Cation nutrition is summarised as follows:
• Basic cations are not metabolised, but they are important in a number of enzymatic reactions and in some cases form the reaction centres of enzymes (Mg in chlorophyll);
• Requirement for K is higher than for Ca and Mg;
• K regulates osmotic concentrations and pH of the cell;
• Mg is involved in chlorophyll functions and in the transfer of phosphate from ATP to organic molecules;
• Ca acts as an osmotic dehydrating ion and as a signal in cell metabolism (calmodulin).
• regulation of photosynthesis (photophosphory-lation) and drought resistance. K regulates the osmotic potential during drought and maintains growth with decreasing water potential (Fisher and Turner 1978). The effect of K on osmoregulation also explains its importance in frost resistance.
• regulation of movements: The function of stomata (see Chap. 2.2.3) is based on oxygen requiring respiratory metabolism, with starch degraded to malate which is then transported into vacuoles and increases their ion content. Simultaneously, malate is decarboxylated in the mitochondria and the ATP produced is available for proton efflux. The HCO3 formed is used again in the degradation of starch. The proton efflux results in an influx of K from the apoplast via the antiport. Some of the protons are regained because of the influx of Cl~. Potassium and chloride are transported into the vacuole, where they balance the ratio of anions/cations to malate. Thus the osmotic potential in the vacuole increases, with a consequential influx of water - and the stomata open. The effect of the stress hormone abscisic acid (ABA) in the regulation of stomata is to close the K channels (Schulze 1994) and thus to stop the movement of K+.
In this reaction ABA has a signal effect on Ca channels, thus increasing the cytosolic Ca concentration which again affects the K and anion channels of the plasmalemma. K uptake is inhibited and thus stomata are not able to open (Schroeter et al. 2001). Closure of stomata is affected by malate and an efflux of K. A corresponding mechanism affects other movements, e.g. of leaves of Mimosa pudica and Dionae mus-cipula on touching, and the orientation of legumes to the sun.
• phloem transport: potassium is the most important cation regulating the pH in the phloem and sucrose loading.
• regulation of fruit and tuber (potato) ripening occurs because of the effect of K on the cation/anion balance and on sugar/starch metabolism.
Potassium occurs predominantly in silicate rocks and becomes reversibly bound to exchangers in the soil, particularly to clay minerals. Because of its ionic radius potassium fits optimally into the intermediate layers of clayey minerals and is therefore preferentially accumulated there (specific adsorption). In some soils, accumulation of K+ in clay minerals which are able to swell results in contraction of these clays which thus "fix" K+ so strongly that it is no longer available for plants. The ionic radius and physical characteristics of K+ are similar to ammonium, so that these two ions are easily interchangeable. Excess of ammonium leads to exchange of K (and other anions) and if these cations are leached this stimulates acidification of soils. K and ammonium are both used to determine the cation concentration of the exchanger.
Deficiency and Excess
With K deficiency and excess the following symptoms occur:
• K deficiency: reduced growth and increased remobilisation from ageing organs, disturbed water relations (drying of tips), wilting appearance particularly at the edge of older leaves, yellowing and early shedding of needles.
• K excess: with K excess (e.g. on granite containing muscovite), Ca and Mg uptake are competitively influenced, thus increasing Mg deficiency at these sites.
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