ABSTRACT
Several physiological and biochemical mechanisms occur in plants that may protect them against the toxic effects of pollutants absorbed into the tissue. For example, onion has been shown to vary in sensitivity to O3 depending on its cultivars. Engle and Gabelman (1966) showed that, following exposure to O3, the stomata of the resistant cultivar of the onion were closed with no appreciable injury, whereas the stomata of the sensitive cultivar remained open, manifesting obvious injury.
