Effects of Low Concentrations of Sulphur Dioxide on Net Photosynthesis and Dark Respiration ofVicia faba

Abstract

Rates of net photosynthesis, P_N, 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, P_N 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, P_N 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 P_N, 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.