In order to access the occurrence of phytotoxic ozone effect at urban and rural sites, two-year old seedlings of ozone-sensitive (Quercus robur L.) and ozone-tolerant (Quercus rubra Michx.) tree species were observed. The seedlings were exposed to ambient air for three month period (June-August), 2008 in two monitoring sites -in the city of Sofia, beside a big road with dense traffic, and in the Plana Mountain. The relationship between ozone pollution and ozone-induced effects on leaf area index and water regime were examined. The results for leaf area parameters are too preliminary due to the short period of the examinations. Strong ozone impact was found at the urban site with higher ozone exposure, corresponding to transpiration rate. The ozone induced alterations in physiological activity of tree seedlings may serve as yearly indication for leaf injuries, which could be expected in the further studies. The obtained results are discussed in relation to AOT40 index.
Effects of ozone on the sensitive tree species Fraxinus excelsior L. exposed to ambient air were investigated. The dynamics of photosynthesis, transpiration, stomatal conductance and the activity of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT) in three-year-old ash seedlings were studied during a four-month period (June-September). Seedlings were exposed to ambient ozone in an urban (the Central City Park of Sofia - Borisova Gradina) and a mountain (Plana Mountain) area in Bulgaria. The sites were located near climate monitoring stations, providing data on ozone concentrations and meteorological parameters. Ozone exposure at the mountain site (AOT40) was more than two times higher compared to the urban site. Significantly higher values of sun radiation, transpiration, stomatal conductance and enzyme activity at the mountain site were also observed. At the urban site higher values of temperature and air humidity were registered. Effects of the measured variables on ash seedlings were complex and interdependent. No direct effect of ozone concentration in ambient air on the leaf physiology and biochemistry could be proved. However, intensified SOD and CAT activity in the presence of elevated ozone suggested antioxidant reaction in response to ozone uptake.
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