Photosynthetic response of field-grown soybeans to fumigations with sulphur dioxide.
Field-grown soyabeans were exposed to 3 levels of SO2 to determine the effects of the exposure upon photosynthesis and stomatal resistance. In plots where the mean SO2 conc. averaged c. 790 p.p.b., photosynthesis was reduced to 37-63% of the control during fumigation. In plots where SO2 conc. averaged c. 300 p.p.b., photosynthesis was reduced to 63-83% of the control, and in plots where SO2 conc. averaged 120 p.p.b., photosynthesis was 90-137% of the control. Stomatal resistance generally increased in the treated plants; however, the response lagged considerably behind the photosynthetic response, suggesting that stomatal closure did not cause the observed reduction in photosynthesis. Visible injury of the leaf tissue was observed only in the high treatment plot. The significant depression of photosynthesis in plots receiving c. 300 p.p.b. SO2 suggests that SO2 may reduce productivity without visible symptoms of injury.