INCORPORATION OF ATMOSPHERIC (NO2)-N-15-NITROGEN INTO FREE AMINO-ACIDS BY NORWAY SPRUCE PICEA-ABIES (L) KARST

During spring and autumn 1991, potted 6-year-old spruce trees (Picea a bies (L.) Karst.) were fumigated with 60 nl . l-1 (NO2)-N-15 for 4 day s under controlled conditions in constant light. Current and previous flush needles, the bark and the fine roots were analysed for total N-1 5 content and incorporation of N-15 into the alpha-amino nitrogen of f ree amino acids. In addition, in vitro nitrate reductase activity and stomatal conductance of the needles were measured. Nitrate reductase a ctivity was significantly higher in the needles of fumigated trees com pared to control trees exposed to filtered air. With an average of 9. 1 % N-15, free glutamate was the pool with the most label. Taking into account the time-course of the labelling of this pool, this figure ca n be taken as an estimate of the minimum contribution of NO2 to the N nutrition of the needles. N-15-labelled amino acids were also detected in the bark and the roots, indicating export from the needles.