Photosynthetic responses of two eucalypts to industrial-age changes in atmospheric [CO2] and temperature

The unabated rise in atmospheric [CO2] is associated with increased air temperature. Yet, few CO2-enrichment studies have considered pre-industrial [CO2] or warming. Consequently, we quantified the interactive effects of growth [CO2] and temperature on photosynthesis of faster-growing Eucalyptus saligna and slower-growing E. sideroxylon. Well-watered and -fertilized tree seedlings were grown in a glasshouse at three atmospheric [CO2] (290, 400, and 650 mu L L-1), and ambient (26/18 degrees C, day/night) and high (ambient + 4 degrees C) air temperature. Despite differences in growth rate, both eucalypts responded similarly to [CO2] and temperature treatments with few interactive effects. Light-saturated photosynthesis (A(sat)) and light- and [CO2]-saturated photosynthesis (A(max)) increased by similar to 50% and similar to 10%, respectively, with each step-increase in growth [CO2], underpinned by a corresponding 6-11% up-regulation of maximal electron transport rate (J(max)). Maximal carboxylation rate (V-cmax) was not affected by growth [CO2]. Thermal photosynthetic acclimation occurred such that A(sat) and A(max) were similar in ambient- and high-temperature-grown plants. At high temperature, the thermal optimum of A(sat) increased by 2-7 degrees C across [CO2] treatments. These results are the first to suggest that photosynthesis of well-watered and -fertilized eucalypt seedlings will remain strongly responsive to increasing atmospheric [CO2] in a future, warmer climate.