Demonstration of a diel trend in sensitivity of Gossypium to ozone: a step toward relating O(3) injury to exposure or flux

Plant injury by ozone (O(3)) occurs in three stages, O(3) entrance through stomata, overcoming defences, and attack on bioreceptors. Concentration, deposition, and uptake of O(3) are accessible by observation and modelling, while injury can be assessed visually or through remote sensing. However, the relationship between O(3) metrics and injury is confounded by variation in sensitivity to O(3). Sensitivity weighting parameters have previously been assigned to different plant functional types and growth stages, or by differentially weighting O(3) concentrations, but diel and seasonal variability have not been addressed. Here a plant sensitivity parameter (S) is introduced, relating injury to O(3) dose (uptake) using three independent injury endpoints in the crop species, Pima cotton (Gossypium barbadense). The diel variability of S was determined by assessment at 2h intervals. Pulses of O(3) (15min) were used to assess passive (constitutive) defence mechanisms and dose was used rather than concentration to avoid genetic or environmental effects on stomatal regulation. A clear diel trend in S was apparent, with maximal sensitivity in mid-afternoon, not closely related to gas exchange, whole leaf ascorbate, or total antioxidant capacity. This physiologically based sensitivity parameter provides a novel weighting factor to improve modelled relationships between either flux or exposure to O(3), and O(3) impacts. This represents a substantial improvement over concentration- or phenology-based weighting factors currently in use. Future research will be required to characterize the variability and metabolic drivers of diel changes in S, and the performance of this parameter in prediction of O(3) injury.