H+ fluxes at plasmalemma level: in vivo evidence for a significaATPase and for the involvement of its activity in the abscisic acid-induced changes in the Egeria leaves.

The features of Ca2+ fluxes, the importance of the Ca2+ pump-mediated H+/Ca2+ exchanges at plasmalemma level, and the possible involvement of Ca2+-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 Ca2+ flux was evident, a conspicuous Ca2+ influx (about 1.1 ìmol g-1 FW h-1) occurred. The concomitant efflux of Ca2+ was markedly reduced by treatment with 5 ìM eosin Y (EY), a specific inhibitor of the Ca2+-ATPase, that completely blocked the transport of Ca2+ after the first 20 - 30 min. The decrease in Ca2+ 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 [Ca2+] from 0.3 to 0.2 mM (reducing Ca2+ uptake by about 30 %) and the hindrance of Ca2+ influx by La3+ were accompanied by progressively higher -ÄH+ increases, in agreement with a gradual decrease in the activity of a mechanism counteracting the Ca2+ influx by an nH+/Ca2+ exchange. The ABA-induced decreases in -ÄH+ and pHcyt were accompanied by a significant increase in Ca2+ 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 Ca2+-ATPase. These results substantially stress the high sensitivity and efficacy of the plasmalemma Ca2+ pump in removing from the cytoplasm the Ca2+ taken up, and the importance of the contribution of Ca2+ pump-mediated H+/Ca2+ fluxes in bringing about global changes of H+ fluxes at plasmalemma level.