Effects of abscisic acid and cytoplasmic pH on potassium and chloride efflux in Arabidopsis thaliana seedlings

The effects of ABA, isobutyric acid (IBA) and nicotine on K+ and Cl- efflux were studied in Arabidopsis thaliana seedlings, and the role of pH(cyt) and E-m in the regulation of the efflux of these ions was discussed. The data show that treatments with IBA and nicotine influenced in opposite directions the efflux of either K+ or Cl-: K+ efflux was increased by nicotine and reduced in the presence of IBA, whereas Cl- efflux was stimulated by IBA and decreased by nicotine treatment. Under all the conditions tested ABA induced cytoplasmic acidification and inhibition of K+ and Cl- net efflux, Experiments aimed to estimate the individual contribution of pH(cyt) and E-m in modulating K+ efflux indicated that, within the range of acidic pH(cyt) values, a regulation of K+ efflux was imposed by pH(cyt) on the control exerted by E-m, the efflux being inhibited by lower pH(cyt) values, Conversely, in the alkaline side of pH(cyt) K+ efflux seemed linked only to the E-m values. These results are consistent with the hypothesis that the decrease in K+ efflux observed in non-stomatal tissues in the presence of ABA may be mediated by the cytoplasmic acidification induced by the hormone.