Stomatal response and leaf injury of Pisum sativum L. with SO2 and O3 exposures. I. Influence of pollutant level and leaf maturity

Plants of Pisum sativum L. 'Alsweet' were grown under a controlled environment and exposed to SO2 and O3 to determine whether changes in stomatal aperture during exposure were related to subsequent leaf injury. Stomata consistently closed with injurious levels of SO2 and O3. Measurements with diffusion porometers demonstrated ≃75 and 25% lower conductance with SO2 and O3 exposures, respectively, compared to the conductance of control plants. Stomata also showed a closing response with noninjurious levels of SO2 but an opening response with noninjurious levels of O3. Stomata closed to the same degree with combinations of SO2 plus O3 as with SO2 alone. Stomata of expanding leaves closed more during pollutant exposures than stomata of expanded leaves. The abaxial and adaxial stomata both exhibited closure with SO2 and combinations of SO2 plus O3, but abaxial stomata tended to close and adaxial stomata tended to open with exposure to O3 alone. The changes in stomatal aperture were not closely correlated with the amount of leaf injury produced by different pollutant levels. Stomata closed, not only with exposure to pollutant levels that caused severe necrosis, but also with levels that caused only a trace of injury. There was no evidence of a reduced amount of closure or even stomatal opening with combinations of SO2 and O3 compared to plants exposed to the pollutants alone to explain the large amount of injury to plants exposed to pollutant combinations.