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) an d two ozone (O-3) concentrations from seedling emergence to maturity i n open-top chambers. Under elevated CO2 concentrations, the green leaf area index of the main shoot was increased, largely due to an increas e in green leaf area duration. Biomass increased linearly in response to increasing CO2 (ambient, 550 and 680 ppm). At anthesis, stem and ea r 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 p pm treatment as compared to the ambient control. Grain numbers per spi kelet and per ear were increased by 0.2 and 5 grains, respectively, an d this, coupled with a higher number of ears bearing tillers, increase d grain yield by up to 33%. Exposure to a 7 h daily mean O-3 concentra tion of 60 ppb induced premature leaf senescence during early vegetati ve growth (leaves 1-7) under ambient CO2 concentrations. Damage to the main shoot and possible seedling mortality during the first 3 weeks o f exposure altered canopy structure and increased the proportion of ti llers 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 O-3 damage, and mean grain yield per ear was not affected. Interactions be tween elevated CO2 and O-3 influenced the severity of visible leaf dam age (leaves 1-7), with elevated CO2 apparently protecting against O-3- induced premature senescence during early vegetative growth. The data suggest that the flag leaf of Minaret, a major source of assimilate du ring grain fill, may be relatively insensitive to O-3 exposure. Possib le mechanisms involved in damage and/or recovery are discussed.