FIGURE 3.

Effect of salt stress on the PPi-dependent proton transport activity in tonoplast vesicles isolated from potato leaves. (A) Representative traces of proton pump activity in tonoplast vesicles isolated from leaves of the cultivar Desiree. The V-H⁺-PPase was activated by the addition of increasing concentrations of PPi to the assay medium containing 20 μg protein of membrane vesicles. After equilibration 1 mM NH₄Cl was added to release the quench. (B) Dose response curves of the initial rates of proton pump activity of the V-H⁺-PPase in dependence of the substrate PPi concentration for both cultivars treated with and without 60 mM NaCl. The curves were fitted with the Michaelis–Menten equation with the aid of SigmaPlot software package. The average K_m and V_max values as shown in the table were calculated from the fit of each independent experiment (n = 3 ± SEM). Different letters indicate a statistically significant treatment effect (one-way ANOVA; P < 0.05). A two-way ANOVA showed a treatment × cultivar interaction effect for V_max (P < 0.05).