Rapid, Reversible Inhibition of Nitrate Influx in Barley by Ammonium

The rate of influx of nitrate into the roots of intact barleyplants was measured over a period of 3–5 min from externalnitrate concentrations of 1–150 mmol m^–3, using¹³N-labelled nitrate as tracer. Ammonium at external concentrationsof 0.005–50 mol m^–3 inhibited nitrate influx ina manner which did not conform to a simple kinetic model butincreased approximately as the logarithm of the ammonium concentration.At any particular ammonium concentration, inhibition of nitrateinflux reached its full extent within 3 min of the ammoniumbeing supplied and was not made more severe by up to 17 minpre-treatment with ammonium. On removing the external ammonium,nitrate influx returned to its original rate within about 3min. Potassium at 0.005–50 mol m^–3 did not reproducethe rapid effect of ammonium on nitrate influx. Net uptake of nitrate also decreased when ammonium was supplied,over a similar timescale and to a similar extent as nitrateinflux. The decrease in nitrate influx caused by ammonium wassufficient to account for the observed reduction in net uptake,without necessitating any acceleration of nitrate efflux. Key words: Hordeum vulgare, roots, ion transport, short-lived isotopes, ¹³N