Nitrate Uptake in Catharanthus Roseus Cells: Electrophysiological Effects

Data on electrophysiological parameters of nitrate uptake are rather rare in plant cells. The distinction between the electrical effect of NO3 − at the plasmalemma and at the tonoplast turns out to be too complex for a heterogeneously polarized system, especially because the localization of the microelectrode tip in the cytosol or vacuole is often unknown. Transcellular potential changes connected to an external alkalinization upon addition of nitrate have often been considered as an indication of an unbalanced stoichiometry of a nitrate cotransport mechanism located at the plasma membrane. The wide-spread neglect of the positive trans-tonoplast potential in studies of nitrate effects on membrane potential can lead to errors in interpreting hyperpolarization phenomena (Rona et al. 1980 a,b; Barbier-Brygoo et al. 1985; Chedhomme and Rona 1988). For cell hyperpolarization, it has been suggested that NO3 −/OH− antiport functions as the main transport mechanism for nitrate across the plasmalemma (Thibaud and Grignon 1981; Monestiez et al. 1987), in the case of cell depolarization upon addition of nitrate a NO3 −/2H+ symport (Ullrich and Novacky 1981; Ullrich 1987) has been proposed. Nevertheless, it has been reported in the literature that the electropositive gradient across the tonoplast is also affected by nitrate uptake into the vacuole, causing a dissipation of the passive trans-tonoplast potential (Poole and Blumwald 1987; Leigh and Pope 1987) and a decrease of the positive electrogenic component in connection with the partial inhibition of the tonoplast ATPase (Bennett and Spanswick 1984; Sze 1984; Jochem et al. 1984; Griffith et al. 1986; Chedhomme and Rona 1986).