Ammonium Uptake by Rice Roots (III. Electrophysiology)

The transmembrane electrical potential differences (delta]psi]) were measured in epidermal and cortical cells of intact roots of 3-week-old rice (Oryza sativa L. cv M202) seedlings grown in 2 or 100 mu]M NH4+ (G2 or G100 plants, respectively). In modified Johnson's nutrient solution containing no nitrogen, delta]psi] was in the range of -120 to -140 mV. Introducing NH4+ to the bathing medium caused a rapid depolarization. At the steady state, average delta]psi] of G2 and G100 plants were -116 and -89 mV, respectively. This depolarization exhibited a biphasic response to external NH4+ concentration similar to that reported for 13NH4+ influx isotherms (M.Y. Wang, M.Y. Siddiqi, T.J. Ruth, A.D.M. Glass 1993] Plant Physiol 103: 1259-1267). Plots of membrane depolarization versus 13NH4+ influx were also biphasic, indicating distinct coupling processes for the two transport systems, with a breakpoint between two concentration ranges around 1 mM NH4+. The extent of depolarization was also influenced by nitrogen status, which was larger for G2 plants than for G100 plants. Depolarization of delta]psi] due to NH4+ uptake was eliminated by a protonophore (carboxylcyanide-m-chlorophenylhydrazone), inhibitors of ATP synthesis (sodium cyanide plus salicylhydroxamic acid), or an ATPase inhibitor (diethylstilbestrol). The results of these observations are discussed in the context of the mechanisms of NH4+ uptake by high- and low-affinity transport systems operating across the plasma membranes of root cells.