Nitrate storage and retrieval in Beta vulgaris: Effects of nitrate and chloride on proton gradients in tonoplast vesicles.

The fluorescent probes acridine orange and oxonol-V were used as indicators of pH gradients (DeltapH) and membrane potential differences (DeltaPsi), respectively, in membrane vesicles believed to be derived from the tonoplast of Beta vulgaris L. Low concentrations of nitrate (1-5 mM) caused a partial dissipation of both DeltapH and DeltaPsi at vesicle transport sites distinct from the H(+)-ATPase. In contrast, chloride dissipated only DeltaPsi. A model is proposed in which nitrate and chloride enter the plant cell vacuole in response to a potential generated by the tonoplast H(+)-ATPase. Nitrate but not chloride may then be retrieved for metabolic use by the operation of a nitrate/proton symport at the tonoplast.