Genotypic differences in leaf biochemical, physiological and growth responses to ozone in 20 winter wheat cultivars released over the past 60 years

Ozone (O₃) concentrations in periurban areas in East Asia are sufficiently high to decrease crop yield. However, little is known about the genotypic differences in O₃ sensitivity in winter wheat in relation to year of cultivar release. This paper reports genotypic variations in O₃ sensitivity in 20 winter wheat cultivars released over the past 60 years in China highlighting O₃-induced mechanisms. Wheat plants were exposed to elevated O₃ (82 ppb O₃, 7 h day⁻¹) or charcoal-filtered air (<5 ppb O₃) for 21 days in open top chambers. Responses to O₃ were assessed by the levels of antioxidative activities, protein alteration, membrane lipid peroxidation, gas exchange, leaf chlorophyll, dark respiration and growth. We found that O₃ significantly reduced foliar ascorbate (-14%) and soluble protein (-22%), but increased peroxidase activity (+46%) and malondialdehyde (+38%). Elevated O₃ depressed light saturated net photosynthetic rate (-24%), stomatal conductance (-8%) and total chlorophyll (-11%), while stimulated dark respiration (+28%) and intercellular CO₂ concentration (+39%). O₃ also reduced overall plant growth, but to a greater extent in root (-32%) than in shoot (-17%) biomass. There was significant genotypic variation in potential sensitivity to O₃ that did not correlate to observed O₃ tolerance. Sensitivity to O₃ in cultivars of winter wheat progressed with year of release and correlated with stomatal conductance and dark respiration in O₃-exposed plants. O₃-induced loss in photosynthetic rate was attributed primarily to impaired activity of mesophyll cells and loss of integrity of cellular membrane as evidenced by increased intercellular CO₂ concentration and lipid peroxidation. Our findings demonstrated that higher sensitivity to O₃ in the more recently released cultivars was induced by higher stomatal conductance, larger reduction in antioxidative capacity and lower levels of dark respiration leading to higher oxidative damage to proteins and integrity of cellular membranes.