EFFECT OF ATMOSPHERIC AMMONIA ON THE NITROGEN-METABOLISM OF SCOTS PINE (PINUS-SYLVESTRIS) NEEDLES

Four-year-old seedlings of Scots pine (Pinus sylvestris L.) were exposed to filtered air (FA), and to FA supplemented with NH3 (60 and 240 mu g m(-3)) in controlled-environment chambers for 14 weeks. Exposure to the higher NH, concentration resulted in an increased activity of glutamine synthetase (GS, EC 6.3.1.2), and an increase in the concentrations of soluble proteins, total nitrogen, free amino acids and leaf pigments in the needles. The GS activity (mu mol g(-1) fresh weight h(-1)) in the needle extract increased to levels 69% higher than in FA and the soluble protein concentration to levels 22% higher. Total nitrogen concentration in the needles was 42% higher than in FA, while the free amino acid concentration was 300% higher, which was caused by an increase in arginine, glutamate, aspartate and glutamine. Chlorophyll a, chlorophyll b and carotenoid concentrations were 29, 38 and 11% higher, respectively. Neither the glutamate dehydrogenase (GDH, EC 1.4.1.2) activity nor the concentrations of free NH4+ and glucose in the needles were affected by exposure to NH3. After NH3 fumigation at 240 mu g m(-3), the starch concentration decreased by 39% relative to the FA. The results indicate that the metabolism of Scots pine acclimates to concentrations of NH3 which an 3 to 10 times higher than the average concentration in areas with intensive stock farming. The possible mechanisms underlying acclimation to NH3 are discussed.