EFFECTS OF DEUTERIUM-OXIDE ON GROWTH, PROTON EXTRUSION, POTASSIUM INFLUX, AND INVITRO 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.