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

The unabated rise in atmospheric [CO 2] is associated with increased air temperature. Yet, few CO 2-enrichment studies have considered pre-industrial [CO 2] or warming. Consequently, we quantified the interactive effects of growth [CO 2] 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 [CO 2] (290, 400, and 650 μL L -1), and ambient (26/18 °C, day/night) and high (ambient + 4 °C) air temperature. Despite differences in growth rate, both eucalypts responded similarly to [CO 2] and temperature treatments with few interactive effects. Light-saturated photosynthesis (A sat) and light- and [CO 2]-saturated photosynthesis (A max) increased by ∼50% and ∼10%, respectively, with each step-increase in growth [CO 2], 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 [CO 2]. 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 °C across [CO 2] treatments. These results are the first to suggest that photosynthesis of well-watered and -fertilized eucalypt seedlings will remain strongly responsive to increasing atmospheric [CO 2] in a future, warmer climate.