LATENT OXIDATIVE STRESS RESPONSES OF OZONE-FUMIGATED CUCUMBER PLANTS ARE ENHANCED BY SIMULTANEOUS COLD EXPOSURES

Cucumber plants (Cucumis sativus L.) were grown under controlled condi tions and fumigated with either O-3, diluted automobile exhaust or a c ombination of both. The ratio of variable to maximum chlorophyll fluor escence (F-v/F-m) was estimated as a measure of PSII activity. Activit ies of the enzymes catalase, glutathione reductase and guaiacol-depend ent peroxidase and contents of the antioxidants ascorbate and glutathi one were assayed as potential indicators of oxidative stress. The beha vior of catalase and of PSII are of particular diagnostic interest bec ause they require continuous repair in light. Exposures of up to 13 da ys to moderate concentrations of the pollutant gases alone did not ind uce striking changes in any of the activities that were assayed. Also when the plants were subjected to an additional stress treatment by ex posing them to 4 short cold treatments (2h each at 0-4 degrees C in li ght on days 12-15 after sowing) which induced marked declines of the F -v/F-m ratio, the chlorophyll content and the catalase activity, these cold-induced symptoms of photodamage were not significantly enhanced by the fumigation treatments. However, increases of the activities of glutathione reductase and peroxidase observed during a period of recov ery following the cold-exposures were markedly higher in O-3-fumigated plants, as compared to plants grown in filtered air or fumigated with car exhaust alone. The results emphasize that effects of moderate pol lutant exposures may be latent or delayed over long time periods and t hat defence responses can be enhanced when plants are exposed to addit ional, naturally occurring stress situations.