Ammonium repression of nitrate reductase formation in Lemna minor L.

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.