Induction of nitrate reductase in needles of Scots pine seedlings by NO X and NO 3 −

The possibility to induce nitrate reductase (NR; EC 1.6.6.2) in needles of Scots pine ( Pinus sylvestris L.) seedlings was studied. The NR activity was measured by an in vivo assay. Although increased NR activities were found in the roots after application of NO 3 − , no such increase could be detected in the needles. Detached seedlings placed in NO 3 − solution showed increasing NR activities with increasing NO 3 − concentrations. Exposure of seedlings to NO x (70–80 ppb NO 2 and 8–12ppb NO) resulted in an increase of the NR activity from 10–20 nmol NO 2 − (g fresh weight) −1 h −1 to about 400 nmol NO 2 − (g fresh weight) −1 h −1 . This level was reached after 2–4 days of exposure, thereafter the NR activity decreased to about 200 nmol NO 2 − (g fresh weight) −1 h −1 . Analyses of free amino acids showed low concentrations of arginine and glutamine in NO x ‐fumigated seedlings compared to corresponding controls.