Responses of gas exchange and the antioxidant system of soybean cultivars to ozone and/or sulfur dioxide

Soybean cultivars (Glycine max (L.) Merr.), "Dare", "Williams" and "Essex", with differential sensitivity to ozone (O₃) based on visible injury were exposed once to 0.20 µl⁻¹ O₃ and/or 0.70 µl 1⁻¹ sulfur dioxide (SO₂) for 4 hr. The cv Dare was considered sensitive, cv Williams intermediate and cv Essex tolerant to O₃. Cultivars exposed to filtered air served as controls. Gas exchange measurements were conducted and antioxidant metabolites (reduced and oxidized glutathione: GSH, GSSG; reduced and oxidized ascorbate: AA, dHAA) concentrations and enzymes (glutathione reductase: GRase; ascorbate peroxidase: APase; superoxide dismutase: SOD) activities were analyzed.

Gas exchange rates of all cultivars were significantly inhibited by pollutants exposure. The cv Essex maintained significantly higher net photosynthesis (Pn) at the end of 03 exposure, during the SO₂ exposure and in the first 2 hr of O₃/SO₂ fumigation. During O₃/SO₂ exposure, the estimated pollutant fluxes were 50% and integrated doses were 25-30% of O₃ or SO₂ when fumigated singly. During O₃ fumigation, cv Dare exhibited a higher O₃ flux and integrated O₃ dose. However, cv Essex showed a higher O₃/SO₂ peak flux in O₃/SO₂ fumigation. The cv Williams showed the lowest pollutant flux in all treatments.

Ozone fumigation imposed a substantial, but statistically insignificant, effect on some antioxidant components. Relative to the controls, O₃ exposure resulted in increases of GRase and SOD activity in cv Dare. In cv Williams, decreases of GSSG and SOD total activity and increase of dHAA were observed. Decreases of AA and SOD activity and increases of GSSG, dHAA and APase activity in cv Essex were found. The SO₂ exposure resulted in increases of glutathione, particularly GSSG, in all cultivars. The cv Dare responded with increases in AA, APase specific activity and SOD activity. In cv Williams, AA and SOD specific: activity increased and APase activity decreased. Decline of SOD activity in cv Essex was found after SO₂ exposure. The O₃/SO₂ fumigation resulted in increases of glutathione, particularly GSSG, and GRase activity, in all cultivars. Declines of dHAA and SOD activity in cv Williams were found. The cv Essex responded with a decline of AA and increases of dHAA and specific activity of APase and SOD.

Ozone and SO₂, singly or in combination, inhibited gas exchange rates in all cultivars, however, cv Essex was the least affected. Stomatal conductance was inhibited greater by O₃ than by SO₂ fumigation. Conversely, Pn was suppressed more by SO₂ than by O₃. The O₃/SO₂ fumigation, however, suppressed Pn and Cs substantially and to a greater extent than individual pollutants. Under the pollutant dose and fumigation profile used in these studies, no consistent responses of different antioxidant components to O₃ and/or SO₂ correlated with differential sensitivity of these soybean cultivars as determined from foliar symptomology.