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

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 (Feff) and effective dose (Deff). 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 (gs) and ambient [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 Deff, which factors in sensitivity, by up to 19.0%. The gs scenarios differed in midday magnitude and nocturnal closure. Cumulative gs varied by 65.2%, which was attenuated in D to 49.2% and in Deff to 51.1%. A simulation of hourly [O3], F, and Feff was run using Monte Carlo techniques with a full month of ambient [O3] data. Resulting diel timecourses of [O3], F, and Feff were realistic, with the principal sources of uncertainty in the physiological parameters, gs and S. is of hourly values from the scenarios and the simulation output demonstrated significant correlation among the O3 metrics. However, the uncertainty in both F and Feff predicted from [O3] was large and proportional to [O3], yielding greatest uncertainty under conditions of high [O3] and potential phytotoxicity. In contrast, Feff 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. results suggest that F, which incorporates gs, represents a substantial improvement over ambient [O3], which does not. Feff, which incorporates S, was closely related to F, which does not use S. The substantial effort required to measure or model S and Feff may not be justified under some conditions. Further research to obtain additional timecourses of S and to explore additional [O3] and gs scenarios is urgently required.