Reduced photosynthetic thermal acclimation capacity under elevated ozone in poplar (Populus tremula) saplings

Abstract The sensitivity of photosynthesis to temperature has been identified as a key uncertainty for projecting the magnitude of the terrestrial carbon cycle response to future climate change. Although thermal acclimation of photosynthesis under rising temperature has been reported in many tree species, whether tropospheric ozone (O 3 ) affects the acclimation capacity remains unknown. In this study, temperature responses of photosynthesis (light‐saturated rate of photosynthesis ( A sat ), maximum rates of RuBP carboxylation ( V cmax ), and electron transport ( J max ) and dark respiration ( R dark ) of Populus tremula exposed to ambient O 3 (AO 3 , maximum of 30 ppb) or elevated O 3 (EO 3 , maximum of 110 ppb) and ambient or elevated temperature (ambient +5°C) were investigated in solardomes. We found that the optimum temperature of A sat ( T optA ) significantly increased in response to warming. However, the thermal acclimation capacity was reduced by O 3 exposure, as indicated by decreased T optA , and temperature optima of V cmax ( T optV ) and J max ( T optJ ) under EO 3 . Changes in both stomatal conductance ( g s ) and photosynthetic capacity ( V cmax and J max ) contributed to the shift of T optA by warming and EO 3 . Neither R dark measured at 25°C ( ) nor the temperature response of R dark was affected by warming, EO 3 , or their combination. The responses of A sat , V cmax , and J max to warming and EO 3 were closely correlated with changes in leaf nitrogen (N) content and N use efficiency. Overall, warming stimulated growth (leaf biomass and tree height), whereas EO 3 reduced growth (leaf and woody biomass). The findings indicate that thermal acclimation of A sat may be overestimated if the impact of O 3 pollution is not taken into account.