Incorporation of atmospheric 15NO2-nitrogen into free amino acids by Norway spruce Picea abies (L.) Karst.

During spring and autumn 1991, potted 6-yearold spruce trees (Picea abies (L.) Karst.) were fumigated with 60 nl1–1 15NO2 for 4 days under controlled conditions in constant light. Current and previous flush needles, the bark and the fine roots were analysed for total 15N content and incorporation of 15N into the -amino nitrogen of free amino acids. In addition, in vitro nitrate reductase activity and stomatal conductance of the needles were measured. Nitrate reductase activity was significantly higher in the needles of fumigated trees compared to control trees exposed to filtered air. With an average of 9.1% 15N, free glutamate was the pool with the most label. Taking into account the time-course of the labelling of this pool, this figure can be taken as an estimate of the minimum contribution of NO2 to the N nutrition of the needles. 15N-labelled amino acids were also detected in the bark and the roots, indicating export from the needles

The Relationship of Nitrate Reductase Activity to Uptake and Assimilation of Atmospheric 15NO2 -Nitrogen in Needles of Norway Spruce (Picea abies [L.] Karst.)

Using NO2, relations between nitrate reductase activity and stomatal conductance, N-uptake and N-glutamate were studied in the two youngest needles flushes of potted Norway spruce (Picea abies [L.] Karst.). There were linear correlations between the stomatal conductance and the N-uptake and between the N-uptake and nitrate reductase (E.C. 1.6.6.1/1.6.6.2) activity. The N labelling of free glutamate shows the assimilation of NO2 from the atmosphere in addition to the nitrogen from the soil. The portion of glutamate originating from NO2 was linearly related to nitrate reductase activity in spring experiments. This indicates that this enzyme activity reflected the rate of NO2-assimilation