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

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Abstract
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−.