Plant material: 8-year-old saplings of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karsten) were exposed for three growing seasons to ambient (AC; 385 μmol(CO 2 ) mol -1 ) and elevated CO 2 concentrations (EC; 700 μmol(CO 2 ) mol -1 ) using glass domes at the experimental research site Bílý Kříž in the Beskydy Mts. (908m a.s.l.). The climate is moderately cold and humid with an annual mean air temperature 6.8 °C, average relative humidity 83 % and average precipitation 1356 mm per year. Date of measurement: 13 – 15 July (beech), 20 – 22 July 2010 (spruce), sunny days Fig. 3: Temperature response curve of light saturated CO 2 assimilation rate (A max ) in Fagus sylvatica and Picea abies grown at ambient (AC) and elevated atmospheric CO 2 concentration (EC), temperature optimum (dashed line) and shift due to EC condition (arrow). The Weibull function (line) was fitted to the data. Light-saturated CO 2 assimilation rate (A max ) was stimulated by EC treatment in both species (Fig. 3). However, differences in A max were negligible at low sub-optimal leaf temperatures. Temperature optima of A max were found at 27±1.1 and 30 ±1.1°C in beech trees grown under AC and EC, respectively and at 21±1.9 and 27±0.5°C in spruce trees grown under AC and EC, respectively. The highest stimulation of A max by EC was observed at 30°C in beech (by 52 %) and at 27°C in spruce (by 60 %). Fig. 4: Temperature response curve of maximum rate of carboxylation of Rubisco (V Cmax ) in Fagus sylvatica and Picea abies grown at ambient (AC) and elevated atmospheric CO 2 concentration (EC), temperature optimum (dashed line) and shift due by EC condition (arrow). The Weibull function (line) was fitted to the data. Maximum rate of in vivo Rubisco carboxylation (V Cmax ) responded to temperature similarly in beech and spruce under AC (Fig. 4). EC treatment led to slight (non-significant) decrease in sensitivity of V Cmax to temperature. Under AC conditions, temperature optima of V Cmax were achieved at 36±2.0 and 32±1.6°C in beech and spruce, respectively. Under EC conditions, T opt was achieved at 35±0,6°C in spruce and it was higher by 16 % than under AC. In beech, the maximum of V Cmax was not found under EC conditions. Methods: Gas-exchange measurements: Li-6400 (Li-Cor, USA); The target leaf temperature was controlled by integrated Peltier thermoelectric coolers. a) light response curve of CO 2 assimilation rates (A/PPFD) → light-saturated CO 2 assimilation rate (A max ), dark respiration rate (R D ), light-saturated transpiration rate (E max ), water use efficiency (WUE; A max / E max ) b) CO 2 response curve of CO 2 assimilation rates (A/C i )→ maximum rate of in vivo Rubisco carboxylation (V Cmax ) c) A/PPFD and A/C i response curves were estimated at 5 different leaf temperatures (T leaf ) within the range 15–40°C → temperature response curve of assimilatory parameters d) temperature response curves were expressed by 1. Arrhenius function → coefficient Q 10 (the rate of change of a biological system as a consequence of increasing the temperature by 10 °C) 2. Weibull function → estimation of temperature optimum (T opt ) 3. Exponential function for decreasing relation → exponential parameter (a) Hypothesis: Growth of tree species under elevated CO 2 concentration leads to an acclimation of photosynthetic apparatus to higher temperature that will result in a shift of temperature optima of the carbon assimilation processes as well as in modified temperature sensitivity of these processes. Presumptions: elevated CO 2 → decrease in stomatal conductance → decrease in transpiration (decline in output of latent heat) → increase in leaf temperature Conclusions: Based on the presented results, beech seems to be more temperature- sensitive than spruce. Elevated CO 2 concentration led to the stimulation of CO 2 assimilation rate (A max ). However, this effect is negligible at low temperatures (<18°C) and increases with rising temperature, when photorespiratory processes are suppressed by EC. The temperature optima of A max were shifted by 3 and 6°C in beech and in spruce, respectively under EC as compared with AC conditions. Maximum rate of Rubisco carboxylation in vivo was slightly reduced by EC, but the growth under EC conditions allowed an increase of V Cmax to higher temperature by 3°C in spruce and more than 4°C in beech. Our results thus support the hypothesis that photosynthetic apparatus of plants grown under elevated CO 2 concentration is acclimated to higher temperature. 6°C 3°C ? Global Change Research Centre AS CR, v. v. i. Background: Global climate models predict that atmospheric CO 2 concentration may be double at the end of this century and global temperature may increase by 4.5°C. The temperature significantly affects plant productivity and species distribution. Also CO 2 concentration has direct effects on plant physiology that could alter some plant responses to temperature or contrary temperature can affect responses to CO 2. As forest ecosystems form a critical component of the global carbon budget, changes in the photosynthesis, as a result of CO 2 and temperature influence, have the potential to substantially modify in the global carbon cycle. Q 10 = 2,1±0,14 Q 10 = 2,2±0,09 Q 10 = 2,01±0,22 Q 10 = 2,06±0,33 Fig. 1: Temperature response curve of dark respiration rate (R D ) in Fagus sylvatica and Picea abies grown at ambient (AC) and elevated atmospheric CO 2 concentration (EC). The Arrhenius exponential function (line) was fitted to the data. Results: Temperature sensitivity of dark respiration rate (R D ) was slightly higher in beech than in spruce. EC treatment did not influence temperature sensitivity of R D in both tree species studied (Fig. 1). Q 10 = 2,05±0,11 Q 10 = 2,26±0,28 Q 10 = 1,84±0,17 Q 10 = 1,84±0,16 Fig. 2: Temperature response curve of light saturated rate of transpiration (E) in Fagus sylvatica and Picea abies grown at ambient (AC) and elevated atmospheric CO 2 concentration (EC). The Arrhenius exponential function (line) was fitted to the data. Light-saturated rate of transpiration (E max ) correlated positively with temperature (Fig. 2). Beech was more sensitive to temperature (higher Q 10 ) than spruce. At 40°C, E max was higher by 69 % and by 44 % in beech compare to spruce under AC and EC conditions, respectively. E max was reduced by EC treatment in beech; however, the sensitivity to temperature slightly increased under EC (non-significant). No effect of EC on E max and its temperature-sensitivity was observed in spruce. Fig. 5: Temperature response curve of water use efficiency (WUE) in Fagus sylvatica and Picea abies grown at ambient (AC) and elevated atmospheric CO 2 concentration (EC). The Exponential decay function (line) was fitted to the data. Water use efficiency (WUE) decreased with increasing temperature. Higher temperature-sensitivity of WUE was found in beech than spruce. At 20°C, WUE was higher in beech as compared with spruce by 31 % and by 34 % under AC and EC conditions respectively. However, differences disappeared above 25°C. WUE was stimulated by EC treatment in both species. The sensitivity to temperature was slightly decreased by EC in beech. Q 10 = 2,14±0,13 Q 10 = 2,11±0,23 Q 10 = 2,21±0,37 Q 10 = 1,99±0,22 Acknowledgement: This work is part of the research supported by grants IAA (GA AV), Czecos/ICOS (MSMT – LM ), the Research Intentions AV0Z (ISBE), and by project CzechGlobe – Centre for Global Climate Change Impacts Studies, Reg.No. CZ.1.05/1.1.00/ Long-term effect of elevated CO 2 concentration on temperature optimum of photosynthetic CO 2 assimilation in two tree species Petra Holišová 1, Ladislav Šigut 2, Karel Klem 1, Otmar Urban 1, Vladimír Špunda 1,2 1 Global Change Research Centre, Academy of Sciences of the Czech Republic, Bělidla 986 / 4a, CZ Brno, Czech Republic 2 Department of Physics, Faculty of Science, Ostrava University, 30. dubna 22, CZ Ostrava 1, Czech Republic a = 0,11±0,01 a = 0,09±0,01 a = 0,08±0,01 a = 0,07±0,02