Growth and photosynthetic responses to ozone of Siebold's beech seedlings grown under elevated CO and soil nitrogen supply.

Ozone (O) is a phytotoxic air pollutant, the adverse effects of which on growth and photosynthesis are modified by other environmental factors. In this study, we examined the combined effects of O, elevated CO, and soil nitrogen supply on Siebold's beech seedlings. Seedlings were grown under combinations of two levels of O (low and two times ambient O concentration), two levels of CO (ambient and 700 ppm), and three levels of soil nitrogen supply (0, 50, and 100 kg N ha year) during two growing seasons (2019 and 2020), with leaf photosynthetic traits being determined during the second season. We found that elevated CO ameliorated O-induced reductions in photosynthetic activity, whereas the negative effects of O on photosynthetic traits were enhanced by soil nitrogen supply. We observed three-factor interactions in photosynthetic traits, with the ameliorative effects of elevated CO on O-induced reductions in the maximum rate of carboxylation being more pronounced under high than under low soil nitrogen conditions in July. In contrast, elevated CO-induced amelioration of the effects of O on stomatal function-related traits was more pronounced under low soil nitrogen conditions. Although we observed several two- or three-factor interactions of gas and soil treatments with respect to leaf photosynthetic traits, the shoot to root dry mass (S/R) ratio was the only parameter for which a significant interaction was detected among seedling growth parameters. O caused a significant increase in S/R under ambient CO conditions, whereas no similar effects were observed under elevated CO conditions. Collectively, our findings reveal the complex interactive effects of elevated CO and soil nitrogen supply on the detrimental effects of O on leaf photosynthetic traits, and highlight the importance of taking into consideration differences between the responses of CO uptake and growth to these three environmental factors.