Effect of salinity on the plasma membrane ATPase from tomato (Lycopersicon esculentum Mill.) leaves

Experiments were conducted to determine if salt-induced changes in the plasma-membrane H⁺-ATPase could account for decreased shoot growth in Lycopersicon esculentum Mill. cv Heinz 1350. The plasma membrane was isolated from mature and young leaves of untreated (control) and salt-stressed leaves using the aqueous polymer two-phase technique. Analysis of ATP hydrolytic activity showed that salt stress reduced the V_max of H⁺-ATPase specific activity in mature leaves but had no effect in young, growing leaves. These findings are consistent with measurements of adenine nucleotide levels. We observed a large decrease in adenylate energy charge (AEC) in mature leaves following salt stress. AEC in young leaves remained unaffected. In addition, a dramatic increase in K, the adenylate kinase mass action ratio, from 0.18 to 1.53 was measured in mature leaves following salt stress. These results indicate that salt stress did not reduce growth of young, growing leaves by decreasing H⁺-ATPase activity and support our earlier hypothesis that salt stress affects mature leaves and, as a consequence, their ability to supply essential metabolites for growing regions of both shoot and roots.