Effects of deuterium oxide on growth, proton extrusion, potassium influx and in vitro plasma membrane activities in maize root segments

Elongation of subapical segments of maize (Zea mays) roots was greatly inhibited by (H2O)-H-2 in the incubation medium. Short-term exposure (30 min) to (H2O)-H-2 slightly reduced O2 uptake and significantly increased ATP levels. (H2O)-H-2 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] (H2O)-H-2). Experiments on plasma membrane vesicles showed that H+-pumping and ATPase activities were greatly inhibited by (H2O)-H-2 (about 35% at 50% [v/v] (H2O)-H-2); NADH-ferricyanide reductase and 1,3-beta-glucan synthase activities were inhibited to a lesser extent (less than 1 5%). ATPase activities present in both the tonoplast-enriched and submitochondrial particle preparations were not affected by (H2O)-H-2. Therefore, the effect of short incubation time and low concentration of (H2O)-H-2 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 (H2O)-H-2 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 (E(m)) was slightly hyperpolarized by (H2O)-H-2 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.