Growth under elevated CO2 concentration affects the temperature response of photosynthetic rate

In this study, we tested the hypothesis that an elevated CO 2 concentration leads to a shift of the temperature optimum of the photosynthetic rate in trees. Since the elevated CO 2 treatment usually leads to a significant decrease in stomatal conductance and consequently to decrease in transpiration followed by an increase in leaf temperature, we hypothesized that elevated CO 2 results in the acclimation of photosynthetic apparatus to the higher temperature. We studied the two most common species in the Czech Republic – European beech ( Fagus sylvatica L.) and Norway spruce ( Picea abies (L.) Karsten). The eight-year-old trees were grown in glass domes at the experimental research site Bílý Kříž in the Beskydy Mts. and they were exposed to ambient (AC; 385 µmol CO 2 mol -1 ) and elevated CO 2 concentrations (EC; 700 µmol CO 2 mol -1 ). Based on the gas-exchange and fluorescence techniques we determined the basic photosynthetic characteristics in the range of leaf temperatures from 10 to 45 °C. The changes in temperature acclimation of tree species studied were evaluated on the basis of the shift of temperature optima. EC treatment led indeed to an increase of leaf temperature up to 3.5 °C in both species studied. The temperature optimum of the light-saturated rate of CO 2 assimilation, measured at the growth CO 2 concentration (i.e. 385 for AC and 700 µmol CO 2 mol -1 for EC trees), was higher by 3 °C in both tree species of the EC variant. Although the shift in photosynthetic temperature optimum disappeared when the AC plants were exposed to an elevated CO 2 concentration, an acclimation of Rubisco carboxylation and electron transport to higher temperature was observed.