GROWTH AT ELEVATED CO2 LEADS TO DOWN-REGULATION OF PHOTOSYNTHESIS ANDALTERED RESPONSE TO HIGH-TEMPERATURE IN QUERCUS-SUBER L SEEDLINGS

The effects of growth at elevated CO2 on the response to high temperat ures in terms of carbon assimilation (net photosynthesis, stomatal con ductance, amount and activity of Rubisco, and concentrations of total soluble sugars and starch) and of photochemistry (for example, the eff iciency of excitation energy captured by open photosystem II reaction centres) were studied in cork oak (Quercus suber L.). Plants grown in elevated CO2 (700 ppm) showed a down-regulation of photosynthesis and had lower amounts and activity of Rubisco than plants grown at ambient CO2 (350 ppm), after 14 months in the greenhouse. At that time plants were subjected to a heat-shock treatment (4 h at 45 degrees C in a ch amber with 80% relative humidity and 800-1000 mu mol m(-2) s(-1) photo n flux density). Growth in a CO2-enriched atmosphere seems to protect cork oak leaves from the short-term effects of high temperature. Eleva ted CO2 plants had positive net carbon uptake rates during the heat sh ock treatment whereas plants grown at ambient CO2 showed negative rate s. Moreover, recovery was faster in high CO2-grown plants which, after 30 min at 25 degrees C, exhibited higher net carbon uptake rates and lower decreases in photosynthetic capacity (A(max) as well as in the e fficiency of excitation energy captured by open photosystem II reactio n centres (F-v/F-m) than plants grown at ambient CO2. The stomata of e levated CO2 plants were also less responsive when exposed to high temp erature.