Effects of ozone on leaf nitrogen assimilation and nitrogen utilization in photosynthetic apparatus of Fagus crenata seedlings grown under different atmospheric CO2 and soil nitrogen conditions

Abstract

Ozone (O₃) impairs photosynthesis and growth of plants, depending on atmospheric CO₂ and soil N levels. Previously, we found that CO₂ exposure mitigated O₃-induced reduction in maximum carboxylation rate (V_cmax) of Fagus crenata seedlings in September. This effect was more pronounced with soil N supply in July. To better understand these physiological mechanisms, we investigated leaf N assimilation and its utilization in the photosynthetic apparatus of F. crenata seedlings. Seedlings were grown with two O₃ levels (low and twice ambient), two CO₂ levels (ambient and 700 µmol mol⁻¹), and three soil N levels (0, 50, and 100 kg N ha⁻¹ year⁻¹). CO₂ exposure mitigated the negative effects of O₃ on ribulose bisphosphate carboxylase/oxygenase (Rubisco) concentration rather than on the ratio of V_cmax to Rubisco concentration in September. Furthermore, such mitigative effects tended to be more pronounced with soil N supply in July, although a three-factor interaction among O₃, CO₂, and soil N was not statistically significant. Additionally, in July and September, the degrees of O₃-induced changes in Rubisco concentration under each CO₂ and N treatment composition were similar to those of the ratio of Rubisco concentration to total soluble protein (TSP) concentration and leaf N allocation to Rubisco, in contrast to the TSP concentration and N assimilation process. Therefore, changes in the N utilization characteristics as proteins involved in the photosynthetic apparatus and the Rubisco quantity in response to O₃, CO₂, and soil N could cause the interactive effect on the V_cmax of F. crenata seedlings.