Foliar responses of five plant species to ozone and a sulphur dioxide/ozone mixture after a sulphur dioxide pre-exposure

Plants that were pretreated with 0.15 ppm SO₂ for either 2 or 3 d before an intermittent O₃ or SO₂/O₃ exposure exhibited differences in per cent visible foliar injury that often varied significantly from plants that received no SO₂ pretreatment. Differences that were observed could not be explained on the basis of leaf diffusive resistance alone. The SO₂ pretreatment for 2 d caused a decrease in visible O₃ injury in white bean, an increase in visible O₃ injury in cucumber and radish, and had no effect on soybean and tomato. The same pretreatment caused an increase in visible injury from the SO₂/O₃ mixture in white bean, a decrease in visible injury for cucumber and tomato, and had no effect on soybean and radish. However, these same trends did not occur when the SO₂ pretreatment was increased to 3 d. Pretreatment with sub-acute levels of SO₂ can significantly alter a plant's response to O₃ or a mixture of SO₂/O₃, the change in response being sensitive to the pretreatment SO₂ dosage. The nature of the visible injury symptoms from O₃ or the pollutant mixture were not altered for any of the plant species regardless of the length of time of the SO₂ pretreatment and, generally, the onset of visible injury was not greatly altered.