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Metabolism and Enzymology

Role of Nitrate and Nitrite in the Induction of Nitrite Reductase in Leaves of Barley Seedlings ¹

Plant Growth Laboratory and Department of Agronomy and Range Science, University of California, Davis, California 95616

The role of NO₃^– and NO₂^– in the induction of nitrite reductase (NiR) activity in detached leaves of 8-day-old barley (Hordeum vulgare L.) seedlings was investigated. Barley leaves contained 6 to 8 micromoles NO₂^–/gram fresh weight x hour of endogenous NiR activity when grown in N-free solutions. Supply of both NO₂^– and NO₃^– induced the enzyme activity above the endogenous levels (5 and 10 times, respectively at 10 millimolar NO₂^– and NO₃^– over a 24 hour period). In NO₃^–-supplied leaves, NiR induction occurred at an ambient NO₃^– concentration of as low as 0.05 millimolar; however, no NiR induction was found in leaves supplied with NO₂^– until the ambient NO₂^– concentration was 0.5 millimolar. Nitrate accumulated in NO₂^–-fed leaves. The amount of NO₃^– accumulating in NO₂^–-fed leaves induced similar levels of NiR as did equivalent amounts of NO₃^– accumulating in NO₃^–-fed leaves. Induction of NiR in NO₂^–-fed leaves was not seen until NO₃^– was detectable (30 nanomoles/gram fresh weight) in the leaves. The internal concentrations of NO₃^–, irrespective of N source, were highly correlated with the levels of NiR induced. When the reduction of NO₃^– to NO₂^– was inhibited by WO₄^2–, the induction of NiR was inhibited only partially. The results indicate that in barley leaves NiR is induced by NO₃^– directly, i.e. without being reduced to NO₂^–, and that absorbed NO₂^– induces the enzyme activity indirectly after being oxidized to NO₃^– within the leaf.

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