Effects of elevated carbon dioxide and ozone on the growth and yield of spring wheat (Triticum aestivum L.)

Spring wheat cv. Minaret was grown under three carbon dioxide (CO2) and two ozone (O3) concentrations from seedling emergence to maturity in open-top chambers. Under elevated CO2 concentrations, the green leaf area index of the main shoot was increased, largely due to an increase in green leaf area duration. Biomass increased linearly in response to increasing CO2 (ambient, 550 and 680 ppm). At anthesis, stem and ear dry weights and plant height were increased by up to 174%, 5% and 9 cm, respectively, and biomass at maturity was 23% greater in the 680 ppm treatment as compared to the ambient control. Grain numbers per spikelet and per ear were increased by 0.2 and 5 grains, respectively, and this, coupled with a higher number of ears bearing tillers, increased grain yield by up to 33%. Exposure to a 7 h daily mean O3 concentration of 60 ppb induced premature leaf senescence during early vegetative growth (leaves 1-7) under ambient CO2 concentrations. Damage to the main shoot and possible seedling mortality during the first 3 weeks of exposure altered canopy structure and increased the proportion of tillers 1 and 2 which survived to produce ears at maturity was increased; as a result, grain yield was not significantly affected. In contrast to the older leaves, the flag leaf (leaf 8) sustained no visible O3 damage, and mean grain yield per ear was not affected. Interactions between elevated CO2 and O3 influenced the severity of visible leaf damage (leaves 1-7), with elevated CO2 apparently protecting against O3-induced premature senescence during early vegetative growth. The data suggest that the flag leaf of Minaret, a major source of assimilate during grain fill, may be relatively insensitive to O3 exposure. Possible mechanisms involved in damage and/or recovery are discussed.