Effects of treatment with low ozone concentrations on stomatal behavior, growth, and susceptibility to acute ozone injury
Creators: name: Rosen, Peter Mark
Publisher: name: University of British Columbia
Year: 2010

description: Sub-acute ozone exposures (doses below the threshold for visible injury) have been found to affect plant growth, stomatal behavior, and the extent of visible injury in response to subsequent acute exposures (doses above the threshold for visible injury). The interrelationships of these effects varied according to the magnitude and timing of the pretreatment dose and the particular species and cultivar tested. When seedlings of bush bean (Phaseolus vulgaris L. cv Pure Gold Wax) were raised from seed in filtered air and then pretreated with 0.02 parts per million (ppm) ozone for six days (six hours per day), beginning at eight days after sowing, susceptibility to acute injury (the extent of visible injury after an acute exposure) was greater than that of controls maintained throughout in filtered air. Seedlings exposed to this low ozone treatment for an additional seven days became less susceptible to acute injury than controls maintained in filtered air. Treatment differences in acute susceptibility were found to correspond to differences in stomatal behavior. At the early stage of increased susceptibility, ozone-p're-treated plants and controls maintained in filtered air had the same stomatal resistance when measured in filtered air, but ozone-pretreated stomata had a reduced tendency, to close in response to an acute ozone exposure (0.4 ppm). At the later stage of decreased acute susceptibility, ozone-pretreated stomata still had this reduced sensitivity to ozone-induced closure, but ozone pretreatment also resulted in an overall increase in stomatal resistance in filtered air, prior to acute exposures. Stomatal entry of ozone may thus provide, in part, an explanation for the differences in susceptibility resulting from these treatments in bean. Exposure of mint cuttings (Mentha arvensis L.) to 0.02 ppm ozone increased their acute susceptibility slightly. The ozone tolerance of mint was approximately six times greater than that of bean seedlings raised in filtered air (based on the dosage required for equivalent degrees of visible injury). In experiments with potted grapevines (Vitis labruscana, Bailey) in open-top field chambers exposed to ambient air (containing "naturally ocurring" ozone) or charcoal-filtered air (containing reduced ozone levels) for an entire growing season, growth was found to be greater in ambient air in two of three cultivars tested. This growth stimulation was greatest in leaves, but occurred also in trunks and roots (both dry weight and fresh weight). Within the three cultivars ('Ives', 'Concord', and 'Delaware') the growth stimulation was directly related to the sensitivity of field-grown vines to ozone injury; the most sensitive cultivar, 'Ives', showed the greatest growth increase in ambient air as compared to filtered air. Only one of the grapevine cultivars. showed any statistically significant effect of chronic exposure to ambient ozone on acute susceptibility. In that case, 'Delaware', the cultivar most resistant to ozone injury in the field, was predisposed to acute injury by chronic pretreatment with low ozone. With grapevines, there were no statistically significant effects of low ozone pretreatment on stomatal behavior, but trends in the data indicated that ozone pretreatment tended to decrease both stomatal resistance when measured in filtered air and closure in response to acute ozone exposures (0.75 ppm). In experiments with grapevines, unlike those with beans, decreased susceptibility appeared to be more closely related to an increase in overall vigor, associated with a lower stomatal resistance in filtered air, rather than to a stomatal mechanism regulating ozone entry into the leaf. Although there were differences between species in the role that stomata play in the response to pretreatment with sub-acute ozone doses, there was the suggestion of a general pattern of increased tendencies toward a protective effect of ozone pretreatment in more ozone-sensitive species (bean versus mint) or cultivars ('Ives' and 'Concord' versus 'Delaware').