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Articles

Effect of Light and Glucose on the Induction of Nitrate Reductase and on the Distribution of Nitrate in Etiolated Barley Leaves ¹

Department of Biology, McMaster University, Hamilton, Ontario, Canada, L8S 4K1, Department of Agronomy and Range Science, University of California, Davis, California 95616

Barley seedlings grown in the dark with 10 mM KNO₃ have low levels of nitrate reductase activity even though large amounts of No₃^– accumulate in the leaves. When the leaves are excised and transferred to the light, there is an increase in nitrate reductase activity both in the presence and absence of exogenous NO₃^–. When the leaves are transferred to a glucose solution (0.05 M) but kept in the dark, induction of nitrate reductase activity occurs only when fresh NO₃^– is added to the system.

In dark-grown leaves, there are small traces of NO₃^– in a "metabolic pool." Addition of glucose does not alter this distribution. Light, on the other hand, results in an appreciable accumulation of NO₃^– in the metabolic pool. There is a linear correlation between nitrate reductase activity and the size of the metabolic NO₃^– pool. Our results thus suggest that NO₃^– accumulates in a storage pool when seedlings are grown in continuous darkness. The transfer of this NO₃^– to an active metabolic pool is mediated by light but not by glucose. We believe that this transfer of NO₃^– leads to the induction of nitrate reductase. When NO₃^– is included in the medium, both light and glucose increase its incorporation into the metabolic pool. The results suggest two mechanisms for regulating the metabolic NO₃^– pool: (a) a transfer from the storage pool which requires light; and (b) a transfer from the external medium which requires either glucose or light.

¹ This work was supported in part by Grant A-2818 from the National Research Council of Canada. and in part by the Kearney Foundation of Soil Science.

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