The mobility of nutrients in plants is largely determined by their solubility in phloem sap. Since the phloem sap is alkaline, irons that are poorly soluble at high pH, e.g., Fe and Mn, are considered (phloem) immobile. They can only be transported in the phloem in appreciable amounts by chelating them.

@Hans Lambers

Accepted that ions like Fe, Mn etc form insoluble hydroxides at higher pH. How do you explain the mobility of elements like calcium, boron, sulphur in plants(All of them being immobile)

Calcium, like Fe and Mn, is also fairly immobile in plants. Hence it it can be used to calculate remobilisation of N and P from senescing leaves.

Sulfur is transported in an organic form, so the relative immobility of inorganic sulfur at high pH does not enter the equation.

Nutrient movement in soil is characterized by 3 mechanisms namely massflow, diffusion and contact (root/soil). The availability of any one nutrient depend on its relative solute concentration indeed affected the soil pH (affecting solubility) and relative soil moisture, presence/ absence precipitating elements (eg. Al, Ca) and of coarse the inherent ionic exchange property conferred by the soil minerals and texture, and the organic matter content (quality and quantity) and finally and not the least soil micro-organisms that participate in the release and immobilization of essential nutrients.

From a plant perspective, I believe nutrients move up the plant principally via the transpiration stream in the xylem to feed the active growing point and from there can be translocated or intercepted by and when required to other plant parts favoring again growing points when there is sup-optimal condition. In fact studies have demonstrated the relative mobility of the different plant nutrient where the "immobile one such as Ca,S" remains permanently in the dead organ indicating that their active movement is hindered or else they are fixed to permanent tissue such as cell wall and their dissolution will only happen if these structures are first degraded in situ. Consequently I assume that the active transports of assimilate such as sugars follows a different mechanism compared to nutrient movement in the plant. Indeed this does not exclude co-movement eg. complex form with chelating substrate leading to movement of the "immobile" plant nutrient together with assimilate for example the so-called mobile nutrient. It is worth mentioning that nutrient distribution from the main supply depend on the cellular/tissue requirement, which at this end is controlled by active pump.

There is increasing evidence from isotopic studies (natural isotope ratio variations and labeling/tracing) that Mg and Ca are transported up the xylem through ion exchange and not mass flow (with transpiration). Natural isotope variation studies show isotopic fractionation for both Mg and Ca occurring all along the xylem and tracer studies have shown that Mg and Ca move much less faster than water and even nitrogen in the xylem.

See reference

Augusto L, Zeller B, Midwood AJ, Swanston C, Dambrine E, Schneider A, Bosc A (2011) Two-year dynamics of foliage labelling in 8-year-old Pinus pinaster trees with (15)N, (26)Mg and (42)Ca-simulation of Ca transport in xylem using an upscaling approach. Ann For Sci 68(1): 169-178

I also have a paper that shows very similar results except that we measured the gradient of tracers in the xylem from the stump to the canopy in 35 yr-old beech trees.

Also, I have been very interested lately in Calcium "immobility". There are many theories about Ca in plant which I believe need to be revisited. For instance, it is said that Ca cannot be taken up by the foliage of a plant but there are old studies using Ca-45 radio isotope that showed that Ca is absorbed into the leaf. What they didn't see is Ca translocation from the leaf to the woody parts of the tree.

see reference

Bukovac MJ, Wittwer SH (1957) Absorption and Mobility of Foliar Applied Nutrients. Plant physiology 32(5): 428-435

Some heartwooded tree species such as oak have been proven to remobilize Ca in woody tissues during heartwood formation :

Penninckx V, Glineur S, Gruber W, Herbauts J, Meerts P (2001) Radial variations in wood mineral element concentrations: a comparison of beech and pedunculate oak from the Belgian Ardennes. Ann For Sci 58(3): 253-260

Finally, Ca often precipitates in plant tissues as Ca-oxalate. This process has many functions in plants, probably the first one being to reduce the concentrations of Ca in tissues but from evolution Ca-oxalate participate in different functions like the plant defense system. There is also evidence that these Ca-oxalate crystals can be dissolved by the plants when Ca availability becomes low.

See the review:

Franceschi VR, Nakata PA (2005) Calcium oxalate in plants: Formation and function. In: Annual Review of Plant Biology. Annual Review of Plant Biology. Annual Reviews, Palo Alto. p 41-71

and also

Nakata PA (2003) Advances in our understanding of calcium oxalate crystal formation and function in plants. Plant Science 164(6): 901-909

and

Volk GM, Lynch-Holm VJ, Kostman TA, Goss LJ, Franceschi VR (2002) The Role of Druse and Raphide Calcium Oxalate Crystals in Tissue Calcium Regulation in Pistia stratiotes Leaves. Plant Biology 4(1): 34-45

oups... I clicked too fast and forgot my conclusion:

Ca may not be as immobile as we previously thought and more effort to investigate Ca mobility is needed and currently on the way. In forest ecosystems, this hypothesis is an important one because the biological control of Ca by storing and releasing could be a key process in soil, or should I say, "ecosystem fertility" especially in forests growing on nutrient poor soils.

@ Gregory Van der Heijden

Thanks for providing a nice piece of information. But you have confined to Calcium only. What about other nutrients.

Thank you Gregory for your enlighten contribution. I would like to know from the ions exchange model what is the driving mechanism for ions to move from point A to B if it is not in the water movement or by diffusion? Similarly, how come water stress affects nutrient use efficiency indices? Any impact of xylem/phloem pH on nutrient mobility? How to explain the hierarchy of deficiency symptoms?

@ Jean Paul: These are excellent questions! I'm afraid that I do not have the answers. Then again I am not a tree physiologist expert but I do think that it's also and mainly due to the fact that we're close to the current frontiers of our knowledge. And that's exciting!

What will then impact the speed of transport... Some ideas:

There are of course many different factors that influence the transport of the cations up the xylem vessels and the water flux is one of them. Diffusion will also play a role but I would think that diffusion of cations mainly plays a role in the horizontal transport (from the xylem to surrounding tissues).

The different cations will not have the same affinity for the ion exchanger. Results from our experiment show or at least suggest that Ca is transported more slowly than Mg along the ion exchanger. K probably has little interaction with the exchanger and K flux is probably mostly by the water mass flow. Nitrogen is mainly mass flow too.

I have read a couple of papers that suggest that plants may be able to complex the cation metals in the xylem sap. This would enable the plant to transport non-charged compounds (organic acids) and avoid the ion exchange processes. But how much of the flux is complexed cations? That is a good question. Can or do plants regulate that complexation in agreement with nutrient demand?

We also thought about xylem pH. If like in the soil, decreasing pH desorbs metal cations from the exchanger then the plant has another way of regulating metal cation supply by using those sources when the soil sources are low. The pH of the soil solution taken up may be directly the controlling factor. I don't know of any study that had measured sap pH but I think that it should be done!

@Aabid:

Thanks!

I mostly talked about calcium because I have mainly worked with Mg and Ca up until now. Ca is particular because although it is taken up, the plant needs to keep Ca concentrations low in the cells and tissues because Ca is "toxic" at high concentrations because it precipitates with phosphate. This is why living organisms have "come up" with inventive ways to use the calcium for their benefit: like vertebrate animals store the Ca in the bones.

Other nutrients such as Mg, N and K are more mobile in plant tissues because they can be dissolved the cytosol in cells. However, I am not a cell biologist so my knowledge stops here and I don't know the details of diffusion, cell wall transport of these nutrients in the phloem for example.

Thank you Gregory

Million thanks Gregory

I am glad if I have been of any kind of help! I figure that is what researchgate is all about!

Very good info provided by Gregory. I would like to add physiology facts that certain transporter/promoter proteins (like ZIP proteins) also help in the translocation of Cu, Fe, Mn and Zn within plant system. This fact has been confirmed in studies of mobility of cations such as Fe and Zn in cereals (Like wheat) because Zn and Fe cations are transgressed at the same QTL (site) of gene governing the synthesis of same promoter proteins.Cell to cell transfer and pholem loading and unloading further facilitate the ionic movement to take the nutients to the site of accumulation(fruit/grain) within plant system.

Some excellent information has been given. I would add that ion concentration and movement within the plant is primarily driven by movement of ions across biological membranes. Active transport allows the movement of ions into the plant root cell, against the concentration gradient. Other places where membranes influence the movement of ions are the movement of ions into to vacuole where they can be stored, phloem cells where loading and unloading occurs, and exudation from the cytosol to apoplast which includes the xylem. Once exuded into the cell wall, ion movement would probably be mainly under physical constraints, similar to those in soils, rather than biological ones although the information from Gregory suggests that the plant may have some control over movement in the xylem. The flux of ions across membranes can be influenced by temperature, the biological demand for certain ions, and by stresses that the plant might be under.

I think that mobility also is related to what compound the ion becomes incorporated into within the plant. For instance, most K remains in ionic form, and so is easy for the plant to move from one place to another. On the other hand, Mg is incorporated into the chlorophyll molecule, and so cannot move unless the molecule is broken down.

Thanks everybody for that interesting information.

I just want to add that some nutrients can be transported using non active transport based on transmembran proteins as aquaporins. This proteins are a key role in water and nutrient transport between cells and organelles and are essential in the cell to cell pathway, but only are known to transport non ionic forms as urea, NH3, boric acid, silicic acid, etc... at least in plants (in animals, few aquaporins are known to transport ions) and always in favour of gradient. In case of Boron and Silica, the transport into the plant require a combination of active transporters, wich load them into the xylem, generating the gradient needed for aquaporins (passive transporters) to continue with absorbtion from soil. Other interesting point is that boron can be moved in complex with sugars through phloem (this way can be used for other nutrients?) or can be fixed in cell walls, were this nutrient influence the ions transport and membrane stability, helping the movement of other nutrients between apoplastic and cell to cell pathways.

Hope this new perspectives help us to get an overview about the amazing possibilities of nutrients transport in plants!

As far as i know, for tomato plants even if Ca concentration is sufficient in the root zone and in the plant itself, there can still be symptoms of Ca deficiency in the fruits. This happens when there is high transpiration rate from the surface of the leafs, reducing xylem flow towards the fruits. This causes insuficcient Ca uptake by the developing fruits, resulting in bad development of the vascular tissue in the tip of the tomato. This leads to abnormal development of that part, eventually leading to blossom end rot.

Experiments have shown that reducing the tranpiration rate during the development of the fruit, increases xylem flow towards the fruits, leading to higher Ca transfer towards the fruits. This has also been confirmed with the use of ABA in experiments and the results were impressive (dont remember what was the paper, but i think it was UCD). Ca transfer in the fruit greatly increased and blossom end rot pressence % was greatly reduced.

ABA is produced by the root in order to cause stomata clossure only when the plant is under water stress. In this case it was used to promote stomata closure while the plant was well watered and observe the effect on Ca mobilization towards fruits.

Hope this info can help somehow.

I spent years working on membrane transport of ions in plants.  Unfortunately, the field has gone out of fashion and the people who understood the bioenergetics and biophysics are rapidly ageing.  I suggest you look at PS Nobel's books on Biophysical Plant Physiology.  I am afraid many wheels will have to be reinvented.

I don't work with ion transport, and surely shouldn't comment on these issues, but I'd like to add to the question three considerations, all related to biophysics, and in this aspect I suppose Dr. Ritchie may be right: 1. The Donnan Free Space in cell wall. It's negatively charged, and so interacts with ions charges, altering their mobility and cell wall storage capability. 2. The transmembrane proton motive force. It's maintained by cell ATP, and higher in high activity cells, and also generates the electrochemical potential responsible for ion exchange through membranes. 3. Membranes acts as non polar molecular filters, but they are supposed to be more or less permeable depending on the particle (ion) radius and charge. In this respect, the membrane could be totally impervious to Ca++, but somewhat permeable to K+, and more even to Na+, as its radius is smaller than K+ (116 to 152). I would greatly appreciate comments.

Dear Jeferson - what you are saying is basically right but undergraduates get very little of this today.  Hence, they have to learn it for themselves.  Two papers I wrote in 1982 on the cation exchange properties of cell walls still get cited and I published a lot on pmf as well.  The crucial thing that students need to be told about that is not in the older literature is aquaporins and their role in transport of not only water but all sorts of small uncharged molecules including arsenite and aluminium hydrates.  For example in the 1990s I did a study of glyerol transport in coral algal symbionts - I now know that what I was looking at was the activity of an aquaporin.

Thanks, Dr. Ritchie, I really don't know much about aquaporines, but it surprised me very much that a water transporting protein also transports many other things. Shouldn't they be specialized? And also, if they transport more than one substance,  how would they be regulated? Wouldn't it be much more simple to have other specific transporters? Are they mass flow controllers? I really must read more.

Dear Aabid,

You may be interested in our studies of intra- and interspecies variability in the minerals, innorganic anions and sugar composition of spring sap of various tree species. So, i invite You to read the following articles:

https://www.researchgate.net/publication/307977207_Evaluation_of_the_content_of_inorganic_anions_in_tree_saps

https://www.researchgate.net/publication/303122484_Mineral_content_of_tree_sap_from_the_subcarpathian_region

https://www.researchgate.net/publication/315855907_Time-related_variability_of_the_mineral_content_in_birch_tree_sap

https://www.researchgate.net/publication/316981573_Variability_of_minerals_content_as_a_factor_limiting_health_properties_of_birch_saps

https://www.researchgate.net/publication/260527220_Sugar_content_in_the_sap_of_birches_hornbeams_and_maples_in_southeastern_Poland

I wish You pleasant reading!

Article Evaluation of the content of inorganic anions in tree saps [...

Article Mineral content of tree sap from the subcarpathian region [Z...

Article Time-related variability of the mineral content in birch tre...

Article Variability of minerals content as a factor limiting health ...

Article Sugar content in the sap of birches, hornbeams and maples in...

 Some outstanding responses , which are educative even today for most of us. I am adding a bit , with a hope , it will be useful...

Variable-charge (v-c) and permanent-charge (p-c) soils differ fundamentally with regard to many nutrient-cycling processes. Variable-charge soils are more common in the tropics than in temperature zones because their formation requires desilication, which proceeds fastest in warm, moist climates. The dynamics of nutrient mobility tend to be more complex in v-c than in p-c soils. For example, theory predicts that, as pH of v-c soils decreases, cation exchange capacity (CEC) also decreases and anion exchange capacity (AEC) increases. If AEC exceeds CEC, cations such as ammonium and potassium will be more mobile than anions such as nitrate; this is the reverse of the situation in p-c soils, on which most of our knowledge of nutrient cycling is based. Variable-charge surfaces sorb phosphorus, creating plant nutritional problems throughout large areas of the humid tropics. Desilication, the same process that creates v-c surfaces, results also in stable aggregation, creating soils that retain water, yet drain rapidly and resist erosion.Source ; Sollins, P., Robertson, G.P. & Uehara, G. Biogeochemistry (1988) 6: 181. doi:10.1007/BF02182995