H+ Extrusion and Potassium Uptake Associated with Potential Hyperpolarization in Maize and Wheat Root Segments Treated with Permeant Weak Acids

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Abstract
The rapid uptake of weak acids permeant in the uncharged form is accompanied in maize and wheat root segments by a hyper polarization of the transmembrane electrical potential and an increase in K+ uptake, suggesting a stimulation of the plasmalemma H+ pump. The evaluation of weak acid-induced H+ extrusion must take into account the alkalinization of the medium due to the rapid uptake of the uncharged form of the acid, partially masking the proton pump-mediated extrusion of H+. The data corrected for this interference show that the lipophilic butyric acid and trimethyl acetic acid induce in maize and in wheat root segments a significant increase in ''real'' H+ extrusion, roughly matching the increase in net K+ uptake. The presence of K+ significantly increases the rate of uptake of the weak acid, possibly as a consequence of an alkalinization of the cytosol associated with K+ absorption. In maize root segments, the effects of fusicoccin and those of butyric acid on both K+ uptake and H+ extrusion are clearly synergistic, thus suggesting distincts modes of action. These results support the view that the activity of the plasmalemma H+ pump is regulated by the value of cytosolic pH.