Induction of nitrate reductase activity in red spruce needles by NO₂ and HNO₃ vapor

Abstract

The induction of the enzyme nitrate reductase in needles may be a prerequisite for the assimilation of foliar-absorbed nitrogen oxide pollutants by red spruce (Picearubens Sarg.) trees. To test for induction of nitrate reductase, 1-year-old red spruce seedlings were exposed to NO 2 , HNO 3 vapor, or acid mist containing nitrate, and the activity of nitrate reductase in needles was measured. One day after exposure to NO 2 (75 nL•L −1 ) began, nitrate reductase activity was three times greater than that of unexposed control plants. One day after exposure ended, the nitrate reductase activity returned to the control level. Older red spruce seedlings that had been excavated from a spruce–fir stand exhibited a similar pattern of response, but the level of nitrate reductase activity was much lower than that of the 1-year-old seedlings. Nitric acid vapor (75 nL•L −1 ) also induced nitrate reductase in red spruce needles, and the pattern of response was similar to that with NO 2 , except that the nitrate reductase activity did not return to control levels until 2 days after exposure ended. Exposure of seedlings to acid mist containing nitrate (pH 3.5 and 5.0) did not result in a change in nitrate reductase activity. These results indicate that red spruce is capable of assimilating NO 2 and HNO 3 vapor and that hypotheses of forest decline based on foliar assimilation of pollutant nitrogen oxides are tenable.