PHOTOSYNTHETIC RESPONSE OF FIELD-GROWN SOYBEANS TO FUMIGATIONS WITH SULFUR-DIOXIDE

Field-grown soybeans were exposed to three levels of SO2 to determine the effects of the exposure upon photosynthesis and stomatal resistance. In plots where the mean SO2 concentration averaged about 790 ppb, photosynthesis was reduced to 37-63% of the control during fumigation. In plots, where SO2 concentration averaged about 300 ppb, photosynthesis was reduced to 63-83% of the control and, in plots where SO2 concentration averaged 120 ppb, photosynthesis ranged from 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 about 300 ppb SO2, suggests that SO2 may reduce productivity without visible symptoms of injury.