Activation of nitrate reductase in extracts of water stressed wheat

Nitrate reductase (EC 1.6.6.1) activity (NRA) in leaves of wheat (Triticum aestivum cv. C306) seedlings declined significantly following polyethylene glycol (PEG)-induced water stress. During initial stages of stress, the in vitro NRA was affected to a greater extent than the in vivo activity. Treatment with potassium ferricyanide of the leaf extracts of plants stressed for 2 and 4 days elevated NRA to the levels almost comparable with that in non-stressed control plants, and in plants stressed for 4 days NADH was also effective. Treatment with EDTA, which promotes conversion of phosphorylated inactive NR to its non-phosphorylated active form, however did not stimulate NRA in these extracts. These results suggest that the decrease in NRA during the initial stages of water stress was due to conversion of the enzyme to an inactive form, which can readily be reactivated in vitro by potassium ferricyanide or NADH.