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

The uptake of K⁺ by cucumber plants decreased markedly duringCa²⁺ starvation. A plasma membrane-enriched fraction, judgedfrom the distribution of marker enzymes, was prepared from controland Ca²⁺-starved roots. The Mg²⁺- and K⁺-Mg²⁺-ATPase activitiesassociated with the plasma membrane-enriched fraction of controlroots were maxima at pH 6.5. Various monovalent cations andpotassium salts of monovalent anions stimulated Mg²⁺-ATPaseactivity. Vanadate, DES and DCCD inhibited K⁺- Mg²⁺-ATPase activity.Of the divalent cations and phosphate esters tested, Mg²⁺ andATP 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 plasmamembrane enriched fraction of Ca²⁺-starved roots were much lowerthan those of control roots. K_m values of K⁺-Mg²⁺-ATPase forATP were comparable for control and Ca²⁺-starved roots. The K⁺-stimulated activity of Mg²⁺-ATPase in Ca²⁺-starved rootswas approximately one fourth that of the control, whereas therate of stimulation was only slightly lower in Ca²⁺-starvedroots. (Received May 9, 1984; Accepted September 17, 1984)