Phytotoxicity of Air Pollutants: Evidence for the Photodetoxification of SO(2) but Not O(3).

Pisum sativum L. cv Alsweet (garden pea) and Lycopersicon esculentum flacca Mill. (tomato) were used to evaluate the phytotoxicity of SO(2) and O(3) in the light and dark. Plants were grown in controlled environment chambers and exposed to SO(2) or O(3) in the light or dark at the same environmental conditions at which they were grown. The pea plants were treated with fusicoccin to ensure open stomata in the dark; the stomata of the tomato mutant remained open in the dark. Both species exhibited 64% to 80% less foliar necrosis following exposure to SO(2) (0.5 to 1.0 microliter per liter for 2 hours) in the light than in the dark. The decrease in SO(2) injury for light versus dark exposed plants was greater in fully expanded than expanding leaves. Both species exhibited 30% greater foliar necrosis following exposure to O(3) (0.2 microliter per liter for 2 hours) in the light than dark. The increase in O(3) injury in the light versus dark was similar for leaves at all stages of expansion. Leaf conductance to water vapor was 7% to 11% and 23% higher in the light than dark for fusicoccin-treated peas and tomato plants, respectively, indicating greater foliar uptake of both pollutants in the light than dark. Thus, the decreased SO(2) toxicity in the light was not associated with pollutant uptake, but rather the metabolism of SO(2). In contrast, the increased toxicity of O(3) in the light was at least in part associated with increased uptake or could not be separated from it.