Abstract
Elongation of subapical segments of maize (Zea mays) roots was greatly inhibited by 2H2O in the incubation medium. Short-term exposure (30 min) to 2H2O slightly reduced O2 uptake and significantly increased ATP levels. 2H2O inhibited H+ extrusion in the presence of both low (0.05 mm) and high (5 mm) external concentrations of K+ (about 30 and 53%, respectively at 50% [v/v] 2H2O). Experiments on plasma membrane vesicles showed that H+-pumping and ATPase activities were greatly inhibited by 2H2O (about 35% at 50% [v/v] 2H2O); NADH-ferricyanide reductase and 1,3-β-glucan synthase activities were inhibited to a lesser extent (less than 15%). ATPase activities present in both the tonoplast-enriched and submitochondrial particle preparations were not affected by 2H2O. Therefore, the effect of short incubation time and low concentration of 2H2O is not due to a general action on overall cell metabolism but involves a specific inhibition of the plasma membrane H+ -ATPase. K+ uptake was inhibited by 2H2O only when K+ was present at a low (0.05 mm) external concentration where absorption is against its electrochemical potential. The transmembrane electric potential difference (Em) was slightly hyperpolarized by 2H2O at low K+, but was not affected at the higher K+ concentrations. These results suggest a relationship between H+ extrusion and K+ uptake at low K+ external concentration.
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