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.