Effects of Low Concentration of Ozone, Singly, and in Combination with Sulphur Dioxide on Net Photosynthesis Rates of Vicia faba L.

Net photosynthesis rates (P_N) of Viciafaba plants were measuredin chambers containing either clean air or air containing between50 and 300 parts 10^–9 ozone (O₃) with or without 40 parts10^{– 9} SO₂. Exposure to O₃ concentrations greater than 50 parts 10^–9 for periods of 4 h resulted in reductions in PN with greaterreductions at higher concentrations. After exposure ceased,recovery of pre-exposure PN depended on O₃ concentration. Exposureto less than 90. 100 parts 10^{– 9} was followed by completerecovery after 20 h whereas 200-300 parts 10^{– 9} resultedin visible injury and irreversible depression of PN. The additionof 40 parts 10^{– 9} SO₂ to O₃ significantly decreased Psbut the effect of SO₂ declined with increasing O₃. The additionof SO₂ had no significant affect on recovery patterns. It is postulated that Vicia faba plants are able to toleratethe absorption of O, up to a threshold above which progressivedecreases in PN occur due to effects on photosynthetic processes.The slow and sometimes incompletely reversible effects of O₃on PN indicate cellular differences in the injury mechanismand repair capacity compared with the action of SO₂ alone. Lowconcentrations of O₃ with SO2 result in additive injury to PNsuggesting independent action of the pollutants, but the reductionin SO₂ action with increasing O₃ indicates a limit for potentialphysiological injury More the onset of visible injury.