Cytoplasmic pH and Abscisic Acid Transport across Guard Cell Membranes

The cytoplasmic pH value o f guard cells o f Valerianella locusta was determined from the distribution o f the weak acid [l4C]ABA between the medium and the cell, which was shown to be 8.1 ±0.1. Treatment o f guard cells in media with low pH (pH 5.0) and different concentra­ tions o f sodium acetate (0 -5 m o lm '3) caused a considerable acidification o f the cytoplasm (6 .6-7 .7). Using the compartmental analysis we determined also the fluxes o f ABA at the plas­ ma membrane o f guard cells (co) and at the tonoplast o f guard cells (cv) under steady state conditions. The flux o f ABA from the cytoplasm into the vacuoles (tonoplast flux) was de­ pendent upon the cytoplasmic pH. The data give evidence that A B A “ is transported across the tonoplast by a saturable transport component. The ABA flux at the plasma membrane seemed to be independent o f the cytoplasmic pH.