Gas exchange, antioxidants and foliar injuries in saplings of a tropical woody species exposed to ozone

Ozone (O 3) reaches phytotoxical concentrations in the tropics, but the sensitivity of tropical plant species to O 3 remains unknown. Visible foliar injuries, carbon assimilation ( A sat), stomatal conductance, superoxide dismutase enzyme (SOD) activity and ascorbic acid concentration (AA) were evaluated in different-aged leaves of Psidium guajava ‘Paluma’ saplings. We hypothesized that the old leaves are less capable of combating the stress induced by O 3 and hence exhibit more severe leaf injuries. Three O 3 exposure experiments were performed with ‘Paluma’ saplings in sites with high O 3 concentration and also under filtered air conditions. The exposure experiments corresponding to the seasons spring/2006, summer and autumn/2007. The decrease of A sat was greater in old leaves of saplings exposed to O 3, except in the second experiment, when the AA concentrations were more pronounced than in the other experiments. In second experiment, O 3 uptake was similar to that of the first experiment, but the injuries were less severe, probably due to the high AA concentrations. It was not possible to identify a pattern of superoxide dismutase enzyme (SOD) activity due to the high variability in the results from O 3 exposed and reference saplings. O 3 uptake/ A sat was higher in leaves exhibiting greater injury, suggesting that decrease in A sat may have been the main feature associated with the visible foliar symptons.