Comparative induction of nitrate reductase by nitrate and nitrite in barley leaves

The comparative induction of nitrate reductase (NR) by ambient NO3 - and $\text{NO}_{2}{}^{-}$ 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 $\text{NO}_{2}{}^{-}$-fed leaves. When the leaves were supplied cycloheximide or tungstate along with $\text{NO}_{2}{}^{-}$, 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 $\text{NO}_{2}{}^{-}$ until the ambient $\text{NO}_{2}{}^{-}$ concentration was 0.5 mM. In fact, NR induction from $\text{NO}_{2}{}^{-}$ solutions was not seen until NO3 - was detected in the leaves. The amount of NO3 - accumulating in $\text{NO}_{2}{}^{-}$-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 $\text{NO}_{2}{}^{-}$, was highly correlated with the amount of NR induced. The evidence indicated that NO3 - was a more likely inducer of NR than was $\text{NO}_{2}{}^{-}$.