Repression of nitrate reductase in cucumber leaves caused by calcium deficiency

Growth of young cucumber plants was strongly inhibited, whencalcium was removed from the culture solution. The activitiesof nitrate reductase, glutamate dehydrogenase and glutaminesynthetase were investigated after the removal of calcium. Thoughthe activities of glutamine synthetase and glutamate dehydrogenasewere not altered much, nitrate reductase activity, measuredby in vitro and in vivo assays, decreased dramatically. Theloss of nitrate reductase activity coincided with the levelof nitrate in the leaves. When nitrate was supplied to the cucumberswith a nitrate deficiency, the plants induced nitrate reductasetogether with a distinct accumulation of nitrate. However, cucumberstreated for both calcium and nitrate deficiency failed to inducenitrate reductase and to accumulate nitrate on the additionof large amounts of nitrate. Leaf sections that had been treatedfor both calcium and nitrate deficiency could induce nitratereductase when floated on nitrate solution under the light.This indicates that the drastic loss of nitrate reductase causedby the removal of calcium was due mainly to the deficiency ofnitrate as the inducer in leaves. (Received December 19, 1979; )