Effect of ammonium on nitrate utilization by roots of dwarf bean.

The effect of exogenous NH(4) (+) on NO(3) (-) uptake and in vivo NO(3) (-) reductase activity (NRA) in roots of Phaseolus vulgaris L. cv Witte Krombek was studied before, during, and after the apparent induction of root NRA and NO(3) (-) uptake. Pretreatment with NH(4)Cl (0.15-50 millimolar) affected neither the time pattern nor the steady state rate of NO(3) (-) uptake.When NH(4) (+) was given at the start of NO(3) (-) nutrition, the time pattern of NO(3) (-) uptake was the same as in plants receiving no NH(4) (+). After 6 hours, however, the NO(3) (-) uptake rate (NUR) and root NRA were inhibited by NH(4) (+) to a maximum of 45% and 60%, respectively.The response of the NUR of NO(3) (-)-induced plants depended on the NH(4)Cl concentration. Below 1 millimolar NH(4) (+), the NUR declined immediately and some restoration occurred in the second hour. In the third hour, the NUR became constant. In contrast, NH(4) (+) at 2 millimolar and above caused a rapid and transient stimulation of NO(3) (-) uptake, followed again by a decrease in the first, a recovery in the second, and a steady state in the third hour. Maximal inhibition of steady state NUR was 50%. With NO(3) (-)-induced plants, root NRA responded less and more slowly to NH(4) (+) than did NUR.Methionine sulfoximine and azaserine, inhibitors of glutamine synthetase and glutamate synthase, respectively, relieved the NH(4) (+) inhibition of the NUR of NO(3) (-)-induced plants. We conclude that repression of the NUR by NH(4) (+) depends on NH(4) (+) assimilation. The repression by NH(4) (+) was least at the lowest and highest NH(4) (+) levels tested (0.04 and 25 millimolar).