Effects of ozone on nitrogen metabolism in the leaves of Fagus crenata seedlings under different soil nitrogen loads

To clarify the effects of ozone (O-3) on nitrogen (N) metabolism in the leaves of Fagus crenata seedlings under different N loads, the combined effects of O-3 and N load on N enzyme activity, amino acid profiles and soluble protein concentrations were investigated. The seedlings were grown in potted andisol supplied with N at 0 (N0), 20 (N20) and 50 kg ha(-1) year(-1) (N50) and were exposed to charcoal-filtered air or O-3 at 1.0, 1.5 and 2.0 times the ambient concentration in open-top chambers from April 2004 to October 2005. The average 24-h concentrations of O-3 during the two growing seasons were 11.8, 42.7, 63.3 and 83.7 ppb, respectively. In July 2005, exposure to O-3 did not significantly affect the concentration of total free amino acids and activities of nitrite reductase and glutamine synthetase in any N treatment. Exposure to O-3 significantly increased the relative content of acidic amino acids in all N treatments while it significantly reduced concentration of total soluble protein (TSP) and ratio of TSP concentration to leaf N concentration in the N50 treatment, but not in the N0 and N20 treatments. Based on the results obtained in the present study, we concluded that exposure to O-3 reduces the allocation of N to soluble protein in addition to O-3-induced degradation of protein in the leaves of seedlings grown under a relatively high N load, with the result that the degree of O-3-induced reduction in the soluble protein was greater under a relatively high N load than under a relatively low N load.