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The addition of ammonium to nitrate induced plants of Lemna minor L. brings about a rapid loss in extractable nitrate reductase activity. This inactivation is reversible both in vivo and in vitro. Inhibitors of RNA and protein synthesis do not protect nitrate reductase against ammonium inactivation. It is suggested that factors, in addition to ammonium ions, are components of the inactivating system. Inactivation may involve some form of protein-protein interaction. The physiological significance of rapid ammonium inactivation of nitrate reductase is discussed.

Low levels of nitrate reductase can be detected in plants of Lemna minor grown on some organic nitrogen sources. Nitrogen-starvation does not lead to a derepression of nitrate reductase activity. Nitrate ions are necessary for the development of maximum enzyme activity and the maintenance of high enzyme levels. Nitrogen-starvation of ammonia-grown plants increases the subsequent rate of nitrate-mediated induction. It is suggested that ammonium ions, either directly or indirectly modulate the rate of nitrate reductase induction. The pattern of control regulating nitrate reductase levels in Lemna is contrasted with that in some species of algae.

The presence of ammonium is shown to inhibit the nitrate-promoted formation of nitrate reductase in Lemna minor L. The ammonium inhibition does not result from an inhibition of nitrate accumulation. The kinetics of the ammonium inhibition suggest it is not a direct effect of ammonium. The inhibition could result from the build up of a product of ammonium assimilation or an ammonium induced regulatory protein. It is suggested that both the nitrate stimulation and the 'ammonium' inhibition of nitrate reductase formation can be accounted for by a transcriptional control system.