Membrane Potential and Proton Cotransport of Alanine and Phosphate as Affected by Permeant Weak Acids in Lemna gibba

The treatment of Lemna gibba plants with the weak acids (trimethylacetic acid and butyric acid), used as tools to decrease intracellular pH, induced a hyperpolarization of membrane potential, dependent on the concentration of the undissociated permeant form of the weak acid and on the value of the resting potential. Measurements were carried out both with `high potential' and `low potential' plants and the maximum values af acid induced hyperpolarizations were about 35 and 71 millivolts, respectively. Weak acids influenced also the transient light-dark membrane potential changes, typical for photosynthesizing material, suggesting a dependence of these changes on an acidification of cytoplasm. In the presence of the weak acids, the membrane depolarization induced by the cotransport of alanine and phosphate with protons was reduced; the maximum reduction (about 90%) was obtained with alanine during 2 millimolar trimethylacetic acid perfusion at pH 5. A strong inhibition of the uptake rates (up to 48% for ¹⁴C]alanine and 68% for ³²P-phosphate) was obtained in the presence of the weak acids, both by decreasing the pH of the medium and by increasing the concentration of the acid. In these experimental conditions, the ATP level and O₂ uptake rates did not change significantly. These results constitute good evidence that H⁺/solute cotransport in Lemna, already known to be dependent on the electrochemical potential difference for protons, is also strongly regulated by the cytoplasmic pH value.