Determination of Essentiality

Numerous solution culture and pot experiments with K⁺-free substrates have shown that plants do not grow without K⁺. As soon as the potassium reserves of the seed are exhausted, plants die. This condition may also occur on strongly K⁺-fixing soils. In contrast to other plant nutrients such as N, S, and P, there are hardly any organic constituents known with K⁺ as a building element. Potassium ions activate various enzymes, which may also be activated by other univalent cationic species with a similar size and water mantle such as NH₄⁺, Rb⁺, and Cs⁺ (7). These other species, however, play no major role under natural conditions as the concentrations of Cs⁺, Rb⁺, and also NH₄⁺ in the tissues are low and will not reach the activation concentration required. In vitro experiments have shown that maximum activation is obtained within a concentration range of 0.050 to 0.080M K⁺. Ammonium may attain high concentrations in the soil solution of flooded soils, and ammonium uptake rates of plant species such as rice (Oryza sativa L.) are very high. In the cytosol, however, no high NH₄ ⁺ concentrations build up because NH₄⁺ is assimilated rapidly, as was shown for rice (8). Activation of enzymes in vivo may occur at the same high K⁺ concentration as seen in in vitro experiments, as was shown for ribulose bisphosphate carboxylase (9).

It is assumed that K⁺ binds to the enzyme surface, changing the enzymic conformation and thus leading to enzyme activation. Recent research has shown that in the enzyme dialkyl-glycine carboxylase, K⁺ is centered in an octahedron with O atoms at the six corners. As shown in Figure 4.1, these O atoms are provided by three amino acyls, one water molecule, and the O of hydroxyl groups of each of serine and aspartate (10). As compared with Na⁺, the K⁺ binding is very selective because the dehydration energy required for K⁺ is much lower than for Na⁺. If the latter binds to the enzyme, the natural conformation of the enzyme is distorted, and the access of the substrate to the binding site is blocked. Lithium ions (Li⁺) inactivate the enzyme in an analogous way. It is supposed that in most K⁺-activated enzymes, the required conformation change is brought about by the central position of K⁺ in the octahedron, where its positive charge attracts the negative site of the O atom located at each corner of the octahedron.

This conformation is a unique structure that gives evidence of the unique function of K⁺. In this context, it is of interest that the difference between K⁺ and Na⁺ binding to the enzyme is analogous to the adsorption of the cationic species to the interlayer of some 2:1 clay minerals, where the adsorption of K⁺ is associated with the dehydration of the K⁺, thus leading to a shrinkage of the mineral; Na⁺ is not dehydrated and if it is adsorbed to the interlayer, the mineral is expanded.

FIGURE 4.1 Potassium complexed by organic molecules of which the oxygen atoms are orientated to the positive charge of K+. (Adapted from K. Mengel and E.A. Kirkby, Principles of Plant Nutrition. 5th ed. Dordrecht: Kluwer Academic Publishers, 2001.)

It is not yet known how many different enzymes activated by K⁺ possess this octahedron as the active site. There is another enzyme of paramount importance in which the activity is increased by K⁺, namely the plasmalemma H⁺-ATPase. This enzyme is responsible for excreting H⁺ from the cell. As can be seen from Table 4.1 the rate of H⁺ excretion by young corn (Zea mays L.) roots depends on the cationic species in the outer solution, with the lowest rate seen in the control treatment, which was free of ions. The highest H⁺ release rate was in the treatment with K⁺. Since the other cationic species had a promoting effect on the H⁺ release relative to pure water, the influence of K⁺ is not specific. However, a quantitative superiority of K⁺ relative to other cations may have a beneficial impact on plant metabolism since the H⁺ concentration in the apoplast of root cells is of importance for nutrients and metabolites taken up by H⁺ cotransport as well as for the retrieval of such metabolites (11). The beneficial effect of cations in the outer solution is thought to originate from cation uptake, which leads to depolarization of the plasma membrane so that H⁺ pumping out of the cytosol requires less energy. This depolarizing effect was highest with K⁺, which is taken up at high rates relative to other cationic species. High K⁺ uptake rates and a relatively high permeability of the plasmalemma for K⁺ are further characteristics of K⁺, which may also diffuse out of the cytosol across the plasma membrane back into the outer solution.