AMMONIUM UPTAKE BY RICE ROOTS .3. ELECTROPHYSIOLOGY

The transmembrane electrical potential differences (Delta Psi) were me asured in epidermal and cortical cells of intact roots of 3-week-old r ice (Oryza sativa L. cv M202) seedlings grown in 2 or 100 mu M NH4+ (G 2 or G100 plants, respectively). In modified Johnson's nutrient soluti on containing no nitrogen, Delta Psi was in the range of -120 to -140 mV. Introducing NH4+ to the bathing medium caused a rapid depolarizati on. At the steady state, average Delta Psi of G2 and G100 plants were -116 and -89 mV, respectively. This depolarization exhibited a biphasi c response to external NH4+ concentration similar to that reported for (NH4+)-N-13 influx isotherms (M.Y. Wang, M.Y. Siddiqi, T.J. Ruth, A.D .M. Class [1993] Plant Physiol 103:1259-1267). Plots of membrane depol arization versus (NH4+)-N-13 influx were also biphasic, indicating dis tinct coupling processes for the two transport systems, with a breakpo int between two concentration ranges around 1 mM NH4+. The extent of d epolarization was also influenced by nitrogen statirs, which was large r for G2 plants than for G100 plants. Depolarization of Delta Psi due to NH4+ uptake was eliminated by a protonophore (carboxylcyanide-m-chl orophenylhydrazone), 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.