In vitro effect of different Na+/K+ ratios on plasma membrane H+-ATPase activity in maize and sugar beet shoot

Plant growth is impaired primarily by osmotic stress in the first phase of salt stress, whereas Na+ toxicity affects the plant growth mainly in the second phase. Salinity leads to increased Na+/K+ ratio and thus displacement of K+ by Na+ in the plant cell. Relatively higher cytosolic Na+ concentrations may have an effect on the activity of plasma membrane (PM) H+-ATPase. A decreased PM-H+-ATPase activity could increase the apoplastic pH. This process could limit the cell-wall extensibility and thus reduce growth according to the acid growth theory. To compare the effect of Na+ on PM H+-ATPase activity in salt-sensitive maize (Zea mays L.) and salt-resistant sugar beet (Beta vulgaris L.) shoot, PM vesicles were isolated from growing shoots of both species and ATPase activity was determined by assaying the Pi released by hydrolysis of ATP. The H+ pumping activity was measured as the quenching of acridine-orange absorbance. An increased Na+/K+ ratio decreased the PM H+-ATPase activity in vesicles of maize as well as of sugar beet shoots. Nevertheless, the detrimental effect of increased Na+/K+ ratio was more severe in salt-sensitive maize compared to salt-resistant sugar beet. At 25 mM Na+ concentration, hydrolytic activity was not affected in sugar beet. However, a significant decrease in hydrolytic activity was observed in maize at pH 7. In maize and sugar beet, reduction in active H+ flux was 20% and 5% at 25 mM Na+ concentration in the assay, respectively. The active H+ flux was decreased to 80% and 60%, when 100 mM K+ were substituted by 100 mM Na+. We conclude that PM H+-ATPases of salt-resistant sugar beet and maize shoot are sensitive to higher concentration of Na+. However, sugar beet PM-H+-ATPases are relatively efficient and may have constitutive resistance against lower concentration (25 mM) of Na+ as compared to that of salt-sensitive maize.