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 mu L L(-1)) or elevated (700 mu L L(-1)) CO2 concentrations, either well watered or without water additions, and subjected to a daily, 3-h high-temperature (45 degrees C, maximum) and high-light (1250 mu mol 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.