Differential effects of ozone on photosynthesis of winter wheat among cultivars depend on antioxidative enzymes rather than stomatal conductance.

Five modern cultivars of winter wheat (Triticum aestivum L.): Yangmai16 (Y16), Yangmai 15 (Y15), Yangfumai 2 (Y2), Yannong 19 (Y19) and Jiaxing 002 (J2) were investigated to determine the impacts of elevated ozone concentration (E-O3) on photosynthesis-related parameters and the antioxidant system under fully open-air field conditions in China. The plants were exposed to E-O3 at 1.5 times the ambient ozone concentration (A-O3) from the initiation of tillering to final harvest. Pigments, gas exchange rates, chlorophyll a fluorescence, antioxidants contents, antioxidative enzyme activity and lipid oxidation were measured in three replicated plots throughout flag leaf development. Results showed that significant O3 effects on most variables were only found during the mid-grain filling stage. Across five cultivars, E-O3 significantly accelerated leaf senescence, as indicated by increased lipid oxidation as well as faster declines in pigment amounts and photosynthetic rates. The lower photosynthetic rates were mainly due to non-stomatal factors, e.g. lower maximum carboxylation capacity and electron transport rates. There were strong interactions between O3 and cultivar in photosynthetic pigments, light-saturated photosynthesis rate and chlorophyll a fluorescence with O3-sensitive (Y19, Y2 and Y15) and O3-tolerant (J2, Y16) cultivars being clearly differentiated in their responses to E-O3. E-O3 significantly influenced the antioxidative enzymes but not antioxidant contents. Significant interactions between O3 and cultivar were found in antioxidative enzymes, such as SOD and CAT, but not in stomatal conductance (gs). Therefore, it can be concluded that antioxidative enzymes rather than gs or antioxidants are responsible for the differential responses to E-O3 among cultivars. These findings provide important information for the development of accurate modeling O3 effects on crops, especially with respect to the developmental stage when O3 damage to photosynthesis becomes manifest.