Ammonium uptake by rice roots. II. Kinetics of 13NH4+ influx across the plasmalemma

Wang, M.Y.; Siddiqi, M.Y.; Ruth, T.J.; Glass, A.D.M.

Ammonium uptake by rice roots. II. Kinetics of 13NH4+ influx across the plasmalemma

1993

Wang, M.Y. | Siddiqi, M.Y. | Ruth, T.J. | Glass, A.D.M.

Short-term influxes of (13)NH4+ 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 micromolar NH4+. Below 1 mM external concentration 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 provision during growth and root [NH4+]. Between 1 and 40 mM [NH4+]o influx showed a linear response due to a low-affinity transport system (LATS). The (13)NH4+ 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 Q(10) (the ratio of rates at temperatures differing by 10 degrees C) for HATS were greater than 2.4 at root temperatures from 5 to 10 degrees C and were constant at approximately 1.5 between 5 and 30 degrees C for the LATS. Influx of 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.

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