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

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 (O₃ ) 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 O₃ (AO₃ , maximum of 30 ppb) or elevated O₃ (EO₃ , 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 O₃ exposure, as indicated by decreased T_optA , and temperature optima of V_cmax (T_optV ) and J_max (T_optJ ) under EO₃ . 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 EO₃ . Neither R_dark measured at 25°C ( $$ ) nor the temperature response of R_dark was affected by warming, EO₃ , or their combination. The responses of A_sat , V_cmax , and J_max to warming and EO₃ were closely correlated with changes in leaf nitrogen (N) content and N use efficiency. Overall, warming stimulated growth (leaf biomass and tree height), whereas EO₃ reduced growth (leaf and woody biomass). The findings indicate that thermal acclimation of A_sat may be overestimated if the impact of O₃ pollution is not taken into account.