Non-Stomatal Limitation to Photosynthesis in Cinnamomum camphora Seedings Exposed to Elevated O(3)

Ozone (O(3)) is the most phytotoxic air pollutant for global forests, with decreased photosynthesis widely regarded as one of its most common effects. However, controversy exists concerning the mechanism that underlies the depressing effects of O(3) on CO(2) assimilation. In the present study, seedlings of Cinnamomum camphora, a subtropical evergreen tree species that has rarely been studied, were exposed to ambient air (AA), ambient air plus 60 [ppb] O(3) (AA+60), or ambient air plus 120 [ppb] O(3) (AA+120) in open-top chambers (OTCs) for 2 years. Photosynthetic CO(2) exchange and chlorophyll a fluorescence were investigated in the second growing season (2010). We aim to determine whether stomatal or non-stomatal limitation is responsible for the photosynthesis reduction and to explore the potential implications for forest ecosystem functions. Results indicate that elevated O(3) (E-O(3)) reduced the net photosynthetic rates (P (N)) by 6.0-32.2%, with significant differences between AA+60 and AA+120 and across the four measurement campaigns (MCs). The actual photochemical efficiency of photosystem II (PSII) in saturated light (F(v) (′)/F(m) (′)) was also significantly decreased by E-O(3), as was the effective quantum yield of PSII photochemistry (Φ(PSII)). Moreover, E-O(3) significantly and negatively impacted the maximum rates of carboxylation (V (cmax)) and electron transport (J (max)). Although neither the stomatal conductance (g (s)) nor the intercellular CO(2) concentration (C (i)) was decreased by E-O(3), P (N)/g (s) was significantly reduced. Therefore, the observed reduction in P (N) in the present study should not be attributed to the unavailability of CO(2) due to stomatal limitation, but rather to the O(3)-induced damage to Ribulose-1,5-bisphosphate carboxylase/oxygenase and the photochemical apparatus. This suggests that the down-regulation of stomatal conductance could fail to occur, and the biochemical processes in protoplasts would become more susceptible to injuries under long-term O(3) exposure, which may have important consequences for forest carbon and water budget.