Responses of photosynthetic and defence systems to high temperature stressin Quercus suber L-seedlings crown under elevated CO2

Growth in elevated CO2 led to an increase in biomass production per plant a s a result of enhanced carbon uptake and lower rates of respiration, compar ed to ambient CO2-grown plants. No down-regulation of photosynthesis was fo und after six months of growth under elevated CO2. Photosynthetic rates at 15 degrees C or 35 degrees C were also higher in elevated than in ambient C O2-grown plants, when measured at their respective CO2 growth condition. St omata of elevated CO2-grown plants were less responsive to temperature as c ompared to ambient CO2 plants. The after effect of a heat-shock treatment ( 4 h at 45 degrees C in a chamber with 80% of relative humidity and 800-1000 mu mol m(-2) s(-1) photon flux density) on A(max) was less in elevated tha n in ambient CO2-grown plants. At the photochemical level, the negative eff ect of the heat-shock treatment was slightly more pronounced in ambient tha n in elevated CO2-grown plants. A greater tolerance to oxidative stress cau sed by high temperatures in elevated CO2-grown plants, in comparison to amb ient CO2 plants, is suggested by the increase in superoxide dismutase activ ity, after 1 h at 45 degrees C, as well as its relatively high activity aft er 2 and 4 h of the heat shock in the elevated CO2-grown plants in contrast with the decrease to residual levels of superoxide dismutase activity in a mbient CO2-grown plants immediately after 1 h at 45 degrees C. The observed increase in catalase after 1 h at 45 degrees C in both ambient and elevate d CO2-grown plants, can be ascribed to the higher rates of photorespiration and respiration under this high temperature.