Diel trend in plant sensitivity to ozone: Implications for exposure- and flux-based ozone metrics: Fid-Bau Portal

Diel trend in plant sensitivity to ozone: Implications for exposure- and flux-based ozone metrics

Grantz, David A.

Abstract Plant sensitivity to ozone (O3) is critical to modeling impacts of air pollution on vegetation. A diel timecourse of sensitivity (S) was recently determined in Pima cotton (Grantz et al., 2013). The sensitivity parameter serves as a weighting factor for stomatal uptake (ozone flux, F), or cumulative F (dose, D). Previous approaches used various weighting schemes to modify ozone concentration ([O3]) or cumulative [O3] (exposure, E). Use of the S parameter allows calculation of effective flux (F eff) and effective dose (D eff). Though theoretically sound, the practical significance of S has not been evaluated due to the previous lack of available data. Here, the newly available S parameter is used to explore the relationships between exposure- and flux-based O3 metrics in response to scenarios of contrasting stomatal conductance (g s) and ambient [O3]. The O3 scenarios were similar but differed in timing of peak [O3]. E varied by up to 13.7%, D by up to 15.4%, and D eff, which factors in sensitivity, by up to 19.0%. The g s scenarios differed in midday magnitude and nocturnal closure. Cumulative g s varied by 65.2%, which was attenuated in D to 49.2% and in D eff to 51.1%. A simulation of hourly [O3], F, and F eff was run using Monte Carlo techniques with a full month of ambient [O3] data. Resulting diel timecourses of [O3], F, and F eff were realistic, with the principal sources of uncertainty in the physiological parameters, g s and S. Analysis of hourly values from the scenarios and the simulation output demonstrated significant correlation among the O3 metrics. However, the uncertainty in both F and F eff predicted from [O3] was large and proportional to [O3], yielding greatest uncertainty under conditions of high [O3] and potential phytotoxicity. In contrast, F eff was significantly correlated with F, with low variability that was not proportional to F. As a result, uncertainty was low and prediction potentially useful under conditions of likely injury. These results suggest that F, which incorporates g s, represents a substantial improvement over ambient [O3], which does not. F eff, which incorporates S, was closely related to F, which does not use S. The substantial effort required to measure or model S and F eff may not be justified under some conditions. Further research to obtain additional timecourses of S and to explore additional [O3] and g s scenarios is urgently required.

Highlights • The practical significance of plant sensitivity to ozone is evaluated. • Stomatal flux and dose of ozone are not well predicted by ambient ozone concentration. • Flux or dose is likely to be better related to ozone impacts than is concentration. • Effective ozone flux (incorporating plant sensitivity) is well predicted by flux. • Flux may be more cost effective than effective flux in predicting ozone-induced injury.