| Summary: | The primary leaves of bush bean plants pretreated daily with non-injurious, low levels of ozone (0.02 or 0.05 ppm) pass through stages of varying susceptibility to a subsequent acute dose. Previous work showed that this variation in response to acute dose could only partly be accounted for by stomatal behaviour. Present studies indicate that the oxy-radical scavenger superoxide dismutase (SOD) appears to play no role in the phenomenon. No observed changes in SOD levels following various low ozone pretreatment regimes were related to susceptibility to acute injury in comparisons with plants maintained in filtered air as controls. The only significant change in SOD levels which appeared to be related to ozone occurred simultaneously with the appearance of visible symptoms of injury following exposure to O₃ concentrations greater than 0.1 ppm. The nature of the effect on SOD levels and the degree of visible injury was related to plant age and temperature. Younger plants or plants fumigated at a lower temperature (23°C) showed less injury after acute exposure to ozone than older plants or ones exposed at a higher temperature (33°C). SOD activity was higher than controls when a lower temperature growth regime (23°/16°C) was used, but dropped to below control levels when higher temperatures (33°/23°C) were used. The anti-ozone agent "ethylene diurea", EDU, provided protection against acute injury, but had no effect on SOD levels in either primary or trifoliate leaves prior to exposure to an acute dose. SOD levels in the first and second leaves of radish plants pretreated daily with low ozone (0.02 ppm) were not substantially different from controls before or after an acute exposure to ozone. In contrast to bean leaves, pretreatment of radish with low ozone also had no effect on the amount of injury induced by an acute dose. Low SO₂, pretreatment (0.1 ppm) of the primary leaves of bush bean or the first two leaves of radish did not affect SOD activity. Visible injury after an acute SO₂ exposure (2.0 ppm) was also not affected by subacute pretreatment in bean. However, pretreated radish leaves were predisposed to acute injury. In contrast with the effect of ozone, acute SO₂ exposure of the primary leaves of bean resulted in a decrease in SOD activity compared with plants grown in filtered air, regardless of pretreatment regime. A similar trend was observed in the first two leaves of radish only if pretreated with subacute SO₂. SOD activity in leaves pretreated with filtered air was not affected by acute SO₂ fumigation. === Land and Food Systems, Faculty of === Graduate
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