Regulation of nitrate reductase activity by ammonium in wheat

Ammonium nitrate-induced nitrate reductase activity was more than potassium nitrate induced activity in excised shoot tips of wheat. This effect was more pronounced in the dark than in light. There was no effect of NH4H2PO4 alone, however, in the presence of nitrate, it enhanced nitrate reductase activity. When shoot tips were transferred to NH4H2PO4 alone after 18 h of KNO3 treatment, the enzyme activity increased slightly and then stabilized. This effect was not noticed with KH2PO4. The nitrate reductase stabilization by ammonium was stopped by the inhibitors of RNA and protein synthesis and tungstate, except actinomycin D which did not show any inhibitory effect. The enzyme activity was stabilized by ammonium in vivo but not in vitro