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

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 (O-3) 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 O-3 exposure (60 nmol mol(-1), during daylight hours) in June, August, and October 2012. We observed O-3-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 O-3 from August, while little effect of O-3 was observed in leaves with lower N-area. On the other hand, there was no difference in the extent of O-3-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 O-3 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 O-3-induced decline of photosynthesis in Siebold's beech and Japanese oak. Based on these results, we emphasize the importance of integration of O-3 effects into the conventional estimation of V-cmax and J(max) from leaf nitrogen content for evaluating canopy photosynthesis under current and future elevated O-3 conditions. (C) 2018 Published by Elsevier B.V.