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H⁺ Extrusion and Potassium Uptake Associated with Potential Hyperpolarization in Maize and Wheat Root Segments Treated with Permeant Weak Acids ¹

Dipartimento di Biologia, Università di Milano, 20133 Milano, Dipartimento di Biologia Vegetale, Universita' di Torino, 10100 Torino, Italy, Dipartmento di Biologia Generale, Universita' di Torino, 10100 Torino, Italy

The rapid uptake of weak acids permeant in the uncharged form is accompanied in maize and wheat root segments by a hyperpolarization 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 distinct modes of action. These results support the view that the activity of the plasmalemma H⁺ pump is regulated by the value of cytosolic pH.

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