The effect of SO 2 on photosynthesis and leaf resistance at varying concentrations of CO 2

Abstract

Photosynthesis, transpiration and stomatal resistance were measured for soybean Glycine max L. before, during and after 2 h fumigations with SO 2 at different levels of CO 2. Sulphur dioxide uptake by leaves was also measured during fumigation. Photosynthesis decreased with increasing SO 2 concentration at 300, 450 and 600 ppm CO 2, with larger reductions occurring at low CO 2. At 1200 ppm CO 2 photosynthesis was not affected by up to 1·1 ppm SO 2. Stomatal conductance to CO 2 declined with increasing SO 2 concentration at 300, 450 and 600 ppm CO 2 but not at 1200 ppm. Residual (mesophyll) conductance to CO 2 declined with increasing SO 2 at low concentrations, did not begin to decline at 600 ppm until the SO 2 concentration exceeded 0·8 ppm, and was not affected at 1200 ppm CO 2 by up to 1·1 ppm SO 2. Total SO 2 uptake for the 2 h period of fumigation increased with increasing SO 2 concentration and decreased with increasing CO 2 concentration. Reduction in photosynthesis was highly correlated with the total amount of SO 2 absorned by the leaves and showed similar trends at 300 and 450 ppm CO 2. At higher concentrations of CO 2 the reduction in photosynthesis for plants fumigated at high SO 2 was less than that observed for plants that obserbed similar amounts of SO 2 but were fumigated at lower concentrations of CO 2. This suggests that elevated CO 2 not only helped in protecting plants from injury by SO 2 via inducing stomatal closure, but also caused a reduction in SO 2 injury when the rate of SO 2 uptake was high.