Ca2+- and Mg2+-ATPase activities in winter wheat root plasma membranes as affected by NaCl stress during growth

Winter wheat seedlings were grown in Hoagland nutrient solution with or without 100 mmol/L NaCl added. Plasma membranes from root cells were prepared by aqueous polymer two phase partitioning and the stimulation of plasma membrane ATPase activity by Mg2+ and Ca2+ was investigated. The enzyme was activated by Mg2+ or Ca2+ and Mg2+-ATPase was further stimulated by K+, whereas no such stimulation was observed with Ca2+-ATPase. Ca2+ inhibited Mg2+-ATPase activity pH optima for Mg2+ and Ca2+-ATPase activities were 6 and 5.6, respectively. ATPase activated by Mg2+ was inhibited by vanadate and DCCD, while Ca2+-ATPase was less sensitive. ATP was the preferred substrate but other nucleotide tri- and diphosphates were also hydrolyzed by the plasma membrane ATPases. NaCl stress during growth had no major effect on the plasma membrane polypeptide components. Simple Michaelis-Menten kinetics were found when Mg2+- or Ca2+-ATP was used as substrate for both control and salt stressed roots. NaCl markedly reduced the V-max of Mg2+- and Ca2+-ATPase activities while it had little effect on the apparent K-m for Mg2+- or Ca2+-ATP. NaCl-induced reduction of V-max of ATPase activity correlated with NaCl-altered plasma, membrane lipid composition and the role of the lipid environment in ATPase activity is discussed.