Abstract
The effects of ammonium application on nitrate utilization were studied in N-limited cultures of Lemna gibba L. G3. Addition of ammonium instantaneously inhibited net nitrate uptake by at least 60%, followed by a slight recovery. The inhibition was equally clear after near-complete inactivation of glutamine synthetase by application of l-methionine-d,l-sulfoximine. Experiments where (13)N-labeled nitrate was used as an influx tracer revealed that ammonium specifically inhibited influx, but did not promote nitrate efflux. Nitrate accumulation was relatively more inhibited than nitrate reduction and net uptake. Nitrate reductase, extracted and assayed in vitro in the presence of the thiol proteinase inhibitor leupeptin, was unaffected by short-term treatment of the plants with either nitrate, ammonium, or ammonium nitrate. Nitrate reductase activity recovered in the absence of leupeptin was considerably lower; however, it was enhanced by all the nitrogen sources, with ammonium as the most potent. It is argued that the effect of ammonium on nitrate utilization in Lemna is due to inhibition of nitrate influx, and that the effect should be attributed to ammonium itself, not to a newly formed nitrogen derivative. The decreased nitrate flux caused a decrease in nitrate reduction, whereas the activity of nitrate reductase per se rather is stabilized by presence of ammonium.