EFFECTS OF LOW CONCENTRATIONS OF SULFUR-DIOXIDE ON NET PHOTOSYNTHESIS AND DARK RESPIRATION OF VICIA-FABA

Rates of net photosynthesis, PN, and dark respiration of Vicia faba plants were measured in the laboratory in clean air and in air containing up to 175 parts 10-9 (500 μg m-3) SO2. At all SO2 concentrations exceeding 35 parts 10-9, PN was inhibited compared with clean air. At light saturation, the magnitude of inhibition depended on SO2 concentration but at low irradiances the inhibition was independent of concentration. Dark respiration rates increased substantially, independent of concentration. When exposures continued for up to 3 days, PN returned to clean air values about 1 h after fumigation ceased: dark respiration recovered after one photoperiod. There were no visible injuries.Reviewing possible mechanisms responsible for the inhibition of PN, it is suggested that SO2 competes with CO2 for binding sites in RuBP carboxylase. Analysis of resistance analogues demonstrates that SO2 altered both stomatal and internal (residual) resistances.A model of crop photosynthesis shows the implications of the observed responses for the growth of field crops in which plants are assumed to respond like laboratory plants. Photosynthesis of the crop would be less sensitive than that of individual plants to SO2 concentration. Daily dry matter accumulation of hypothetical 'polluted crops' would be substantially less than clean air values but would vary relatively little with SO2 concentration. It is concluded that physiological bases exist to account for observed reductions in growth of plants at very low SO2 concentrations, and that thresholds for plant responses to SO2 require reassessment. © 1979 Oxford University Press.