Why total chlorophyll content ( chl a+b) increases under low light stress and why chl a/b ratio decreases?
Dear Darshan
Shade-plants develop acclimation strategies, including larger and thinner leaves which present even a three-fold increase in chlorophylls. The synthesis and degradation of the photosynthetic pigments are associated with the plants adaptability to different environments. The chlorophylls are usually synthesized and photo-oxidized in the presence of light. Nonetheless, the excess of light can cause greater degradation and consequently, a reduction in the levels of total chlorophyll. On the other hand, under deficit light conditions, the plants set a series of compensatory mechanisms into motion such as a substantial increment of the photosynthetic pigments. This response fulfils the function of the photosynthetic antennae absorbing the required light energy considering that the highly pigmented leaves show higher light absorption efficiency per unit of leaf biomass, which may allow the plant to achieve a better carbon balance under light limitation. Besides, it has been reported that the efficiency of photosynthesis during sun flecks is higher in the shade tolerant species.
Therefore, the decreased chl a/b ratio (or increased chl b/a ratio) could also be an adaptive response for the light harvesting maximization.
Hi Darshan,
The reason why the Chla/Chlb ratio decreases is that under low light availability plants develop bigger Light Harvesting Complexes. Both Chla and Chlb are found in LHCs yet only Chla is found in the reaction centers. As a result, bigger LHCs as a response to low light lead to increments of both Chla and Chlb, still the overall Chla/Chlb ratio drops as there is no increase in the number of Chla molecules forming the reaction centers of the photosystems.
Why in particular chl b increases resulting in dropping chl a/b ratio?
what triggers this process?
Darshan Panda Indeed, Chl. b is mainly effective and responsible for blue light absorption. Therefore, under low light environments, the shade adaptive species increase the Chl. b to enhance the light harvesting system. In general Chl. a is the most effective pigment for photosynthesis at the blue and red bands and increasing Chl. b helps in higher photons absorbed by the shade chloroplasts, though some blue light is not absorbed by the Chl b rather by the other pigments such as Carotenes that do not transfer absorbed energy of photons efficiently to chlorophyll, and thus some part of absorbed light in the blue is not used for the photosynthesis.
Chl a and Chl b have somewhat overlapping absorption spectra, but with distinct peaks both in the blue and red regions. Under normal conditions Chl a, and thus its corresponding absorption spectrum, dominates the scene. Ratio of Chl a/b reducing under low light indicates increase in the relative abundance of Chl b. This is an "absorption-peak-broadening" response (Dale and Causton, 1992) ( https://www.jstor.org/stable/2389754?seq=1#metadata_info_tab_contents ). This allows the plant to intercept light in a broader range of the visible spectrum, both in the blue and red regions. Predominant Chl a (in normal leaves under normal light) narrows down the collective range of light absorption because it is sufficient for survival.
chlorophyll absorbs light in the red (long wavelength) and the blue (short wavelength) regions of the visible light spectrum. Green light is not absorbed but reflected, making the plant appear green.the stress is due.to the change of wavelength
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