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+4oC. We simultaneously measured light-saturated CO2 assimilation (Asat) and chlorophyll fluorescence emission.
Results/Conclusions Interspecific differences in intrinsic growth rate did not appreciably affect responses to pCO2. Asat 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.