanthophylls and carotenoids) called accessory pigments that feed light energy to chlorophyll "a" from light. Chlorophyll is almost useless in the green part of the spectrum, and doesn't absorb that colour. That is why most plants are green.

Dear Chandrasekhar Kuppam

Chl inside leaves and Chl separated from leaves are two different nanometric systems.

Optically and electrically their behaviour are different being inside or outside. Inside leaves, Chl, Car, ect behave as an nanometric and composed system.

According to Terashima (ref. below), the green colour is more efficient than the red and blue ones for the photosynthesis process. So, the green colour cannot be considered mainly as a reflection.

More: according to reflection and transmission experiments on leaves, we have only 15% of reflection for the green colour. This does not explain the green colour intensity from leaves. Absorption for the green colour on leaves is about 55%. These experimental results put down many ideas on the photosynthesis process/Chl/charges separation mechanism on leaves.

The question by Ralf is: why leaves didn't make an evolutionary choice to take proffit of the more intense colour of the sunlight? His question is deeper...

In my opinion, the green colour of leaves is strongly related to the spent energy to separate negatives from positives charges. Otherwise we are faced to violations of physical laws. Please, see:

M. Sacilotti et al: World Journal of Nano Science and Engineering, 2012, 2, 58-87,

Interface Recombination & Emission Applied to Explain Photosynthetic Mechanisms for (e–, h+) Charges’Separation at http://www.scirp.org/journal/wjnse/

I. Terashima, T. Fujita, T. Inoue, W. Chow and R. Oguchi,

“Green Light Drives Leaf Photosynthesis More Efficiently

than Red Light in Strong White Light: Revisiting

the Enigmatic Question of Why Leaves Are Green,”

Plant & Cell Physiology, Vol. 50, No. 4, 2009, pp. 684-

697. doi:10.1093/pcp/pcp034

Professeur Marco, Thank you very much for such a valuable suggestions. I am very much happy by the addition information from your side (reference).

If we consider the question from evolutionary and ecological perspective, the answer would be simple; early land plants were evolved from green algae, which were abundant in surface waters. In such a condition, there is a plenty of sunlight and so they didn't need to harvest light in green spectrum. Those alga which live in deeper waters where there is shortage of light has developed pigmentation system (phycocyanin in blue-green algae and phycoerythrin in red algae) to absorb most of sunlight spectrum so simply they are not green!

A question may be raised that why in terrestrial environment such adaptation had not evolved? Two reason may be raised 1- Simply because the genes that had introduced to land plants were from green algae and not red, blue green and brown algae and 2- In terrestrial situation absorption of full spectrum of light could result in extra heating that could be considered a disadvantage causing problems for leaf to get rid of extra heat.

See the figure for absorption spectra of photosynthetic pigments here:

http://www.emc.maricopa.edu/faculty/farabee/biobk/pigment.gif.

Dear Ebrahim Hadavi

The absorption curves you sent in attachement is for proteins separated from leaves. It is not the same thing as on leaves. They don't bring information and do not answer the question proposed by Ralf.

Moreover, according to absorption and reflection experiments on plants, they have only 15% of reflection for the green colour and 55% absorption for green colour. So, plants look green because of its internal machinery, transforming photons with equal or higher energy than green into green. As the green photons is more efficient than red or blue photons (see Terashima work) for the photosynthesis process, the question keep on.

In Addition to Prof. Sacilotti answer; Ralph's question might be extended up to why green algea are green, so on. Do we know the most frequent leaf color 300 mya or more in terretrial life!

It is a question that provides enough food for the thought. Seems very simple but I feel it is very difficult to answer this question. I am although not suitable person to answer this question, but since I have something in my mind and that im just writting as below:

It seems that the answer is not in that the plant utlize only a narrow spectrum of sun light but is that why it utilize only that narrow band of light? If we look critically that what is primary reaction driven by light then it is only the photolysis of water and extitation of the electron for the photophosphoriliation. To achieve this, light of higher energy is required and it seems that this job cannot be accomplished by the energy of most of the visiable spectrum including the green and near green region. For photolysis of water and excitation of electron for the photophosphoriliation higher freqency light is required and that could be the possible reagion that photosynthetic appartus utilize the blue light (a region towards the maximim frequency and higher energy in visible light)

It is an real intersting question and hope has multiple explanations too

Regards

Anil Kumar

Dear Marco Sacilotti,

The absorption curves are for major pigments extracted from chloroplasts. We see that by considering the curve for terrestrial plants the green spectrum remains less-absorbed and we see the chloroplasts as green.

The leaf color is affected by other factors like air spaces and more that is beyond scope of this question for example some plants seem white. This is due to absorption/reflectance by other parts of leaves. This is true for the 55% absorption of green color by leaf. This phenomenon (absorption of spectra by other parts of leaf) is true for all wavelengths not just green. Therefore, if we talk about the chloroplast color it would be more accurate, and regarding chloroplast color it, is resulted by pigments and their cumulative absorption spectra. Of course, this curve shows just major pigments and there are other minor pigments. While 15% of green light is reflected (and 30% Transmitted?) the other wavelengths have lesser reflectance and transmittance ratios and a large portion of their energy is dissipated as heat so the overall output is what we see as green light.

Regarding any machinery in chloroplast that could transform any wavelength to another one I am really in doubt. In fact “transforming photons with equal or higher energy than green into green” seems impossible at least in biological systems. There are some ways proposed in physics forums but none is applicable to plants (see http://www.physicsforums.com/archive/index.php/t-1833.html), so I don’t think that this is what aimed by Terashima’s work.

Green light (together with infra red light emitted from leaves and elsewhere) could be used as a source of energy by leaves in lower parts of canopy and this could be considered as a ecological advantage. Even thought individual leaves do not absorb Full-spectrum of light the land plants can harvest most of light energy by creating a profile of tall plants to above ground annuals and from light adopted leaves in upper canopy to shade tolerant plants which are adopted to low light conditions. This way we see that while when we focus on a given plant we see a waste of light energy but when considering the same plant in its natural habitat we will see that the nature had managed to use light energy with the highest efficiency. In fact the green light (and IR) might be considered as the exchangeable form of light inside canopy levels. This could make green color to reach our eyes more than any other one so the plants seem green.

Dear Ebrahim Hadavi

There is a branch on physics called materials science. In this branch we have the semiconductor theories and devices (experimental results) we have today (lasers, LEDs, detectors, transistosrs, etc). This is the solid state (inorganics) physics. In this field we have what we call "the band gap enineering" to build these devices. Now, considering organics, recently researches developed the band gap engineering too. In these researches they developped organic LEDs, organic solar cells, etc.

The models to describe inorganic devices are utilized to describe organic molecules interactions, concerning the forbiden band gap and their relative position. If the same model can be applied to inorganics and organics (and now to mixed organic/inorganic) I don't see why not to use it to proteins like Chl, Car, etc on leaves.

If the question is "how photons absorbed by leaves (by their proteins and membranes) can have a negative charge separated from a positive cherge?" I do see only one answer: we should use models that work. The models that work uses the concept of band gap engineering. So, negatives charges walking down on the conduction band (LUMO) and positives charges walking down on the valence band (HOMO) of a molecule must see and interact with the conduction bands and valence bands of the nearby molecules. It is exactely the same as for both solid state and organics materials.

People working with plants do not accept these concepts and they keep working with a fixed forbiden band gap and non existing energetic configuration (*). In this way, it is very difficult to model electronics in plants and find out the physics behind it. The physics behind it is: electronic movements, optical (absorption and emeissin) properties, etc.

* the ground state energy representation = the valence band energy level is not known and they represent it at the same energy level for both molecules. This is not correct! Presently we have many groups working to find out the relative position of many organic molecules, diluted on polymers and deposited as nanometric layers. Moreover, energy band bending is now know (experimental measurements). The band bending gives the electric field, necessary to separate and electron from a hole. Otherwise we are faced to the violation of physical laws. Nature don't do that.

Charges don't see the ground state energy level. Charges see energetic steps (up or down) when interacting with their neighborwood molecules. When electrical charges jumps from their molecules to the nearby molecule, both molecules remain in a non equilibrium condition. This non equilibrium conditiion creates an elecric field. This electric field can push away others charges. Only in this condition we can have a net flow of negatives charges to one side and a net flow of positives charges to the opposite direction.

The above explanation is to present you a paper on inorganic/inorganic system or interface, where, for the first time, we have the proof that interface can absorb and emit at approximately the same wavelengh (please see figure 2, transition B2).

I don't see why the same concepts for organic/roganic, inoganic/inorganic and organic/inorganic cannot be usefull for leaves. The physics behind it is the same.

That is why I'm provoking discussions aboutband gap engineering on molecules on plants and trying to show to researches that, if we don't have the appropriate physical condition, we can never propose good models (I'm not a theoretician). By the way, the model to explain FRET (energy transfer between organic molecules) is based on a non existing energetic configuration. What should we expect from a non existing energetic configuration? Charges don't care about models based on non existing physical situation. The models proposed by Forster (60 years old) should be rediscussed.

Coming back to the question by Ralf, I should try: plants don't need the main intensity of the solar spectra today (green). So, plants uses it to waste most of the non-necessary energetic source (absorption and quick emission), by interfaces effects. Note: absorption by interfaces do create energy band bending, so creating electric field, able to separate electrons from holes. The separated (e-, h+) pairs can get back and recombine and emit again (once on the way to separate them).

After reading the paper of Terashima introduced by Marco Sacilotti, not being an expert in this field, if I got it right what he proposes has nothing to do with the physics of light absorption discussed by Marco. Terashima just points out that, 1) because of the inefficiency of RuBisCO, plants need lots of it to get enough CO2 fixed; 2) they need thick leaves to accomodate that RuBisCO in chloroplasts and those chloroplasts accross the whole leaf depth to maximize cell surface are per leaf surface area and ensure efficient CO2 supply to the chloroplasts 3) this poses a "light" problem, as light reaching the lower leaf part (assuming it comes from above the leave) is much less that that reaching the upper leaf part, resuting in either too little light at the bottom in low light (hence, rendering chloroplasts there useless) or too much light at the upper part in high light (hence, causing photoinhibition problems) whateever the chlorophyll concentration in the chloroplasts (and remember any plant experiences both situations along the day and the year) 4) Using a molecule that absorbs parts of the light spectrum available on earth surface more efficiently that other parts of the spectrum (i.e., blue and red vs. green), but still absorving in all this spectrum as pointed out by Marco, the result is that red and blue light can be used efficiently in the upper part, but they do not penetrate in the leaf if chlorophyll concentration is adapted to the mean levels of red and blue; however, green, been less efficiently absorved, does penetrate to the lower parts of the leaf, were a similar chlorphyll concetration would be effective to capture green light, resulting in the light being used efficiently both in the upper and in the lower part of the leaf, either in low or high light (red and blue light in the upper part, green light at the bottom) with no need of two or more pigments specialised in blue or red light in the upper part and green light at the bottom (or any other combination of parts of the spectrum) and being able to do that with a single mollecule (well, in fact two: Chl a and b). In fact, Terashima does not explicitelly takes into consideration chlorophyll concentration, but this extension is obvious to me, so I included it.

My point is that, as far as Terashima goes and with my little knowledge in light absorption physics, this explanation is independent of the actual form of physical light absorption by organic molecules, be it the Foster model or what Marco tries to introduce for us. I see no need for green light having a special role in charge separation or whatsoever, which I do not deny: I am completely ignorant on the subject and just keep repeating the Foster mode as I was taught ;)

But still, I do have a couple of quations for Marco, if he is so gentle: if I got it right, whta you propose is that after a phton is captured by a Chl molecule, what happens in every energy absorption (either phton absorptions or energy transfer to another chlorphyll) is a charge separation and the migration of an electron to the next molecule, which we were taught only happend in the reaction center? Or this only happens exactly in the RC? In either case, do you propose that the needed energy comes from green light being absorbed and released? I probably misunderstood you, because if this is whta happens, first it would not influence the balance in reflection and transmittance measurements, as any photon absorbed and re-emitted in the same wavelength would pass unnoticed, and it would no deliver any energy to any process, as if the wavelenght is the same for the absorbed and emitted phtotns, the energy is also the same, so we keep at draws.

Dear Xavier Aranda

The absorption of a photon does not separate spatially a positive charge from a negative charge. Both are separated in energy. This is the big problem of the present theory. To separate an electron from a positive charges we need certain conditions. This is not provided by the present theory (or model). More, the presently accepted Forster model does not explain this separation mechanism. Trushly speaking, it violates physical laws. Firstly, the Forster model is based on an energetic configuration that does not exist in physics. Electrical charges does not see this energetic configuration proposed by Forster.

As a physicist, I'm still at the begining and trying to understand the mechanism.

Whenever an organic molecule (or inorganic material) absorbs a photon, its physical situation is as that described as an in an 'excited state'. As the absorption of a photon takes an electron from the valence band (HOMO) and put this electron on the conduction band (LUMO), both the empty state (hole) on the valence band and the filled state (electron) keep under attraction. The valence band and conduction bands can be seen as energetic highways (streets) for both: holes and electrons.

For the excited molecule: if there is no 'an appropriate energetic configuration' of the nearby molecule, charges don't move from the excited molecule. In this case both electron and hole recombine, emitting a photon that is characteristic of this excited molecule (the energy band gap).

The problem for the photosynthesis people: they don't know the relative position of the conduction and valence bands of the molecules that are side by side on leaves. This relative position (e.g., between Chl and Car) is very important to know which molecule will receive or give an electric charge to the other. If we don't know these relatives positions we can't know how charges go (or not) to the reaction center. Note: charges are not free to move where they want to. They move only if the energetic configuration allows it to perform the walk on.

This problem has been solved by people working on organic LEDs (charges getting together) and on organic solar cells (charges getting separated).

The big questions are: a) how to separate electrical charges under atraction, without spending energy? b) To separate charges under attraction we need an electric field. Otherwise we are faced to viloation of physical laws. On the present Forster theory (or model), both, the electric field and the spent energy are not present.

Concerning the colour (emission) of plants: it can be associated to the spent energy to separate electrical charges under attraction in a molecule (or ensemble of it). Depending on the energetic relative position between a molecule A and the nearby molecule B, charges can flow from A to B (or vice-versa). The flow can create an electric field. This electric field can separate other electrical charges (negatives from positives).

Only in this case we can have both the spent energy and the elecric field. This energetic relative position can be associated to interface effects, between the molecule A and the molecule B. On leaves we have a lot of interface between different molecules (about 10^20/cm3). This is not taken into account on leaves. But it is taken into account on organic LEDs and organic solar cells. That is why this technological field is so much increasing today.

What I'm proposing is that most of the absorbed photons on plants should be associated to the spent energy, able to separate electrons from holes (the colours of plants = emitted photon). There is to much absorption (about 79% of the visible light). Plants need to get rid of of the exess of absorbed photons (there is to less work to be performed with a so low CO2 level, today).

Note: interface absorption promotes an electron from a molecule A to the nearby molecule B. This electron can return and recombine and emit another photon with almost the same energy as that of the absorbed photon. This can be, for ex., the green photon. Even if it gives absorption and emission nearly with the same energy, it helps others charges to be separated. The interface absorption creates also an electronic desequilibrium, creating an electric field. This electric field can separate electrons from holes (caused by others absorbed photons).

In my opinion, the main green colour we have today is an evolutionary step of plants, that, according to the low level of CO2 disposable, they moved to the more intense (green) spectrum of the solar radiation, to keep doing its job without to much stress.

Increasing the CO2 (and others ambient physical conditions) concentration, may be plants will move to a red shift colour (more work, more stress).

The observations by Terashima (green is more efficient) can be part of the evolutionary step: it goes deep on the leaves thickness, it helps on the photosynthesis process but it is re-emitted without making to much damage to the leaves.

I hope these physical concepts can help people to discuss and to get to the right answers to the photosynthesis mechanism. Just accepting what is written on book, we will never get to the anwser.

Dear Marco

Thank you for your explanation, but if I got it right, you propose that green light is absorbed, helps in the phtosynthesis and is then reemitted? What happens with the energy and second law of thermodynamics, then? If the energy of green light is used, then either less photons are reemitted with the same energy, or they are reemitted with less energy, i.e., they will be red, far-red... By the way, these will not be counted as absorbed iwhen using an integrating sphere, than only sees the balance between absorbed, reflected and trasmitted but cannot tell apart a phton truly traversing the leaf or being directly reflected from a photon suposedly being absorbed and then reemitted in the general direction of incident light ("transmitted") or in the opposite general direction ("reflected).

Asi said, my knowledge in physics is very basic, and your explanation still looks to me that you are going much further than the question: wether you are right or not in the physical mechanism of photon absorption and charge separation, the reason for plants to be green derives from using chlorophylls and no other auxilliary pigmets that would absorb all light rendering leaves black, as discussed by Praveen Rahi. What you propose about green color being an evolutionary step of plants is as naïf as I guess my physics arguments are, because things do not work that way in evolution: plants, or in general living beens, do not "decide" which evolutionary pathway they will follow, it is just natural selection that favors one direction or another, but always on the existing variability. Putting it simple, plants would not be able to stop using a pigment and change to actual chlorophylls. In fact, we do not have any indication this happend in the past, and chlorophylls are the same that used to be when CO2 was much higher in the atmosphere. Hence, there is no reason to see it happening in a future atmosphere with higher CO2 levels; then, how would that "red shift" happen? Plants will not change chlorophyll for other pigments, so theis physico-chemical properties will kepp the same, in this case, they will keep absorbing green light in lower proportion than other wavelengths irrespective of plant "needs": it is much more probalble that other speceis already using different pigments overcome presnt days plants in the fight for light (if this scenario ever comes to reality) than seeing plants change chlorophylls for other pigments.

Also, the present level of CO2 is not so low, it has been in the same range for a long time (although it has been both much higher and lower in the past), but plants seem to perform well with it. In the same line of reasoning, why do you assume than absorbing 79% of visible incident light is too much light? For instance, for a leave down in a fores lisght level is about 5-10% of that received by upper leaves: is that light also too much? And it is absorbed with the same efficieny (in fact, a little bit higher). Moreover, were these numbers come from? The proportion of light absorbed by a leave will depende on the number of chlorophyll (and other fotoreceptors) present inthe leave and the intensity of the light: under intense light (such as at noon in summer in tropical regions), they will absorb much less light than at dawn the same day, and this is very simple to check with an espectroradiometer, and this is what I see every time I have to adjust the integration time of the radiometer according to sunlight intensity...

So, all in all I think you might be right in the aspectes related to photon absorption by organic molecules, but it has nothing to do with which fotons are relatively more absorbed than others, which causes things presenting one color or other, and why there is not a selection pressure to absorb the complete visible spectrum, or why plants are not black as discussed above by other contributors

Dear Xavier Aranda

Interfaces properties and its effects can absorbs and emit photons with nearly the same energy (lets say below 100 meV). According to experimental results, green photons are absorbed by about 55% of the solar spectra. If they are absorbed, leaves should know how to do with them. Note that 55% of the most intense part of the solar spectra (Ralf's question) is a big quantity.

For inorganics, these 'interfaces traped (e-, h+) charges' (please, see ppt file enclosed) can be of the order of few tens of meV, in each valence and conduction bands. So, from the main green colour (535 nm = 2,317 eV), you should extract these few tens of meV to keep respecting energy conservation (second law of thermodynamics). In average, you absorb green photons and you still have green photons (less few tens of meV) emited.

The absorption of green photons create energy band bending (see the curved conduction and valence bands on the ppt file). These band bending created the necessary electric field to separate negatives from positives charges.

For the phrase "the reason for plants to be green derives from using chlorophylls": Chl separated from leaves and on leaves represent different physical systems. The absorption and emission properties are differents. Note: CdTe and CdSe are different materials, with they on optical properties. When we growth one over the other, theier optical properties are completely differents. Why Chl/Car should keep the same optical properties as if they were separated?. Moreover, adding Chl/Car/Chloroplast should change even more they optical properties.

For the others questions, experimental results on reflection and transmission give directely absorption (see reference below). About 79% (calculated by me from these accepted experimental curves) of absorption is too much for leaves. Without an optical 'waste' mechanism, leaves can't afford such a big quantity.

Note: why researches don't have experimental results on polymers layers containing Chl (grown separately) and polymers layers containg Car to see their optical properties? After, growing (polymer + Chl)/(polymer + Car), it should give us the different optical properties. Working with layers (flat interfaces) it is easer than working with 3D structures (which is the case of Chl, Car, etc on leaves). That is the case of organic LEDs and organic solar cells (layers, with flat interfaces) we have today.

S. Seager, E. Turner, J. Schafer and E. Ford, “Vegetation’s

Red Edge: A Possible Spectroscopic Biosignature

of Extraterrestrial Plants,” 2005.

http://Arxiv:astro-phy/0503302v1 and

L. O. Bjorn, G. Papageorgiou, R. Blankenshi and K. Govindjee,

“A View Point: Why Chlorophylla?” Photosynthesis

Research, Vol. 99, No. 2, 2009, pp. 85-98.

Dear Marco Sacilotti,

I think that you agree that we can conclude that plants seem green, obviously because our eyes perceives green light more than other wavelengths.

When we talk about the mechanism behind we may mention as follows;

1. The higher rate of green spectrum reflection by leaves (15% vs roughly 8% for other wavelengths.

This 15% is the share of light that is reflected to same side of light source, but considering the overall sphere like shape of chloroplast, a higher part of light could be reflected towards other side of leaf that is considered as transmittance. Chloroplasts could act like randomly oriented micro mirrors for green light. This way chloroplasts are illuminated from all sides by green spectrum so all chloroplasts could have a minimal absorption compared with a fixed amount of blue and red light which could be absorbed by one side of chloroplast that perceive theses directly. In other words, even thought the green light is weakly absorbed in any given site of chloroplasts, it can multiple more sites due to scattering of photons in leaf. This is the cause for what is stated in Terashima’s work as “green light drove photosynthesis more effectively than red light” and “”…to drive photosynthesis efficiently in all the chloroplasts”. This reflection could be the cause for better penetration of green light into the leaf compared with red or blue light that is stated in Terashima’s work. In strong white light while the sides of chloroplast which perceive red and blue lights are in over-saturation state, more green light could scatter in leaf and drive photosynthesis in parts which do not perceive red and blue light. This way we can understand the role of green light in photosynthesis; It is not strong like blue and red in first hit, but instead it can multiply itself by reflectance and reach to places which red and blue cannot and this way it can create such final strong effect as reported by Teranshima.

2. The higher rate of transmittance for green spectrum through leaves.

This could accomplished partly by reflection from chloroplast in random directions as stated above leading light outward from other side. Obviously this light is consumed by lower leaves in multilevel canopies like forests. As we are practicing monocultures usually with short plants, we cannot benefit from this light source so wrongly we consider it as inefficiency.

For a detailed model for optical properties of leaves see http://www.photobiology.info/Jacq_Ustin.html

3. Fluorescence. The re-irradiation for Far Red light is well known both for pigments (chlorophyll fluorescence) and for other parts of leaf. This light could be a source of energy for lower parts of canopy. Fluorescence in other wavelengths is not reported so far but we can consider the potential. At least by UV light it is very common but needs to be investigated in depth.

4. The Interfaces properties (band bending effect) as put forward by you as a source of green light re-emission by leaves. In this regard, I believe that it is a valuable theory and needs to be proved as being present in leaves. If we accept that it persists now, then we need to compare its share in creating the green light emitted from leaf.

Absorption of light and emitting longer wavelengths as you described could be considered same as Fluorescence, could you elucidate the difference here in plant tissue? Maybe we have fluorescence in other wavelengths like green in addition to Far Red that you consider as interface effect?

Let my add a question to Ebrahim comment: from my poor understanding of the Forster theory, that Marco challenges, I would doubt there is room for other fluorescence emission in the visible range from chlorophylls and, as far as I understand, also from carotenoids from the phostosystems antennae because all absorbed phtons with more energy that red (and Chl and Car do not absorb UV), very quickly loose the energy above that of a red photon, and they loos it as heat as the quantum the loose each time is too small to produce a photon, so all of them (blue, green, yellow...) end finally with the energy of a red photon which, in case of being re-emitted as fluorescence, can only be in the far red (or a little portinon still in the red region, just below 700nm. I always wondered if this small fraction could be reabsorbed by PSI, but thois is another matter). It is true that some flavonoids do absorb UV, and in fact I was never curious enough to ask what did they do with the energy, so I assumed it ended as heat. Is this what you refered to, Ebrahim?

And here is the question: Marco, your explanation of the process of light absorption in organic molecules would affect fluorescence properties, or what I just described would be basically correct (o incorrect if I had it wrong even under Forster therory)?

Dears Ebrahim Hadavi and Xavier Aranda

That is truth, ours yes detect more efficiently green photons. Evolution did this and I don't know the reason for. Moreover I don't know if others animals see (e.g.: caws) it the same way we do. But leaves have others les intense colours like yellow, red, etc. This is, may be, part of the past and future adaption mode they can move to if climatic changes do operate.

By changing the medium where organic molecules are immersed, we can change the colour of plants. Please see:

Y. H. Su, S. L. Tu, S. W. Tseng, Y. C. Chang, S. H.

Chang and W. M. Zhang, “Influence of Surface Plasmon

Resonance on the Emission Intermittency of Photoluminescence

from Gold Nano-Sea-Urchins,” Nanoscale, Vol.

2, No. 12, 2010, pp. 2639-2646. doi:10.1039/c0nr00330a

In this case, aquatic plants change colour depending on the shape and size of gold nanoparticles they are immersed on.

By exposing plants to UV (A+B): the colour and nutritif power can change. Please see:

S. Blitz, “Lettuce Carotenoids Affected by UV Light in

Greenhouse,” 2009.

http://www.ars.usda.gov/is/np/Fnrb/fnrb0409.htm#lettuce

So, the Forster model, antena model, FRET model, GFP, green fluorescent models are far from being solved.

All these models don't propose a physical mechanism to separate negatives from positives charges. Neither, they dont propose a fast mechanism to spend energy in doing the charges separation act.

If plants have a self adaptation system and can survive with only one of these colours: red, green, blue or UV, each one of the A/B molecules pairs must have a charge separation mechanism and spend energy in doing so. For each one of the A/B generic pairs of molecules they should present an appropriated energetic configuration to promote charges separation expending part of it as a waste. For the total solar spectrum, plants must have some of these A/B pairs that is the most appropriate to better survive. May be few of them working at the same time when the sun is on.

The explanation of the process of light absorption in organic molecules pairs (or even the medium like chloroplast) would affect fluorescence properties. Adaptation during the year or during the day should affect absorption and emission fluorescent properties. Exemple given: if, during the day, leaves have to much separated charges, the contrary should take place. In this case, separated charges should return (electrostatic effect) and emit photons at interfaces between molecules, to get rid of the surplus work.

Sorry, MArco, but sea urchins are not plants, but animals. They do not even resemble plant. Ar the paper,a s per its title, speaks about bioluminiscence,light emission, not absorption.

Dear Xavier Aranda and Marco Sacilotti

Stimulating production of phenolic compound specially flavonoids under UV light is a well known process in plant tissue. We considered them as barriers of UV light to protect plant tissue, but the outcome of this discussion and the point regarding flavonoids by Xavier Aranda leads us to the possibility of these compounds not just as protecting plant tissue but as converting UV light to more absorb-able wavelengths. Interestingly I found a work that confirms presence of this fluorescence by anthocyanin which is a common flavonoid in leaves. So may be we have to add this role at least for flavonoids. There maybe other compounds with similar fluorescence function as well.

Please see this work which shows that anthocyanins fluoresce by UV light in 363, 434 and 519 nm:

http://www.sciencedirect.com/science/article/pii/S1011134499000706

On the proposition of Ebrahim about "conversion" of UV light into phtosinthetically efficient light by mean obf absorption and re-emission (fluorescence) by anthocyans, I would only add that fluorescence is usually a minimum proportion of absorbed light (at least for chlorpophylls), which can be very useful for detection and characterization purposes, as in the paper referred, or even to quantify processes in which the mollecule participates, as chlorphyll fluorescence quenching analysis, but I doubt they will be a quantitatively significant energy source, as they would be a minimum part of a small part of the UV-Visible spectrum. But it is just a prejudice that data may confirm or reject

Dear Xavier Aranda

Sea urchin is the shape of the gold nanoparticles that, being absorbed by aquatic plants, change its colour (emission and not reflection). So by modifing the Chl + Car + etc medium, the colour of plants changes. If the green colour of plants is due to a reflection of the green sun light, how to explain the change of colour by just changing the medium where these organic molecules are diluted?

Dear Marco,

Sorry for my displicent previous answer: ok, it is about sea-urchin-shaped nanoparticles, not about sea-urchins (by the way, Bacopa caroliniana, the plant they use in this study, is called water grass in the study, which usually corresponds to another species; anyway, none of them are aquatic, but terrestrial plants despite their name).

I am sure I did not fully understood the paper of Su et al. as it is far away from my expertise, but I did got several things: Basically, they use those gold nanoparticles to emitt blue photons by a mechanism I did not understood but, that is irrelevant for hte grass, as it is a blue photon that, of course, can be absorbed by the chlorophylls and processed in any of the several ways they usually do, one of them is fluoresncence, called bioluminiscence in the paper. In fact, they just use the ordinary behaviour of chlorophylls to prove that the nanaoparticles emit blue light, thats all. In other words, they just introduced blue nanolamps (ok, activated by light and not from mains or a battery, but this is irrelevant) and irradiated chlophylls from inside the plant. But for the rest, there is nothing new in the behaviour of these "in situ" chlorophylls. You would obtain exactly the same results if you irradiate a leave with blue light and, with the pertinent filter, you filter light emitted from the leave accepting only wavelengths longer than blue.

Again, the process by which light is absorbed and re-emitted (in this case, even from the golden nanoparticles) is irrelevant to the colour of the leaves. What matters is the incident light and the chlorophylls. So the original question stays as restated by Vincent Gutschick and Praveen Rahi: why plants do use chlorophylls and not other light-absorbing molecules instead or on top of chlorophylls, absorbing the complete incident spectrum capable of driving photosynthesis? Why they are not black, or red or blue? They look green because they emitt relatively more green light than other wavelengths. What you tried to explain, if I got it right, is that it is not (only?) because they reflect more green light, but (also?) because they have a mechanism of net green light emission, and you propose a putative mechanism for that.

My (provisional) conclusion after reading all contributions and most papers is that just green lgiht differential reflection would be enough to explain why plants look green, but here might be additional mechanisms (even more important that reflection, if you want), but they are still to be proven. But, most important, this does not answer the original question, which is why they do not use green light. Even if you are right and they comletely use green light but extract very few energy from it as they re-emitt it as still green light giving the apparence of being reflected, they questin would remain why don't they use other pigments that use green light more effciently, and I think the reasons have been given above.

As a final note, I will here stick to Ockam's razor and prefer the simplest explanation, i.e., chlorphylls absorb green light worse than other lights and a bigger proportion is reflected and transmitted, making leaves look green. Your alternative mechanisme would have to explain, if it is already present chlophylls extracts, how did it went unnoticed till now (i.e., extracted chlorophylls would also albsorb efficiently green light but use it inefficiently reemitting the absorbed energy still as green light) , or if not present in extracted chlorphylls and presnet in sin situ chlorphylls, it would represent a strange coincidence of a molecule absorbing poorly a specific band (then, reflecting / transmitting in higher proportion when in solution) but then abosorbing it efficiently but reemitting it when "in situ". As I said, unless this mechanism can be proved or at least shown to explain better some experimental results, thus deactivatin Ockam's razzonr, I will stick to it.

Dear Xavier Aranda

The mechanism by which gold sea-urchins chape nanoparticles, absorbed by bacopa caroliniana, emit different colour than the green photons is not yet clear. The important point is that the colour change and Chl is still there. This shows that we should be carefull in proposing that plants are green just because Chl is there and "reflect" green light. As I said, Chl separated from leaves and within leaves are different "nanoworld" systems.

In this nanoworld systems, interface effects should be taken into account. Electrical chagers that move arround their atom and move around their molecules "see" enegetic highways that they can cross or not, they can jump (depending on the energetic step) and they can recombine (case of an electron on the conduction band and a hole on the valence band energetic highways). The recombination gives part of the molecule's colour. Up to now, researches don't talk about pairs of A/B molecules or pairs o molecules/wall can play an important role on optical and electrical properties on plants.

Presently there is a branch of the nanotechnology research groups called functionalization. E.G.: it looks like a sphere (gold for ex.) surface where we can stick some organic molecules. This functionalization gives to the new system (gold-molecules) new properties, including optical and different spectral emission properties. From this, you can imagine that molecules sticking to the chloroplast wall can equally give these different physical properties. We have to consider the interface effect.

Applying it to the leaves' nanoworld, the charges movements across the energetic highways (jumping from one molecule to the nearby molecule) can create the necessary physical tools to allow a net movement of negative charges into one direction and a net movement of positives charges to the opposite direction. This net movements can be statistical (probability to cross or not barriers). This is the mechanism we have to discover and try to apply it to plants. We have to try to show these ideas to people for which, it is enough a photon absorption and charges can go freely to the reaction center. This is false! We read this in books and it violates physical laws. Authors don't like we say that their books (on this context or subject) are not correct. I'm sorry for them but mother Nature do respect physical laws. Part of these books not! By the way, if books or papers are right in every thing and we have nothing more to do, which should be the utility of the present research groups? So, we have still a lot of work to do, including changing minds.

It is a nice discussion that seems to be progressing in right direction. I wrote a short answer to it few days back and did not track this discussion there after. Today I was looking at the views of Dr. Vincent Gutschick who is suggesting that with the absorption of this narrow rangeof spectrum, plants are getting saturated. It is very true for light absorption. But as I understand that this saturation is resulting from the deplition in the concentration of carbondioxide and the RIBSCO working as a oxygenase rather than carboxylase. This is the reason that C4 plants are at advantage beacuse these plants are equiped with the carbon concentrating mechanism. (resulting in mid day deprission in C3 plants and not in C4 plants).

This could be one of the other reason that the evolution of light harvesting system with absorbing a part of the spectrum and can and benifit maximum resulting from lower damage resulting from photooxidation as suggested by Dr Vincent Gutschick

It is good to take part in such types of thought provoking discussions. Otherwise my exect area of work is not photosynthesis. I did not carried out much work in this area. Although plant sciences are of prime interest to me

Anil

Dear Marco,

In think we are getting to a point were I will see which is the point I am not understanding from your explanation: For me it is clear that, in the paper of Su et al., gold nanoparticles absorb UV light amb emit (sometimes?) blue ligth, and then this blue light can be absorbed by chlorophylls and some of it give place to the well know red fluorescence of chlorphylls. Nothing extrage here to me. Notice that, for this to work, there is no need to understand how UV is absorbed by gold nanoparticles, how it is converted to blue light, how it is then absorbed by chlorophysll and how it yields red fluorescence. So, it doesn't matter if what the books say about the mechanism is wrong or right (in fact, I will buy that it is wrong, because I never fully understood the ressonance effect that "magically" allowed energy to jump from one molecule of chlorophyll to the next, but I thought "hey, this is quantum mechanichs, it si full of magic!").This was just my point. When you say that " The recombination gives part of the molecule's colour", I understand it as "the recombination produces a photon of a certain color". Is this right? So again, irrespective of the mechanisme, wha t we have is an UV photon, then a blue photon then a red phton, with the net balance of an UV photon absorbed and a red photon emitted.

If all of this is correct, then there is no "color change". Simply, there is no green light to be reflected there, as illumination is with UV light and it is converted to blue light in nanoparticles, so no green light implicated, only blue, absorbed by chlorophylls, and red, its fluorescence. An in fact ths is what you see in Table 3 of Su et al.: plants with gold nanoparticles and illuminated with UV light look red.

Dear Xavier Aranda

May be, in the case of plants with or without gold nanoparticle inside, have no way to absorb and emit (interface properties) at the same UV photons. But this can be the case for blue, green and red. Note that green plants have also other minors colours intensity (like, yelow and red). The UV photons can be absorbed by gold and molecules present in the leaves. In the case of gold nanoparticles the absorption is by quantum mechanics effects (called plasmonic effects).

Your question: "the recombination produces a photon of a certain color". Is this right?

Yes, it is right and this recombination can be at interfaces or within a molecule.

My proposition that has never been come about is the interface effect that is useful on organic LEDs and organic solar cells.

More: interface effect has never been:

a) associated to the colour of plants,

b) associated to the process of charges separation (electric field, necessary to separate (e-, h+) pairs),

c) associated to the waste energy, necessary to separate an (e-, h+) pair that is under attraction.

The interface effect brings all these parameters, lacking on the Forster, FRET, GFP, etc theories.

Note that interface effects is associated to:

i) absorption and emission at the same photon energy,

ii) it can help on the charges separation mechanism,

iii) it is still far from being undertood by researches because we need to make experimental research to measure the band gap relative position of the molecules inserted within the medium where they are (e. g.: Chl/Car, Chl/chloroplast, Car/Chloroplast, etc). This is yet performed for the organic LEDs and solar cells.

Seems I am late to this discussion; however, I do want to point out that plants do use green light. Any examination of spectral profiles will lead you to this conclusion. For example look at Figure 1 in this paper: http://calmit.unl.edu/people/agitelson2/pdf/2008/AJEV_59_3_299-305.pdf

Even with a peak around 550 nm, you can see that green reflectance is decreasing. The leaves not only absorb more in the blue and red regions, but also in the green. Therefore plants do use green reflectance; however, it is not the primarily absorption peak. This suggests that there is internal scattering of green light and possibly absorption and re-emission at longer wavelengths that can be utilized by the chloroplasts.

Dear Anthony Nguy-Robertson

Thnak you for your participation. I'll check the paper you recommend.

It is true, green light is abosrbed by leaves by about 55%, reflection being 15% (please, see standard curves for R, T and A).

Concerning your phrase: "it is not the primarily absorption peak", may be this can be handled with interfaces absorption/emission, like that shown on the ppt file enclosed.

This mechanism has never been proposed by people working with plants. But it is utilized by people working with organic/organic, organic/inorganic and inorganic/inorganic materials. In my opinion, interfaces effects are more important on plants than simply trying to know optical properties of Chl, Car, etc, separated from leaves.

The "internal scattering of green light" you mention can be absorption and re-emission at nearly the same wavelengh. It should depends on the density of states available between two generic A/B molecules, as proposed on the enclosed ppt file. Note: density of states is like energetic seats available for electrons on each energy bands (valence band = HOMO, conduction band = LUMO).

Note: an energetic interface like this one is able to absorb and re-emit photons. The presented ppt slow motion is only for emission.

The secondary metabolites such as the production of phenolic compound specially flavonoids under UV light is a well known process in plant tissue culture . We considered them as barriers of UV light to protect plant tissue, but the outcome of this discussion and the point regarding flavonoids leads us to the possibility of these compounds not just as protecting plant tissue but as converting UV light to more absorb-able wavelengths as a result of which the colourisation exists. So may be we have to add this role at least for flavonoids. There maybe some other compounds with similar fluorescence function as well resulting the colour development of plant leaves.

Just remember that not any molecule that absorbs lght is able to re-emitt it: for the role you propose for flavonoids it should be proven that they can re-emitt light in the range of 400 to 700nm to be usable by chlorophylls.

Hi, Satyajit and Xavier. While some phenolics / flavonoids fluoresce under UV irradiation, molecules don't need to emit light to transfer energy to Chl; Foerster transfer is a radiationless process, in which the quantum state (rovibronic state) of each molecule shifts, down in energy for the donor molecule and up in energy for the acceptor. That's how carotenoids pass energy to Chl and how Chl molecules pass it to each other in the PSU. I believe that there's no evidence of the phenolics doing Foerster transfer to Chl. Furthermore, the UV protectants are too far away to do the transfer (transfer rate falls off as distance to the 6th power), and there are a lot of constraints. One constraint is that the quantum states have to have the proper wavefunction symmetries. OK, now I raised one possibility for energy transfer and quashed the idea. Still, for either fluorescence or Foerster transfer to work well in getting energy to Chl, the energy of excitation has to be retained well against competing processes (radiationless relaxation, internal conversion to states too low in energy, intersystem crossing). Some phenolics do retain or partition the energy well (e.g., they fluoresce), but others do not. Finally, fluorescence of molecules that aren't strongly ordered in orientation, such as the phenolics in the epidermis, is omindirectional - only a portion goes from epidermis to mesophyll where the Chl is located Overall, then, with UV quantum flux density being only about 3% that of PAR and with the only moderate fluorescence quantum yields and with fluorescence "mis-aiming" toward the mesophyll cells, I think that we should consider that energy absorbed by UV protectants is an insignificant source of energy that is transferred to Chl.

Hi Vincent,

Yes, I completely agree with you. I would summary your explanation to: fluorescence is not the ordinary way light is processed in relation to energy capture, most probably because of the inefficinecy of the process, as it means that a molecule would capture a photon and then release it again with a high chance of not being recaptured by another molecule but escaping the site were light capture and energy use to (photo)chemical processes. I am afraid I introduced the confussion when commenting a paper brought up by Marco were some particles, which captured UV light and emitted fluorescence in the visible light, were introduced in plant tissue to prove some properties of the particles, not of the chlorophylls: it is like introducing a gene in a different cell from the original to prove that it encodes a certain protein: it does not prove anyting about the receiving cell, but about the introduced gene.

Hi Vincent and Xavier

As a physicist, I have some problems in undertanding energy transfer from a molecule A to the nearby molecule B. This is performed by the Forster model (FRET), proposed 60 years ago. Today's model for organic molecules mixed on polymer multilayers, applyed to organic LEDs and organic solar cells, people use de concept of measuring experimentaly the energetic steps between the different molecules within these layers.

These measurements brings to the presently accepted 'bandgap engineering' model. In this way, electrical charges don't see the fixed energy highways (Homo and Lumo) proposed by Forster: the ground state energy as a botton energy level to which charges should obey and transfer (if possible) the received energy to its neighbourhood.

I did propose some of these problems I have in accepting the Forster model (FRET) below.

Some drawbacks on FRET model, see by a physicist:

a) The energetic configuration used to represent FRET does not exist in physics.

b) Electrical charges does not ‘see’ the proposed energetic configuration for FRET.

c) Electrical charges ‘see’  energetic steps (up or down) when travelling (or not) from one material to its neighbourhood.

d) Why should a molecule, after receiving energy, emit at lower energy regardless of the main peak it usually emits light? Note: the maximum of an emission peak corresponds to a maximum of the density of states curve. Neglecting this fact we are violating physical laws and/or neglecting all the proposed explanation

for the recombination/emission mechanisms.

f) If the lower bandgap energy molecule has its actual energetic position higher than the nearby molecule, how to explain FRET without violating physics?

e) The described FRET mechanism does not separate (e-, h+). Why should Nature use a ‘step mother’ to separate (e-, h+) pairs?

f) In others words: If FRET is used to explain energy transfer from a molecule A to the nearby molecule B and this molecule B does not have the necessary physical tools to separate (e-, h+), what is the utility of FRET? Note that the absorption of energy (from photons or from transfer) gives to this molecule B an excitonic state but not a separated (e-, h+) pair.

g) FRET technology suffers from few drawbacks, considering unknown parameters,

correction factors, intensities corrections, etc (1,2).

1- J. Szollosi et al. Cytometry 34, 159-179, 1998. 2- S. Weiss. Science 12 March, 1676-1683, 1999.

Sorry, Marco, I do not have the knowledge to even discuss the subject. As far as I know, you may be completely right or completely wrong and I would not see the difference. Maybe somebody would be able to argue (Vincent?). What I would say is that this discussion is not really related to the original question: as I posted before, wether the FRET model is completely right or completely wrong, the fact is leaves absorb green light and use it, but the net absorption (i.e. incident light minus light leaving the leaf) is less than other wavelengths, be it be reflection or absorption/reemission, as your model proposes. This is the reason they look green as any other green-looking object. In the same manner, as Vincent pointed out, Chl molecules in the antenna of the photosystems transfer energy from one to another, they receive energy form som carotenoids and they can transfer it to other carotenoids (the latter was not in Vincent comment, but it is thought to happen between Chl and xantophylls like Zeaxantine resulten in energy release in form of heat). If the Forster model is right or not, anyway energy transfer exists somehow (or I am wrong also here?). It is up to physicists like yourself to give us the correct answer.

What is of interest to me is: if plants have carotenoids that enlarge the capacity to absorb photons in the range of blue, but also in the green range but with much lower effciciney, why they do not have other molecules that increment furhter photon capture in the green range? In other words, the point of the original question, as I understand it, is why plants do not also use green light in the same manner like other colors? Why don't hey look black (or more properly grey, whcih they should look if they do not absorb all photons but a similar proportion in every wavelength). Which is the selective advantage of not using green light in the same magnitude than others? Or is it something unavoidable given the context of the evolution of light absorption to drive photosynthesis? Or, simply, no molecules exist that can capture grren light and transfer it, no matter the way, to chlorophylls? Even the fact of using this non-absorbed green light in the lower layers of the leaf or the canpopy pointed out in some comments, is a real primary selective advantage, a driver of natural selection, or just a way to take some advantage of an unavoidable characteristics of chlorophylls? In other words, are leaves and most plants multilayer, among other things, because they use chlorophylls or they use chlorophylls because it allows them to be multilayer?

And, again, notice this has to do with the fact they absorb green light differently, not with the way they do so.

Dear Xavier

Thank you for these comments. In this way me and people can learn a bit more.

I do agree with you that we are geting far from the original question. Let's put it again on the center, as you mention: "In other words, the point of the original question, as I understand it, is why plants do not also use green light in the same manner like other colors? "

May be the absorption of green light is performed by leaves but we do not see it. As I do propose, it can be an absorption and emission at almost the same wavelengh (green) and Nature does not need more than the evolution did. This absorption and emission (it is not re-emission or reflection) can be by the interface (energetic) between two organic molecules. As CO2 (and others ambiental conditions) is not enough to go to rer-shifted leaves' colours, we have just the green colour that correcponds to the maximum sunlight intensity. In this way, plants use green photons but with less efficiency.

Hi, everyone,

Yes, resonant energy transfer is a bit off-track, but I'd like to add some counterpoint to Marco's answer. Foerster energy transfer is simply an expression of physics, independent of any molecular orbital model of molecular electronic states (HUMO, LUMO). It expresses the energy transfer probability between two molecules as the product of a density of rovibronic states and transition dipole moments. Bandgap energy models apply to regular arrays, as in solid-state physics, but using effectively the same formulation of pairwise interactions of molecules. Molecules in biological systems don't have those high-level spatial symmetries. One can use intermediate concepts, such as excitons in a semi-ordered array of molecules. Excitons are useful in discussing transfer among identical molecules, such as Chl in a PSU, but the pairwise view of Foerster transfer is useful to formulate rates between dissimilar molecules that have big differences in electronically excited states at the ground state in vibration and rotation. At the base, however, the formulations are different ways to similar answers.

Getting back to using green light: isolated Chl molecules in solution are very poor absorbers of green light, it is true. However, nearby molecules perturb the Chls and their absorption is broadened to fill in some of the green-light gap, though not too much. The carotenoids, as auxiliary pigments, absorb much green light and pass energy to the Chls. Overall, leaves absorb 50% (typical annuals or crops) to 85% (e.g., Ficus) of light at the wavelength of minimum absorption, near 550 nm. That's not too bad, considering that there is no molecule that can absorb well over the whole PAR spectrum, and that molecules are very rare (really, only chlorophylls) that have the right combination of traits for energy capture and efficient transfer: strong absorption over a good part of the spectrum; chemical stability; high rates of internal conversion (from second excited singlet state S2, which covers the higher-energy part, the blue region, to first excited singlet state S1); weak rates of competing processes of radiationless transitions (dumping energy as heat) and intersystem crossing (creating triplet T1 that can exchange energy with ubiquitous oxygen, O2, in its triplet ground state, to make excited singlet oxygen that is a dangerous nonspecific oxidant); and redox states that allow photochemistry to start the whole process of photosynthesis. I am in awe of chlorophyll and of the wild coalescence of photochemistry and evolution that made photosynthesis possible, and so early in the history of life!

Hi Vincent

Thank you for the explanation above. I do keep learning.

But I stiil prefer to stay with the "band gap engineering" performed by people working with complexes organic molecules, to fabricate organic LEDs and organic solar cells. Their band gap relative position are experimentaly measured. In this case absorption and emission depends on the organic molecules on each layer and on the relative position of the band gap at the interface. They dont' consider energy transfer. May be these people (plants biology, organic LEDs and organic solar cells) should get together. ..

You could rephrase and ask why plants are "infrared". Of the entire solar spectrum Infrared is much more reflected by plants than green.

Anyway, plants have evolved mechanisms to detect shade, that are also in part dependent on "green" detection (similar to far red detection).

Hi Filip

Very good point you mention: "why plants are infrared?"

According to experimental results, plants reflects 15% of the green colour and reflects about 38% of the entire infrared side of the solar spectra on the Earth's surface. At the infrared side, the absorption is practicaly zero. May be we should start to question these experimental results! If plants are able to absorbs photons and emit them at the same wavelenght (same energy), that should change many "scientific believes" we have today. Artificially we can do this with solid state materials, by using the "band gap engineering" concept. These concepts are available to people working with organic solar cells and organic LEDs. It is not available to people working with plants. Let's see what people, participating at this RG, could say about "why plants are more infrared than green?". If the "scientific believe" is: "plants are green because Chl does not absorbs green light", should we keep giving the same answer? I should keep saying: Chl and plants are different systems.

I think Filip made a very good poitn here: we see plants green because we cannot see infrared. If you take that into account, a lower use of green light is quantitatively is not so important, the same way we do not pay much attention to the fact that not all red wavelengths are captured with the same efficiency: first, because we cannot see it, and second, because the effect is relatively smaller than that of green light.

However, there is a fundamental difference beyond that: infrared light is not capture at all by chlorophylls or accompanying pigments. To put the question in the same terms than the original question, we could rpobably ask why plants do not have pigmets that capture infrared light and use its energy to drive biochemical processes like photosynthesis? And here I think the answer would be different from green light: infrared light has not enough energy to drive photosyntesis, which is one of the characteristics so well described by Vincent about chlorophylls: a molecule capturing infrared, even far-red light, would not have "redox states that allow photochemistry to start the whole process of photosynthesis".

Hence, if we could see infrared we would probably not ask why plants are green, but we would still legitimately ask why do they capture it less efficiently than other wavelengths that have enough energy to drive photosyntesis.

Now I see Marco answer at almost the same time than me, so let me add something: as I wrote before, for what we are discussing it is no directly relevant if (green) light is direclty reflected, passes tthorugh the leaf with no interaction with any molecule, or if it is absorbed and emitted at ALMOST the same energy as proposed by Marco (I am quite sure, ignorant as I am in light physiscs, they cannot absorb and emit at the SAME wavelength or it would realy change some scientific "believes", such as second law of thermodinamics). What is really relevant is the balance of light leaving the leaf to incident light. If Marco is right, then the question would simply change to why plants emit proportionally more green light than other wavelengths, why don't they keep that energy to drive photosynthesis. If light is in excess, as it is in manay leaves most of the time, why don't they emit similar amounts in all wavelengths? And, please, notice I wrote plants and not chlorophylls because, as Vincent explained, chlorphylls "in situ" do not behave light chlorophylls in an acetone extract: their absorption spectrum is much broadened by the physical and chemical environment were they are located, in the protein complex of the antenna of photosystems I and II, surrounded by several kind of carotenoids, some of which still contribute to broaden the absorption spectrum by directly capturing light (mostly green!) and passing the energy to chlorophylls.

Anyway, would you please add some more information about green light role in shade detection?

Hi, everyone,

Infrared is forgone as an energy source by plants, algae, and bacteria for good reason. If there's one reaction center (OK, two that are very similar in energy level), then the energy of all absorbed photons is degraded to the energy level of the reaction center. This happens in all photosynthesis. To be able to use infrared while having one reaction center type, the energy of all photons, even in the PAR, would have to be degraded to the level of some infrared photon. E.g., if the reaction center had the energy of an 1000 nm photon, then the energy in a blue-light photon would be degraded by 60% (given that E = hc/lambda). In addition to making the use of PAR inefficient, the energy at the reaction center might be insufficient to do CO2 reduction for PS in the style of vascular plants. The other option is to have a separate set of reaction centers for handling lower energy photons. This is akin to multilayer solar cells (very efficient, very costly, only usable with concentrated solar power), and no organism has managed the feat. We're "stuck" with using the PAR only, and that's not so bad. Note that plants are not often energy-limited! In full sun, most PS is supersaturated; plants more often have to deal with excess energy than insufficient energy, as an ecosystem (understory shade plants have it harder, of course). Having enough N or water is more of a resource challenge for plants. There's also a packing problem: it's not possible to pack in enough photosynthetic apparatus to make use of full sun, if only because Rubisco is so slow as the rate-limiting enzyme.

Sorry to skip many of the comments and answers in this trend, but, could it be that green plants do not absorb green light because that could be too much (since green comes from the sun with the highest power)? I mean, could it be that, for adaptative evolution, is better to have just enough and not a lot absorbed radiation?

Dear Ignacio Cortese

May be this is what some people try to propose as the reason why, presently, plants dont' need (question: is it true?) the more intense green light from the sun. May be we dont' have enough CO2 for leaves to work more intensely. The same for water or water vapour on air (do we have enough today?). May be, by just and mostly absorbing red and blue, it is enough to do the job for plants. This job is mainly: absorb the photons, to separate electrons from holes, send them to opposit directions and it is up to each one of these charges to perform its job (expelling O2 and fabricating the biomass).

I do see two big problems: the mechanism for the electron/hole separation and about the absorption of green photons.

For the first, (e-, h+) separation, many questions arrises presently (see below). For the second one, may be leaves absorbs/emits green photons very quickely and we dont' pay enough attention. The question is: why?, the choice for green.

As proposed by a colleague from RG before: if the reason for the greeness is that Chl (really?) dont' absorbs green, plants sould be called infrared colloured, because the IR reflection is 38%, and green reflection is 15%.

For those interested on the charges separation mechanism, there is a beautifull experiment by G. Pollack, showing that the system glass/Nafion/water plus photons do separate electrical charges. So, whater is an important environement for charges separation. This is not taken into account. In the same way, the system: Chloroplast wall/Chl/water is not taken into account on the photosynthesis process. In my opinion, erroneously!.

Note: Nafion is an artificial polymer.

Please see: http://www.youtube.com/watch?v=XVBEwn6iWOo, for the charges separation mechanism with a simple experiment.