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

Ozone (O-3) concentrations in periurban areas in East Asia are sufficiently high to decrease crop yield. However, little is known about the genotypic differences in O-3 sensitivity in winter wheat in relation to year of cultivar release. This paper reports genotypic variations in O-3 sensitivity in 20 winter wheat cultivars released over the past 60 years in China highlighting O-3-induced mechanisms. Wheat plants were exposed to elevated O-3 (82 ppb O-3, 7 h day(-1)) or charcoal-filtered air (< 5 ppb O-3) for 21 days in open top chambers. Responses to O-3 were assessed by the levels of antioxidative activities, protein alteration, membrane lipid peroxidation, gas exchange, leaf chlorophyll, dark respiration and growth. We found that O-3 significantly reduced foliar ascorbate (-14%) and soluble protein (-22%), but increased peroxidase activity (+46%) and malondialdehyde (+38%). Elevated O-3 depressed light saturated net photosynthetic rate (-24%), stomatal conductance (-8%) and total chlorophyll (-11%), while stimulated dark respiration (+28%) and intercellular CO2 concentration (+39%). O-3 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 O-3 that did not correlate to observed O-3 tolerance. Sensitivity to O-3 in cultivars of winter wheat progressed with year of release and correlated with stomatal conductance and dark respiration in O-3-exposed plants. O-3-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 O-3 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.