Effects of temperature on H+ secretion and uptake by excised flexor cells during dark-induced closure of Samanea leaflets

Previous studies reveal that dark-induced closure of Samanea leaflets is accompanied by H+ secretion from flexor motor cells. We now report that flexor tissue excised in the light, incubated in a weakly buffered bathing solution, and then darkened at different temperatures (18°C-30°C) acidified the medium (indicating net H+ efflux) at all temperatures tested, but most rapidly at the highest temperature. However, pH changes reversed direction after 20 to 70 minutes; the lower the temperature, the later pH reversal occurred, and the lower the pH at reversal and after 45 minutes. These data provide a basis for the previously reported promotive effect of low temperature on dark-induced leaflet closure, assuming net H+ and K+ fluxes are opposite in direction. Net H+ efflux at all temperatures tested was greater when the impermeant molecule iminodiacetate replaced small permeant anions in the bathing solution, suggesting that H+ uptake is coupled to anion uptake, probably via a H+/anion symport system. When permeant anions were deficient, the amount of malate in the tissue increased, presumably by new synthesis. Malate synthesis would substitute for H+/anion uptake in charge balance and in providing H+ for cytoplasmic pH regulation.