Influences of nitrogen load on the growth and photosynthetic responses of Quercus serrata seedlings to O3

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.