DIFFERENCES IN THE RESPONSE OF CARBON ASSIMILATION TO SUMMER STRESS (WATER DEFICITS, HIGH LIGHT AND TEMPERATURE) IN 4 MEDITERRANEAN TREE SPECIES

Daily changes in photoprotective mechanisms were studied in sun leaves of Quercus suber L., Quercus ilex L., Olea europaea L. and Eucalyptus globulus Labill. trees during the summer in Portugal. Even though sto matal closure explained most of the diurnal variation in carbon assimi lation along the summer, a decline in the photochemical yield of photo system II (F-v'/F-m') also occurred, as a result of an excess of inter cepted solar radiation when carbon assimilation is limited by stomatal closure due to high vapour pressure deficits and/or soil water defici ts. These changes were accompanied by the conversion of violaxanthin t o antheraxanthin and zeaxanthin which were correlated with thermal dis sipation of excess photon energy. In spite of a common general respons e, differences between species were observed - Olea europaea, which is a slow-growing tree, had the lowest net photosynthetic rates, the hig hest proportion of carotenoids in relation to chlorophyll and the high est rates of de-epoxidation of violaxanthin. This enabled a large ther mal dissipation of the excess intercepted radiation but led to rather small values of light utilisation for photochemistry (ca 20%). In cont rast, in E. globulus, a fast-growing tree, photosynthetic rates were t he highest, thermal dissipation of absorbed radiation the lowest and m aximal values of light utilisation for photochemistry reached ca 50%. The two Quercus species exhibited an intermediate response. A high deg ree of co-ordination is apparent between stomatal behaviour, photosynt hetic capacity and photoprotection mechanisms.