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Metabolism and Enzymology

Comparative Induction of Nitrate Reductase by Nitrate and Nitrite in Barley Leaves ¹

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

The comparative induction of nitrate reductase (NR) by ambient NO₃^– and NO₂^– as a function of influx, reduction (as NR was induced) and accumulation in detached leaves of 8-day-old barley (Hordeum valgare L.) seedlings was determined. The dynamic interaction of NO₃^– influx, reduction and accumulation on NR induction was shown. The activity of NR, as it was induced, influenced its further induction by affecting the internal concentration of NO₃^–. As the ambient concentration of NO₃^– increased, the relative influences imposed by influx and reduction on NO₃^– accumulation changed with influx becoming a more predominant regulant. Significant levels of NO₃^– accumulated in NO₂^–-fed leaves. When the leaves were supplied cycloheximide or tungstate along with NO₂^–, about 60% more NO₃^– accumulated in the leaves than in the absence of the inhibitors. In NO₃^–-supplied leaves NR induction was observed at an ambient concentration of as low as 0.02 mM. No NR induction occurred in leaves supplied with NO₂^– until the ambient NO₂^– concentration was 0.5 mM. In fact, NR induction from NO₂^– solutions was not seen until NO₃^– was detected in the leaves. The amount of NO₃^– accumulating in NO₂^–-fed leaves induced similar levels of NR as did equivalent amounts of NO₃^– accumulating from NO₃^–-fed leaves. In all cases the internal concentration of NO₃^–, but not NO₂^–, was highly correlated with the amount of NR induced. The evidence indicated that NO₃^– was a more likely inducer of NR than was NO₂^–.

¹ Supported in part by National Aeronautics and Space Administration grant NCC2-99.

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