The Effect of Elevated [CO₂] on Growth and Photosynthesis of Two Eucalyptus Species Exposed to High Temperatures and Water Deficits

Two species of eucalyptus (Eucalyptus macrorhyncha and Eucalyptus rossii) were grown for 8 weeks in either ambient (350 μL L^-1) or elevated (700 μL. L^-1) CO₂ concentrations, either well watered or without water additions, and subjected to a daily, 3-h high-temperature (45°C, maximum) and high-light (1250 μmol photons m^-2 s^-1, maximum) stress period. Water-stressed seedlings of E. macrorhyncha had higher leaf water potentials when grown in elevated [CO₂]. Growth analysis indicated that increased [CO₂] may allow eucalyptus species to perform better during conditions of low soil moisture. A down-regulation of photosynthetic capacity was observed for seedlings grown in elevated [CO₂] when well watered but not when water stressed. Well-watered seedlings grown in elevated [CO₂] had lower quantum efficiencies as measured by chlorophyll fluorescence (the ratio of variable to maximal chlorophyll fluorescence [F_v/F_m]) than seedlings grown in ambient [CO₂] during the high-temperature stress period. However, no significant differences in F_v/F_m were observed between CO₂ treatments when water was withheld. The reductions in dark-adapted F_v/F_m for plants grown in elevated [CO₂] were not well correlated with increased xanthophyll cycle photoprotection. However, reductions in the F_v/F_m were correlated with increased levels of nonstructural carbohydrates. The reduction in quantum efficiencies for plants grown in elevated [CO₂] is discussed in the context of feedback inhibition of electron transport associated with starch accumulation and variation in sink strength.