Rapid, Reversible Inhibition of Nitrate Influx in Barley by Ammonium

The rate of influx of nitrate into the roots of intact barley plants was measured over a period of 3–5 min from external nitrate concentrations of 1–150 mmol m−3, using 13N-labelled nitrate as tracer. Ammonium at external concentrations of 0.005–50 mol m−3 inhibited nitrate influx in a manner which did not conform to a simple kinetic model but increased approximately as the logarithm of the ammonium concentration. At any particular ammonium concentration, inhibition of nitrate influx reached its full extent within 3 min of the ammonium being supplied and was not made more severe by up to 17 min pre-treatment with ammonium. On removing the external ammonium, nitrate influx returned to its original rate within about 3 min. Potassium at 0.005–50 mol m−3 did not reproduce the 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 nitrate influx. The decrease in nitrate influx caused by ammonium was sufficient to account for the observed reduction in net uptake, without necessitating any acceleration of nitrate efflux.