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

摘要

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.