To obtain the basic data for evaluating the critical level of ozone (O 3 ) to protect Japanese deciduous broad-leaved forest tree species, the growth and photosynthetic responses of Fagus crenata seedlings to O 3 under different nitrogen (N) loads were investigated. The seedlings were grown in potted andisol supplied with N as NH 4 NO 3 solution at 0, 20 or 50 kg ha -1 year -1 and were exposed to charcoal-filtered air or O 3 at 1.0, 1.5 and 2.0 times the ambient concentration for two growing seasons. The interactive effect of O 3 and N load on the whole-plant dry mass of the seedlings at the end of the second growing season was significant. The O 3 -induced reduction in the whole-plant dry mass of the seedlings was greater in the relatively high N treatment than that in the low N treatment. This interactive effect was mainly due to the difference in the degree of O 3 -induced reduction in net photosynthesis among the N treatments. The degree of O 3 -induced reduction in N availability to photosynthesis was greater in the relatively high N treatment than that in the low N treatment. In conclusion, the sensitivity of growth and photosynthetic parameters of F. crenata seedlings to O 3 become high with increasing amounts of N added to the soil. Therefore, N deposition from the atmosphere should be taken into account to evaluate the critical level of O 3 to protect Japanese deciduous broad-leaved forest tree species.

The objectives of this study were to clarify the influences of nitrogen (N) load on the growth and photosynthetic responses of Quercus serrata seedlings to O 3 and to obtain basic data for evaluating the critical levels of O 3 for protecting Q. serrata forests in Japan. The effects of O 3 and/or N load on growth and photosynthetic activity of Q. serrata seedlings were investigated during the two growing seasons. Two-year-old seedlings were assigned to 12 experimental treatments, which were comprised of the combination of four gas treatments (charcoal-filtered air and three levels of O 3 at 1.0, 1.5 and 2.0 times ambient concentration) and three N treatments (0, 20 and 50 kg ha -1 year -1 ). During the second growing season, no significant interactive effects of O 3 and N load on the growth and net photosynthetic rate of the seedlings were detected. Threrfore, we concluded that N supply to the soil at £50 kg ha -1 year -1 does not significantly influence the growth and photosynthetic responses of Q. serrata seedlings to O 3 . Based on the O 3 exposure-response relationships for the whole-plant growth of the seedlings, the critical level of O 3 for Q. serrata was estimated to be approximately 36 nmol mol -1 as the average 15-h O 3 concentration during the one growing season.

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