The effect of exogenous NH4' on NO3-uptake and in vivo NO3-reductase activity (NRA) in roots of Phascolus vulgaris L. cv Witte Krombek was studied before, during, and after the apparent induction of root NRA and NO3-uptake. Pretreatment with NH41 (0.15-50 milli-molar) affected neither the time pattern nor the steady state rate of N03-uptake. When NH4' was given at the start of NO3-nutrition, the time pattern of NO3-uptake was the same as in plants receiving no NH,4. After 6 hours, however, the NO3-uptake rate (NUR) and root NRA were inhibited by NHI4 to a maximum of 45% and 60%, respectively. The response of the NUR of NO3-induced plants depended on the NH4C1 concentration. Below I millimolar NH4, the NUR declined immediately and some restoration occurred in the second hour. In the third hour, the NUR became constant. In contrast, NH44 at 2 millimolar and above caused a rapid and transient stimulation of N03-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 NO3-induced plants, root NRA responded less and more slowly to NI1-4' than did NUR. Methionine sulfoximine and azaserine, inhibitors of glutamine synthe-tase and glutamate synthase, respectively, relieved the NH.4 inhibition of the NUR of NO3-induced plants. We conclude that repression of the NUR by NH@4 depends on NH,4' assimilation. The repression by NH4' was least at the lowest and highest NHI4 levels tested (0.04 and 25 millimolar). N03-is the main source of N for crop plants, including legumes (23). At present there is much concern about the ecological and nutritional impact of NO3-fertilizers. A better understanding of the physiological processes by which the plant responds to the presence of N03 in its environment may permit a more intelligent use of NO3-. Despite its biological and agricultural significance, detailed studies on the process of N03-acquisition by plants are fairly recent and insight into the regulation of NO3-uptake is still in its infancy (12). The process of NO3-uptake by higher plants differs from that of other inorganic nutrients. For example, the fate of NO3-in the plant may be accumulation as well as conversion and both uptake and reduction have characteristics of induction. Against this background, we started an investigation into the initial events of NO3-utilization in dwarf bean, a species which is very flexible in the choice of its N source. Our previous work has identified NO3-uptake and reduction of NO3-to NO2-in the root system ' M. S. was a student from the Department of Soil Science and Plant Nutrition of the Agricultural University at Wageningen. as two key steps in NO3-utilization by N-depleted dwarf bean (3,6). Utilization of NO3-encompasses all processes that deliver exogenous N03-N to its final destination which may range from deposition in the root to biosynthesis in the shoot. Assimilation of NO3-has a number of obligatory chemical intermediates such as NO2, NH4', amides, and amino acids. Each inte...
