Demonstration of a diel trend in sensitivity of Gossypium to ozone: a step toward relating O3 injury to exposure or flux

Authors: GRANTZ, DAVID - University Of California; VU, HAI-BANG - University Of California; HEATH, ROBERT - University Of California; Burkey, Kent

Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2013
Publication Date: 2/11/2013
Citation: Grantz, D., Vu, H., Heath, R., Burkey, K.O. 2013. Demonstration of a diel trend in sensitivity of Gossypium to ozone: a step toward relating O3 injury to exposure or flux. Journal of Experimental Botany. 64:1703-1713.

Interpretive Summary: Ozone is a toxic air pollutant that injures plants leading to reductions in growth and yield of crops, forests, and natural vegetation. Predicting ozone impacts requires knowledge of a number of interacting factors including ozone concentration in the air, leaf gas exchange that controls ozone uptake into plants, and the inherent defense capacity of the plant. Defense capacity is recognized as being a combination of metabolic and genetic components. In this study with cotton, plant sensitivity to ozone was shown to vary throughout the day with greatest sensitivity observed in the afternoon when ambient ozone concentrations are often the highest. Evidence that plant sensitivity to ozone varies on a relatively short time scale suggests that modeling ozone impacts on vegetation should not assume sensitivity to be constant. Although there has been much speculation that plant sensitivity to ozone may vary over time, this study is the first to demonstrate such short term changes exist and provides the basis for new future research on the underlying mechanisms that determine plant defense capacity.

Technical Abstract: Plant injury by ozone (O3) occurs in three stages, O3 entrance into the leaf through stomata, O3 overcoming metabolic defenses, and O3 attack on bioreceptors to cause injury. Concentration, deposition and uptake of O3 are accessible by observation and modeling. Injury can be assessed visually and through remote sensing. The relationship between O3 exposure and injury is not well characterized, reflecting uncertainties in these parameters and in capacities of O3 detoxification and defense metabolism as they vary between species or environments, and temporally. Attempts have been made to assign sensitivity weighting parameters to different plant functional types and growth stages, but diel and seasonal variability have not been addressed. We have developed a plant sensitivity parameter relating injury to O3 dose. We determined sensitivity at 2 hour intervals throughout the photoperiod, using three independent injury endpoints. All exhibited a clear diel trend, with maximal sensitivity in mid-afternoon, not coincident with gas exchange. We restrict exposure to brief (15 min) pulses of O3 to plants grown in clean air to assess passive (constitutive) defense mechanisms, independent of genetic or environmental effects on stomatal conductance or timecourses of ambient O3 concentration. Stomatal conductance pre- and post-exposure was interpolated to calculate O3 dose during the pulse. O3 was measured continuously during the pulse. Injury was assessed one week later as stomatal conductance, chlorophyll (SPAD), and non-injured leaf area. Sensitivity was not related to whole leaf ascorbate nor total antioxidants. Our sensitivity parameter represents a physiologically-based weighting factor to relate O3 exposure or flux to O3 impacts, a potential improvement over available concentration- or phenology-based weighting factors. Future research will be required to characterize the metabolic drivers of diel changes in sensitivity, their variability within and between plants and environments, and the performance of the sensitivity parameter in prediction of O3 injury.