THE ANTIOXIDANT STATUS OF SOYBEAN (GLYCINE-MAX) LEAVES GROWN UNDER NATURAL CO2 ENRICHMENT IN THE FIELD

The effects of progressively higher CO2 levels on the foliar antioxidant status were studied by growing soybean (Glycine max Merrill cv. Cresir) plants at decreasing distances from natural CO2 sources of geothermal origin in central Italy. When compared with neighbouring controls grown under normal CO2 concentration (C), soybean leaves grown at 2 x C, 7 x C and more than 20 x C showed a substantial reduction in the size of ascorbate pool and in the activity of Cu,Zn-superoxide dismutase; both the content of ascorbic acid and the activity of ascorbate peroxidase declined at 2 x C and 7 x C and recovered to the control values at 20 x C. The foliar titre of glutathione disulfide and the activities of glutathione disulfide reductase and Mn-superoxide dismutase progressively increased as CO2 concentration increased in ambient air. The results obtained suggest that the immanent risk of dioxygen toxicity associated with photosynthetic electron flow could be reduced in the presence of high CO2 levels. On the other hand, depending on both the CO2 exposure regimes and the cell compartment considered, high CO2 could promote oxidative processes which cause GSH oxidation and require an enhanced cellular ability to scavenge superoxide anion and hydrogen peroxide.