CO 2 elevation improves photosynthetic performance in progressive warming environment in white birch seedlings [version 1; referees: 2 approved]

White birch (Betula paperifera Mash) seedlings were exposed to progressively warming in greenhouses under ambient and elevated CO 2 concentrations for 5 months to explore boreal tree species’ potential capacity to acclimate to global climate warming and CO 2 elevation. In situ foliar gas exchange, in vivo carboxylation characteristics and chlorophyll fluorescence were measured at temperatures of 26 oC and 37 oC. Elevated CO 2 significantly increased net photosynthetic rate (Pn) at both measurement temperatures, and Pn at 37 oC was higher than that at 26 oC under elevated CO 2. Stomatal conductance (gs) was lower at 37 oC than at 26 oC, while transpiration rate (E) was higher at 37 oC than that at 26 oC. Elevated CO 2 significantly increased instantaneous water-use efficiency (WUE) at both 26 oC and 37 oC, but WUE was markedly enhanced at 37 oC under elevated CO 2. The effect of temperature on maximal carboxylation rate (Vcmax), PAR-saturated electron transport rate (Jmax) and triose phosphate utilization (TPU) varied with CO 2, and the Vcmax and Jmax were significantly higher at 37 oC than at 26 oC under elevated CO 2. However, there were no significant interactive effects of CO 2 and temperature on TPU. The actual photochemical efficiency of PSII (DF/ Fm’), total photosynthetic linear electron transport rate through PSII (JT) and the partitioning of JT to carboxylation (Jc) were higher at 37 oC than at 26 oC under elevated CO 2. Elevated CO 2 significantly suppressed the partitioning of JT to oxygenation (Jo/JT). The data suggest that the CO 2 elevation and progressive warming greatly enhanced photosynthesis in white birch seedlings in an interactive fashion.