Ammonium nutrition has the following physiological, structural and ecological consequences:
• Supplying only ammonium ions leads to carbohydrate deficiency, as C skeletons are largely required for the assimilation and detoxification of ammonium in the root;
• Ammonium nutrition leads to cation deficiency (especially Mg deficiency), as the uptake of ammonium results in competitive inhibition of the uptake of other cations;
• In ammonium nutrition Cl~ and S042- are the available ions in the soil which are also taken up by symport, acting as counterions to balance charges. Cl~ can become toxic. SOl~ can react with other essential cations, especially Ca2+, leading to the formation of the insoluble calcium sulfate and therefore enhancing cation deficiency;
• In the case of ammonium, an excess of free amino acids (diamines, e.g. putrescine) occurs. The formation of storage proteins would result in smaller ratio of N to C and therefore an increase in the C deficiency. Formation of amino acids leads to changes in leaf structure; they have a higher water content (as a result of the osmotic effects of amino acids) and lower dry weight per area, making the leaves more susceptible to environmental stress (e.g. temperature and pests and diseases);
• Higher concentration of amino acids leads to a greater susceptibility of the plant to plant diseases and pests. Especially damaging are pests that "suck" phloem sap (aphids);
• In the soil (or nutrient solution), feeding ammonium leads to a reduction in pH. Ammonium is stored in the soil as an exchangeable ion. This leads to the release of basic cations and heavy metals that are toxic to plant roots (see Chaps. 1.7 and 1.8). Cations are washed out of the soil horizon. Initially, when plants are placed in an ammonium-rich environment, there is an improvement in cation availability (Kreutzer and Gottlein 1991). The high binding affinity of ammonium is used in soil science to determine the cation-exchange capacity (CEC, determination of the exchangeable basic cations by washing with NH4C1);
• Ammonium nutrition can, with high cell pH, lead to the release of NH3 into the atmosphere, as almost half the ammonium at pH 7.2 (cytosolic pH) is already dissociated, i.e. in neutral conditions, half is already present as NH3. Ammonia (NH3) is poisonous to cells.
Physiologically, nitrate is not toxic and thus may be stored in the root or shoot. Nitrate storage in the leaf has a regulatory (signal) effect on C allocation (shoot-root growth) of plants (Scheible et al. 1997; Klein et al. 2000). Large nitrate concentration in the leaf stimulates shoot growth and inhibits root growth by regulating the sugar transport to the root whereas in the soil it stimulates root growth.
Generally, nitrate transport is via the xylem into the storage parenchyma of the stem, or into the leaf where nitrate is initially stored together with cations in the vacuole. This increases the osmotic concentration and the water content of leaves (e.g. with "crunchy" nitrate-fertilised vegetables). If required, nitrate may be transported back from the vacuole into the cytosol. This occurs in exchange with organic acids formed via PEP-carboxylase in the leaf. For some plant spe cies, oxalic acid is transported in exchange for nitrate into the vacuole. With a large Ca supply this leads, in the vacuole, to the formation of Ca oxalate which is difficult to dissolve (rhaphides, giving the typical taste of rhubarb and banana peel). Nitrate is reduced to nitrite in the cytosol. In the chloroplast, nitrite is reduced to NH4, which is immediately assimilated by the GOGAT system. The turnover rate of nitrite reductase is faster than that of nitrate reductase, as nitrite is toxic to the cell. Some of the cations taken up with nitrate are transported back into the phloem, particularly K+ is relocated into the root, accompanied by associated organic acids. These acids are decarboxylated in the root (e.g. malate) and thus support nitrate uptake.
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