THE EFFECT OF AMMONIUM-IONS ON MEMBRANE-POTENTIAL AND ANION FLUX IN ROOTS OF BARLEY AND TOMATO

The effects of ammonium (0-5 mol m-3) on root hair membrane potential and on the influx of nitrate and phosphate were investigated in roots of intact barley and tomato plants. In both species, addition of ammonium to the medium bathing the roots caused an almost immediate depolarization of the membrane potential; the depolarization was greater at higher concentrations of ammonium. Influx of (NO3-)-N-13 and 32Pi was inhibited over the same time scale and concentration range. In tomato roots, there was little further depolarization of the membrane potential or inhibition of anion influx at ammonium concentrations above 0.4 mol m-3. In barley roots, the inhibition of nitrate influx and the depolarization of the membrane potential did not saturate below 5 mol m-3 ammonium.