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

Ozone (O3) reaches phytotoxical concentrations in the tropics, but the sensitivity of tropical plant species to O3 remains unknown. Visible foliar injuries, carbon assimilation (Asat), 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 O3 and hence exhibit more severe leaf injuries. Three O3 exposure experiments were performed with ‘Paluma’ saplings in sites with high O3 concentration and also under filtered air conditions. The exposure experiments corresponding to the seasons spring/2006, summer and autumn/2007. The decrease of Asat was greater in old leaves of saplings exposed to O3, except in the second experiment, when the AA concentrations were more pronounced than in the other experiments. In second experiment, O3 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 O3 exposed and reference saplings. O3 uptake/Asat was higher in leaves exhibiting greater injury, suggesting that decrease in Asat may have been the main feature associated with the visible foliar symptons.