Enhancement of Nitrate Reductase Activity by Benzyladenine in Agrostemma githago

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Abstract
Nitrate reductase activity in excised embryos of Agrostemma githago increases in response to both NO3− and cytokinins. We asked the question whether cytokinins affected nitrate reductase activity directly or through NO3−, either by amplifying the effect of low endogenous NO3− levels, or by making NO3− available for induction from a metabolically inactive compartment. Nitrate reductase activity was enhanced on the average by 50% after 1 hour of benzyladenine treatment. In some experiments, the cytokinin response was detectable as early as 30 minutes after addition of benzyladenine. Nitrate reductase activity increased linearly for 4 hours and began to decay 13 hours after start of the hormone treatment. When embryos were incubated in solutions containing mixtures of NO3− and benzyladenine, additive responses were obtained. The effects of NO3− and benzyladenine were counteracted by abscisic acid. The increase in nitrate reductase activity was inhibited at lower abscisic acid concentrations in embryos which were induced with NO3−, as compared to embryos treated with benzyladenine. Casein hydrolysate inhibited the development of nitrate reductase activity. The response to NO3− was more susceptible to inhibition by casein hydrolysate than the response to the hormone. When NO3− and benzyladenine were withdrawn from the medium after maximal enhancement of nitrate reductase activity, the level of the enzyme decreased rapidly. Nitrate reductase activity increasd again as a result of a second treatment with benzyladenine but not with NO3−. At the time of the second exposure to benzyladenine, no NO3− was detectable in extracts of Agrostemma embryos. This is taken as evidence that cytokinins enhance nitrate reductase activity directly and not through induction by NO3−.