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

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Abstract
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.