Ozone exposure affects photosynthesis of pumpkin (Cucurbita pepo) plants

. Physiological and biochemical responses of the ozone (O-3)-sensitive pumpkin cultivar (Cucurbita pepo 'Ambassador') to O-3 exposure are reported here to determine critical O-3 concentrations related to duration of pollutant exposure. . Pumpkin plants were exposed to 150 nl l(-1) of O-3 for 5 h d(-1) for 5 d. Gas-exchange, chlorophyll a fluorescence and pigment-content measurements were determined on days 1, 3, 5 for young leaves and days 1, 3, 4 for the mature leaves of control and treated plants. . In both young and mature leaves, O-3 induced a significant decrease in photosynthetic rate through impacts on stomata and photochemistry. A reduction in PSII photochemistry, photochemical quenching and quantum yield, accompanied by a parallel increase in non photochemical quenching was detected. Following O-3 treatment the pool of xanthophyll cycle pigments and the de-epoxidation index underwent a significant increase, whereas chlorophylls and beta -carotene were less affected. . The results showed that most of the parameters were affected by O-3 after 1 d of exposure and indicated an overall effect of O-3 and days of treatment, irrespective of leaf age.