ENERGETICS OF OH- OR H+ DEPENDENT NITRATE UPTAKE BY CATHARANTHUS-ROSEUS CELLS - ELECTROPHYSIOLOGICAL EFFECTS

The electrical potential across the plasmalemma and the tonoplast were recorded, in Catharanthus roseus cells, by pushing a glass microelectrode through a cell with the tip consecutively in the cell wall, the cytoplasm and the vacuole. The electrical potential difference between the cytoplasm and the external medium (E(CO)) was about -71 mV and the mean potential difference at the tonoplast (E(VC)) about +22 mV. In culture conditions, during the first two days following the transfer of the cells into a fresh medium, nitrate uptake by the cells was marked by a hyperpolarization of the plasmalemma (about -18 mV) and a simultaneous alkalinization of the external medium (1.3 pH units). Similar data were also obtained in short experiments (less than 10 min) with cells bathed in nitrate solutions (10 mM NaNO3). Evidence for the existence of nitrate cotransport with H+ or OH- as counterions at the plasmalemma is discussed.