A more complete understanding of the impacts of climate change on photosynthesis will require examination of the full range of modern-day increases in atmospheric CO₂ concentration (pCO₂) and the interactive effects with increasing temperature.  Furthermore, the response of photosynthetic carbon assimilation to climate change has been well studied, but little attention has been paid to the partitioning of absorbed light to photosynthesis and photoprotection.  Here, we grew fast-growing Eucalyptus saligna and slower-growing E. sideroxylon in sun-lit glasshouses at all combinations of 29 Pa, 40 Pa, or 65 Pa pCO₂ and ambient temperature or ambient+4^oC.  We simultaneously measured light-saturated CO₂ assimilation (A_sat) and chlorophyll fluorescence emission. 

Results/Conclusions Interspecific differences in intrinsic growth rate did not appreciably affect responses to pCO₂.  A_sat increased with increasing pCO₂, but was not significantly affected by temperature.  At the elevated temperature, increasing pCO₂ resulted in greater allocation of absorbed light to electron transport.  Across species and treatments, increased light energy partitioning to electron transport was always associated with decreased partitioning to energy dissipation. Thus, photoprotective processes responded to increasing pCO₂ and temperature.  The sensitivity of photosynthetic energy partitioning to rising pCO₂ was temperature dependent and evident because we included a pre-industrial pCO₂ treatment.