K+ is the most abundant cellular cation. The classical reviews by Evans and Sorger (1966) and Suelter (1974) advanced the idea that high concentrations of K+ are required for the active conformation of many enzymes participating in intermediary metabolism and biosynthesis. High concentrations are also needed to neutralize the soluble and macromolecular anions of a cytoplasm which has few organic cations. In this role, K+ contributes much to the osmotic potential. A corollary of these requirements is that K+ must be readily transported along with acidic metabolites, in order to balance their formation.
Transport appears to lie with a K+-activated Mg++- requiring ATPase of cell membranes (Hodges 1973, Poole, 1978). Frequently, K+ appears to be in electrochemical equilibrium indicating passive flux (Higinbotham 1973), but this is not true for low external K+ concentrations, and models based on a H+/K+ exchanging ATPase have been proposed (Hodges 1973, Lin and Hanson 1976, Cheeseman and Hanson 1979). Whatever the mechanism, transport of K+ appears to be implicated in several physiological functions: phloem transport (Ashley and Goodson 1972, Hartt 1970, Mengel and Viro 1974), guard cell turgor (Raschke 1977), leaf movements (Satter et al. 1974, Schrempf et al. 1976), and cell growth (Cram 1974). Thus the nutritional need for K+ centres on four physiological-biochemical roles: enzyme activation, membrane transport processes, anion neutralization, and osmotic potential (Clarkson and Hanson 1980).
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