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 P-i 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. (C) 2011 Elsevier Masson SAS. All rights reserved.