Growth of young cucumber plants was strongly inhibited, when calcium was removed from the culture solution. The activities of nitrate reductase, glutamate dehydrogenase and glutamine synthetase were investigated after the removal of calcium. Though the activities of glutamine synthetase and glutamate dehydrogenase were not altered much, nitrate reductase activity, measured by in vitro and in vivo assays, decreased dramatically. The loss of nitrate reductase activity coincided with the level of nitrate in the leaves. When nitrate was supplied to the cucumbers with a nitrate deficiency, the plants induced nitrate reductase together with a distinct accumulation of nitrate. However, cucumbers treated for both calcium and nitrate deficiency failed to induce nitrate reductase and to accumulate nitrate on the addition of large amounts of nitrate. Leaf sections that had been treated for both calcium and nitrate deficiency could induce nitrate reductase when floated on nitrate solution under the light. This indicates that the drastic loss of nitrate reductase caused by the removal of calcium was due mainly to the deficiency of nitrate as the inducer in leaves.
展开▼
