Photosynthetic activity in relation to a gradient of leaf nitrogen content within a canopy of Siebold's beech and Japanese oak saplings under elevated ozone

Abstract

The primary objective of this study was to describe parameters related to the leaf biochemical assimilation capacity of photosynthesis, such as the maximum rates of carboxylation (V_cmax) and electron transport (J_max), as a function of the leaf nitrogen content throughout a canopy of Siebold's beech and Japanese oak grown under elevated ozone (O₃) conditions during a growing season. To this end, we investigated the relationship between photosynthetic traits and leaf nitrogen content in various canopy positions of two tree species under free-air O₃ exposure (60 nmol mol^-1, during daylight hours) in June, August, and October 2012. We observed O₃-induced reduction in V_cmax and J_max without reduction of leaf nitrogen content in both tree species. In Siebold's beech, V_cmax and J_max in leaves with higher N_area were largely decreased by O₃ from August, while little effect of O₃ was observed in leaves with lower N_area. On the other hand, there was no difference in the extent of O₃-induced reduction in V_cmax and J_max across the range of N_area in leaves of Japanese oak. Reduction of leaf nitrogen content under elevated O₃ conditions was observed only in Siebold's beech in October. These results indicated that the decrease in the efficiency of photosynthetic nitrogen use is in an earlier step in O₃-induced decline of photosynthesis in Siebold's beech and Japanese oak. Based on these results, we emphasize the importance of integration of O₃ effects into the conventional estimation of V_cmax and J_max from leaf nitrogen content for evaluating canopy photosynthesis under current and future elevated O₃ conditions.

Keywords: Canopy nitrogen gradient; Fagus crenata; Free air ozone exposure; Nitrogen use to photosynthesis; Quercus mongolica var. crispula.