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

Abstract

Plant injury by ozone (O3) occurs in three stages, O3 entrance through stomata, overcoming defences, and attack on bioreceptors. Concentration, deposition, and uptake of O3 are accessible by observation and modelling, while injury can be assessed visually or through remote sensing. However, the relationship between O3 metrics and injury is confounded by variation in sensitivity to O3. Sensitivity weighting parameters have previously been assigned to different plant functional types and growth stages, or by differentially weighting O3 concentrations, but diel and seasonal variability have not been addressed. Here a plant sensitivity parameter (S) is introduced, relating injury to O3 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 O3 (15 min) 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 O3, and O3 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 O3 injury.

Fig. 1.

Representative leaves photographed 7 d following midday exposure to a single 15min pulse of O₃, with a dose of (A) 5.0 mmol m^–2 or (B) 23.9 mmol m^–2.

Fig. 2.

Impact of O₃ as relative reduction in chlorophyll determined as SPAD, measured at 6 d post-pulse exposure, in response to O₃ dose (D) at (A) 07:00h and (B) 15:00h. The slope of the dose–response relationship yielded the sensitivity parameter (S_SPAD), determined at discrete 2h intervals during the photoperiod. The best fit regression line was determined from the pooled data set obtained using two exposure technologies.

Fig. 3.

Ozone impact as relative reduction in stomatal conductance (g_s), measured at 6 d post-pulse exposure, in response to O₃ dose (D) at (A) 07:00h and (B) 15:00h. The slope of the dose–response relationship yielded a second sensitivity parameter (S_COND), determined at discrete 2h intervals during the photoperiod. The best fit regression line was determined from the pooled data set obtained using two exposure technologies.

Fig. 4.

Correlation between sensitivity to O₃ assayed as stomatal conductance (S_COND) and sensitivity assayed as SPAD (S_SPAD), both relative to control values at 6 d after the pulse exposure to O₃.

Fig. 5.

Ozone impact as relative reduction in non-injured leaf tissue, measured at 7 d post-pulse exposure, in response to O₃ dose (D) at (A) 07:00h and (B) 17:00h. The slope of the dose–response relationship yielded a third sensitivity parameter (S_NON-INJ), determined at discrete 2h intervals during the photoperiod. The best fit regression line was determined from the pooled data set obtained using two exposure technologies.

Fig. 6.

The diurnal course of plant sensitivity to ozone in Pima cotton. Sensitivity (S) is characterized as (A) chlorophyll pigmentation (S_SPAD), (B) stomatal conductance (S_COND), both relative to control values at 6 d after the pulse exposure to O₃, and (C) non-injured leaf area (S_NON-INJ) relative to total leaf area at 7 d after the pulse exposure. Points with the same letter do not differ at P < 0.05. Error bars are omitted from the 19:00h data points in (A) and (C) as they were very large.

Fig. 7.

The diurnal course of leaf stomatal conductance determined on the growth bench prior to the pulse exposure to O₃, characterized as the mean over both runs. Points with the same letter do not differ at P < 0.05.

Fig. 8.

The diurnal course of (A) ascorbic acid and (B) total antioxidant capacity in whole leaf extracts, expressed as ascorbic acid (AA) equivalents, averaged over both runs. Points with the same letter do not differ at P < 0.05.