Short-term studies of NO 3 (-) uptake in Pisum using (13)NO 3 (-).

Influx, efflux and net uptake of NO 3 (-) was studied in Pisum sativum L. cv. Marma in short-term experiments where (13)NO 3 (-) was used to trace influx. The influx rate in N-limited plants was similar both during net uptake at external concentrations of around 50 μM, and at low external NO 3 (-) concentrations (4-6 μM) when net uptake was practically zero. Efflux could be inferred from discrepancies between influx and net uptake but was never very high in the N-limited plants during net uptake. Close to the threshold concentration for not NO 3 (-) uptake, efflux was high and equalled influx. Thus, the threshold concentration can be regarded as a NO 3 (-) compensation point. The inclusion of NH 4 (+) in the outer medium decreased influx by about 40% but did not significantly affect efflux. The roles of NO 3 (-) fluxes and nitrate-reductase activity in regulating/limiting NO 3 (-) utilization are discussed.