Growth and photosynthesis of two eucalypt species during high temperature stress under ambient and elevated [CO₂]

Abstract

Abstract Two species of eucalypt ( Eucalyptus macrorhyncha and E. rossii ) were grown under conditions of high temperatures (45 °C, maximum) and high light (1500 μmol m −2 s −1 , maximum) at either ambient (350 μL L −1 ) or elevated (700 μL L −1 ) CO 2 concentrations for 8 weeks. The growth enhancement, in terms of total dry weight, was 41% and 103% for E. macrorhyncha and E. rossii , respectively, when grown in elevated [CO 2 ]. A reduction in specific leaf area and increased concentrations of non‐structural carbohydrates were observed for leaves grown in elevated [CO 2 ]. Plants grown in elevated [CO 2 ] had an overall increase in photosynthetic CO 2 assimilation rate of 27%; however, when measured at the same CO 2 concentration a down‐regulation of photosynthesis was evident especially for E. macrorhyncha. During the midday period when temperatures and irradiances were maximal, photosynthetic efficiency as measured by chlorophyll fluorescence ( F v / F m ) was lower in E. macrorhyncha than in E. rossii. Furthermore, F v / F m was lower in leaves of E. macrorhyncha grown under elevated than under ambient [CO 2 ]. These reductions in F v / F m were accompanied by increases in both photochemical ( q P) and nonphotochemical quenching ( q N and NPQ), and by increases in the concentrations of xanthophyll cycle pigments with an increased proportion of the total xanthophyll cycle pool comprising of antheraxanthin and zeaxanthin. Thus, increased atmospheric [CO 2 ] may enhance photoinhibition when environmental stresses such as high temperatures limit the capacity of a plant to respond with growth to elevated [CO 2 ].