Stimulation of weak acid uptake and increase of cell sap pH as an evidence for FC- K+-induced cytosolic alkalinization.

In maize root segments fusicoccin induced a consistent increase in cell sap pH (taken as representative of vacuolar pH). This effect was markedly enhanced by the presence of K' in the medium, whereas in the absence of fusicoccin K' did not significantly influence cell sap pH. Treatment with a weak acid at 2 mm concentration inhibited the uptake of a different ('4C-labeled) weak acid fed at a lower concentration, thus suggesting that acidification of the cytoplasm inhibits weak acid uptake. Fusicoccin and K' increased the rate of uptake of 5,5-dimethyloxazolidine- 2,4-dione, butyric acid, or isobutyric acid slightly when fed separately, strongly when fed in combination. The synergism between fusicoccin and K' in stimulating weak acid uptake was parallel to that observed for the stimulation of H' extrusion. Application of the weak acid distribution method to a condition of 'quasi-equilibrium' indicated that fusicoccin induces a cytosolic pH increase of about 0.14 unit. These results are interpreted as providing circumstantial evidence that fusicoccin- and K'- induced stimulation of H' extrusion led to an alkalinization of the cytosol, and that other early metabolic responses, such as an increase in malate level, are a consequence of the increase in cytosolic pH.