Repression of the K+ Uptake and Cation-Stimulated ATPase Activity Associated with the Plasma Membrane-Enriched Fraction of Cucumber Roots Due to Ca2+ Starvation

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Abstract
The uptake of K+ by cucumber plants decreased markedly during Ca2+ starvation. A plasma membrane-enriched fraction, judged from the distribution of marker enzymes, was prepared from control and Ca2+-starved roots. The Mg2+- and K+-Mg2+-ATPase activities associated with the plasma membrane-enriched fraction of control roots were maxima at pH 6.5. Various monovalent cations and potassium salts of monovalent anions stimulated Mg2+-ATPase activity. Vanadate, DES and DCCD inhibited K+- Mg2+-ATPase activity. Of the divalent cations and phosphate esters tested, Mg2+ and ATP were most effective for the stimulation of ATPase by K+, whereas Ca2+ was ineffective in replacing Mg2+. Mg2+- and K+-Mg2+-ATPase activities associated with the plasma membrane enriched fraction of Ca2+-starved roots were much lower than those of control roots. Km values of K+-Mg2+-ATPase for ATP were comparable for control and Ca2+-starved roots. The K+-stimulated activity of Mg2+-ATPase in Ca2+-starved roots was approximately one fourth that of the control, whereas the rate of stimulation was only slightly lower in Ca2+-starved roots.