Effects of Ozone on Growth and Photosynthesis of Castanopsis sieboldii Seedlings Grown under Different Nitrogen Loads

Abstract

To obtain basic data for evaluating the critical levels of ozone (O3) for protecting Japanese evergreen broad-leaved tree species, we investigated the effects of O3 on the growth and net photosynthesis of Castanopsis sieboldii seedlings grown under different nitrogen (N) loads. Two-year-old seedlings were grown in 12 experimental treatments, which were comprised of four gas treatments (charcoal-filtered air and three levels of O3 at 1.0, 1.5 and 2.0 times the ambient concentration) in combination with three N treatments (0, 20 and 50 kg ha-1 year-1) in open-top chambers during the two growing seasons. The net photosynthetic rate and the whole-plant dry mass of the seedlings were reduced by exposure to O3, while were increased by N load. No significant interaction between O3 and the N load for the whole-plant dry mass of the seedlings was observed. The N load increased the sensitivity of net photosynthetic rate to O3 during the first and second growing seasons. However, the increase in the area of second-flush leaves during the second growing season compensated for the O3-induced reduction in the net photosynthetic rate of the seedlings grown under relatively high N load. We concluded that the N load to the soil at ≤50 kg ha-1 year-1 has no influence on the sensitivity of the whole-plant dry mass of C. sieboldii seedlings to O3. Based on the relationship between AOT40 (accumulated exposure over a threshold of 40 nmol mol-1) of O3 during the daylight hours from April to September and the whole-plant dry mass increment per one growing season, the critical level of O3 for C. sieboldii, which was expressed as AOT40 for one growing season and corresponding to a 5% reduction in the whole-plant dry mass increment, was evaluated as 22 µmol mol-1 h.