RELATIONSHIP BETWEEN CO2-DEPENDENT O-2 EVOLUTION AND PHOTOSYSTEM-II ACTIVITY IN OAK (QUERCUS-PETRAEA) TREES GROWN IN THE FIELD AND IN SEEDLINGS GROWN IN AMBIENT OR ELEVATED CO2

The light-response of the apparent quantum yield of photosynthetic O2 evolution (PHI(O2)) under non-photorespiratory conditions was measured together with the photochemical efficiency Of PS II (DELTAF/F(m)'), the photochemical efficiency of open PS II reaction centers (F(v)'/F(m)') and the photochemical fluorescence quenching (q(p)) of leaf disks punched from oak leaves of seedlings grown in ambient (350 mumol mol-1) or elevated (700 mumol mol-1) CO2 in a greenhouse, and from sunlit leaves of mature oak trees (Quercus petraea (Matt.) Liebl.). There were marked differences between seedlings and trees. In seedlings, CO2 concentration during growth did not modify the light response of photosynthesis or PS II activity. There was a single linear relationship between PHI(O2) and DELTAF/F(m)' in seedling leaves that was independent of the CO2 concentration imposed during growth. In contrast, this relationship was curvilinear in sunlit leaves of adult trees. In seedling leaves, the decrease in q(p) (i.e., the proportion of open PS II reaction centers) largely accounted for the decrease in DELTAF/F(m)', whereas the decrease in DELTAF/F(m)' in sunlit leaves of mature oak trees was dependent on both q(p) and F(v)'/F(m)'.