Ammonium Uptake by Rice Roots (II. Kinetics of 13NH4+ Influx across the Plasmalemma)

Short-term influxes of 13NH4+ were measured in intact roots of 3-week-old rice (Oryza sativa L. cv M202) seedlings that were hydroponically grown at 2, 100, or 1000 mu]M NH4+. Below 1 mM external concentration (NH4+]0), influx was saturable and due to a high-affinity transport system (HATS). For the HATS, Vmax values were negatively correlated and Km values were positively correlated with NH4+ provision during growth and root NH4+]. Between 1 and 40 mM NH4+]0, 13NH4+ influx showed a linear response due to a low-affinity transport system (LATS). The 13NH4+ influxes by the HATS, and to a lesser extent the 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 differing by 10deg]C) for HATS were greater than 2.4 at root temperatures from 5 to 10deg]C and were constant at approximately 1.5 between 5 and 30deg]C for the LATS. Influx of 13NH4+ by the HATS was insensitive to external pH in the range from 4.5 to 9.0, but influx by the LATS declined significantly beyond pH 6.0. The data presented are discussed in the context of the kinetics, energy dependence, and the regulation of ammonium influx.