AMMONIUM UPTAKE BY RICE ROOTS .2. KINETICS OF NH4-N-13 INFLUX ACROSS THE PLASMALEMMA()

Short-term influxes of (NH4+)-N-13 were measured in intact roots of 3- week-old rice (Oryza sativa L. cv M202) seedlings that were hydroponic ally grown at 2, 100, or 1000 muM NH4+. Below 1 mm external concentrat ion ([NH4+]o), influx was saturable and due to a high-affinity transpo rt system (HATS). For the HATS, V(max) values were negatively correlat ed and K(m) values were positively correlated with NH4+ provision duri ng growth and root [NH4+]. Between 1 and 40 mm [NH4+]o, (NH4+)-N-13 in flux showed a linear response due to a low-affinity transport system ( LATS). The (NH4+)-N-13 influxes by the HATS, and to a lesser extent th e LATS, are energy-dependent processes. Selected metabolic inhibitors reduced influx of the HATS by 50 to 80%, but of the LATS by only 31 to 51%. Estimated values for Q10 (the ratio of rates at temperatures dif fering by 10-degrees-C) for HATS were greater than 2.4 at root tempera tures from 5 to 10-degrees-C and were constant at approximately 1.5 be tween 5 and 30-degrees-C for the LATS. Influx of (NH4+)-N-13 by the HA TS was insensitive to external pH in the range from 4.5 to 9.0, but in flux by the LATS declined significantly beyond pH 6.0. The data presen ted are discussed in the context of the kinetics, energy dependence, a nd the regulation of ammonium influx.