Spring wheat cv. Minaret was grown to maturity under three carbon dioxide (CO2) and two ozone (O3) concentrations in open-top chambers (OTC). Green leaf area index (LAI) was increased by elevated CO2 under ambient O3 conditions as a direct result of increases in tillering, rather than individual leaf areas. Yellow LAI was also greater in the 550 and 680 μmol mol–1 CO2 treatments than in the chambered ambient control; individual leaves on the main shoot senesced more rapidly under 550 μmol mol–1 CO2, but senescence was delayed at 680 μmol mol–1 CO2. Fractional light interception (f) during the vegetative period was up to 26% greater under 680 μmol mol–1 CO2 than in the control treatment, but seasonal accumulated intercepted radiation was only increased by 8%. As a result of greater carbon assimilation during canopy development, plants grown under elevated CO2 were taller at anthesis and stem and ear biomass were 27 and 16% greater than in control plants. At maturity, yield was 30% greater in the 680 μmol mol–1 CO2 treatment, due to a combination of increases in the number of ears per m–2, grain number per ear and individual grain weight (IGW).
Exposure to a seasonal mean (7 h d–1) of 84 nmol mol–1 O3 under ambient CO2 decreased green LAI and increased yellow LAI, thereby reducing both f and accumulated intercepted radiation by ≈ 16%. Individual leaves senesced completely 7–28 days earlier than in control plants. At anthesis, the plants were shorter than controls and exhibited reductions in stem and ear biomass of 15 and 23%. Grain yield at maturity was decreased by 30% due to a combination of reductions in ear number m–2, the numbers of grains per spikelet and per ear and IGW. The presence of elevated CO2 reduced the rate of O3-induced leaf senescence and resulted in the maintenance of a higher green LAI during vegetative growth under ambient CO2 conditions. Grain yields at maturity were nevertheless lower than those obtained in the corresponding elevated CO2 treatments in the absence of elevated O3. Thus, although the presence of elevated CO2 reduced the damaging impact of ozone on radiation interception and vegetative growth, substantial yield losses were nevertheless induced. These data suggest that spring wheat may be susceptible to O3-induced injury during anthesis irrespective of the atmospheric CO2 concentration. Possible deleterious mechanisms operating through effects on pollen viability, seed set and the duration of grain filling are discussed.