GROWTH, LIGHT INTERCEPTION AND YIELD RESPONSES OF SPRING WHEAT (TRITICUM-AESTIVUM L.) GROWN UNDER ELEVATED CO2 AND O-3 IN OPEN-TOP CHAMBERS

Spring wheat cv. Minaret was grown to maturity under three carbon diox ide (CO2) and two ozone (O-3) concentrations in open-top chambers (OTC ). Green leaf area index (LAI) was increased by elevated CO2 under amb ient O-3 conditions as a direct result of increases in tillering, rath er than individual leaf areas. Yellow LAI was also greater in the 550 and 680 mu mol mol(-1) CO2 treatments than in the chambered ambient co ntrol; individual leaves on the main shoot senesced more rapidly under 550 mu mol mol(-1) CO2, but senescence was delayed at 680 mu mol mol( -1) CO2. Fractional light interception (f) during the vegetative perio d was up to 26% greater under 680 mu mol mol(-1) CO2 than in the contr ol treatment, but seasonal accumulated intercepted radiation was only increased by 8%. As a result of greater carbon assimilation during can opy development, plants grown under elevated CO2 were taller at anthes is and stem and ear biomass were 27 and 16% greater than in control pl ants. At maturity, yield was 30% greater in the 680 mu mol mol(-1) CO2 treatment, due to a combination of increases in the number of ears pe r m(-2), grain number per ear and individual grain weight (IGW). Expos ure to a seasonal mean (7 h d(-1)) of 84 nmol mol(-1) O-3 under ambien t CO2 decreased green LAI and increased yellow LAI, thereby reducing b oth f and accumulated intercepted radiation by approximate to 16%. Ind ividual leaves senesced completely 7-28 days earlier than in control p lants. At anthesis, the plants were shorter than controls and exhibite d reductions in stem and ear biomass of 15 and 23%. Grain yield at mat urity was decreased by 30% due to a combination of reductions in ear n umber m(-2), the numbers of grains per spikelet and per ear and IGW. T he presence of elevated CO2 reduced the rate of O-3-induced leaf senes cence and resulted in the maintenance of a higher green LAI during veg etative growth under ambient CO2 conditions. Grain yields at maturity were nevertheless lower than those obtained in the corresponding eleva ted CO2 treatments in the absence of elevated O-3. Thus, although the presence of elevated CO2 reduced the damaging impact of ozone on radia tion interception and vegetative growth, substantial yield losses were nevertheless induced. These data suggest that spring wheat may be sus ceptible to O-3-induced injury during anthesis irrespective of the atm ospheric CO2 concentration. Possible deleterious mechanisms operating through effects on pollen viability, seed set and the duration of grai n filling are discussed.