Ozone tolerance and antioxidant enzyme activity in soybean cultivars

The current study confirmed earlier conclusions regarding differential ozone (O-3) tolerances of two soybean cultivars, Essex and Forrest, and evaluated antioxidant enzyme activities of these two varieties based on their performance under environmentally relevant, elevated O-3 conditions. The experiment was conducted in open-top chambers in the field during the 1994 and 1995 growing seasons. Exposure of plants to moderately high O-3 levels (62.9 nl l(-1) air, 2-year seasonal average) caused chlorophyll loss and increased membrane permeability when compared to control plants grown in charcoal filtered air (24.2 nl l(-1) air). The other effects of O-3 treatment were decrease in seed yield, loss of total sulfhydryl groups, reduction of soluble protein content, and increase in guaiacol peroxidase activity in leaves of both cultivars. The O-3-induced increase in guaiacol peroxidase activity was much smaller in cv. Essex leaflets. Cv. Essex had less leaf oxidative damage and smaller reduction in seed yield than cv. Forrest under elevated O-3 conditions. During ozonation, mature leaflets of the more O-3 tolerant cv. Essex had higher levels of glutathione reductase (30%), ascorbate peroxidase (13%), and superoxide dismutase (45%) activity than did mature leaflets of cv. Forrest. Cu,Zn-superoxide dismutase, which represented 95% of total superoxide dismutase activity in the two cultivars, appeared to be increased by O-3 exposure in the leaflets of O-3 tolerant cv. Essex but not in those of cv. Forrest. Cytosolic ascorbate peroxidase activity was also higher in leaflets of cv. Essex than in cv. Forrest regardless of O-3 level. Stromal ascorbate peroxidase and Mn-superoxide dismutase activity did not appear to be involved in the O-3 tolerance of the two soybean cultivars.