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 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. K_m 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.