Examination of pre-industrial and future [CO₂] reveals the temperature-dependent CO₂ sensitivity of light energy partitioning at PSII in eucalypts

We grew faster-growing Eucalyptus saligna Sm. and slower-growing Eucalyptus sideroxylon A. Cunn ex Woolls tree seedlings in sunlit glasshouses at all combinations of 290µLL⁻¹ (pre-industrial), 400µLL⁻¹ (modern) or 650µLL⁻¹ (future) global atmospheric CO₂ ([CO₂]), and ambient or ambient+4°C temperature. To assess photosynthetic performance, we simultaneously measured light-saturated CO₂ assimilation (Asat) and chlorophyll fluorescence emission along with the capacity for photosynthetic O₂ evolution and leaf pigment composition. Photosynthetic response to [CO₂] was similar between species. Increasing [CO₂] but not temperature increased Asat. The response of photosynthetic electron transport to [CO₂] was temperature-dependent and manifested through adjustments in energy partitioning at PSII. Increasing [CO₂] resulted in greater PSII operating efficiencies at the elevated temperature. We observed no associated acclimatory adjustments in the capacity for photosynthetic O₂ evolution or changes in leaf chlorophyll content. Photoprotective energy dissipation responded to increasing [CO₂] and temperature. Across species and treatments, increased energy partitioning to electron transport was always associated with decreased partitioning to energy dissipation. Our results suggest that in response to increasing [CO₂] and temperature, E. saligna and E. sideroxylon meet increased demands for the products of electron transport via adjustments in energy partitioning, not through acclimation of the capacity for photosynthetic electron transport or light absorption.