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Growth at elevated CO2 leads to down-regulation of photosynthesis and altered response to high temperature in Quercus suber L. seedlings

The effects of growth at elevated CO 2 on the response to high temperatures in terms of carbon assimilation (net photosynthesis, stomatal conductance, amount and activity of Rubisco, and concentrations of total soluble sugars and starch) and of photochemistry (for example, the efficiency of excitation energy captured by open photosystem II reaction centres) were studied in cork oak (Quercus suber L.). Plants grown in elevated CO 2 (700 ppm) showed a down-regulation of photosynthesis and had lower amounts and activity of Rubisco than plants grown at ambient CO 2 (350 ppm), after 14 months in the greenhouse. At that time plants were subjected to a heat-shock treatment (4 h at 45°C in a chamber with 80% relative humidity and 800-1000 μmol m -2 s -1 photon flux density). Growth in a CO 2 -enriched atmosphere seems to protect cork oak leaves from the short-term effects of high temperature. Elevated CO 2 plants had positive net carbon uptake rates during the heat shock treatment whereas plants grown at ambient CO 2 showed negative rates. Moreover, recovery was faster in high CO 2 -grown plants which, after 30 min at 25°C, exhibited higher net carbon uptake rates and lower decreases in photosynthetic capacity (A max ) as well as in the efficiency of excitation energy captured by open photosystem II reaction centres (F v /F m ) than plants grown at ambient CO 2 . The stomata of elevated CO 2 plants were also less responsive when exposed to high temperature.