H + Fluxes at Plasmalemma Level: In Vivo Evidence for a Significant Contribution of the Ca 2+ ‐ATPase and for the Involvement of its Activity in the Abscisic Acid‐Induced Changes in Egeria Densa Leaves

The features of Ca 2+ fluxes, the importance of the Ca 2+ pump‐mediated H + /Ca 2+ exchanges at plasmalemma level, and the possible involvement of Ca 2+ ‐ATPase activity in ABA‐induced changes of H + fluxes were studied in Egeria densa leaves. The results presented show that, while in basal conditions no net Ca 2+ flux was evident, a conspicuous Ca 2+ influx (about 1.1 ìmol g −1 FW h −1 ) occurred. The concomitant efflux of Ca 2+ was markedly reduced by treatment with 5 íM eosin Y (EY), a specific inhibitor of the Ca 2+ ‐ATPase, that completely blocked the transport of Ca 2+ after the first 20 ‐ 30 min. The decrease in Ca 2+ efflux induced by EY was associated with a significant increase in net H + extrusion (−ÄH + ) and a small but significant cytoplasmic alkalinization. The shift of external [Ca 2+ ] from 0.3 to 0.2 mM (reducing Ca 2+ uptake by about 30 %) and the hindrance of Ca 2+ influx by La 3+ were accompanied by progressively higher −ÄH + increases, in agreement with a gradual decrease in the activity of a mechanism counteracting the Ca 2+ influx by an n H + /Ca 2+ exchange. The ABA‐induced decreases in −ÄH + and pH cyt were accompanied by a significant increase in Ca 2+ efflux, all these effects being almost completely suppressed by EY, in line with the view that the ABA effects on H + fluxes are due to activation of the plasmalemma Ca 2+ ‐ATPase. These results substantially stress the high sensitivity and efficacy of the plasmalemma Ca 2+ pump in removing from the cytoplasm the Ca 2+ taken up, and the importance of the contribution of Ca 2+ pump‐mediated H + /Ca 2+ fluxes in bringing about global changes of H + fluxes at plasmalemma level.