IMPACT OF ELEVATED ATMOSPHERIC CO2 AND O-3 ON GAS-EXCHANGE AND CHLOROPHYLL CONTENT IN SPRING WHEAT (TRITICUM-AESTIVUM L.)

Stands of spring wheat grown in open-top chambers (OTCs) were used to assess the individual and interactive effects of season-long exposure to elevated atmospheric carbon dioxide (CO2) and ozone (O-3) on the ph otosynthetic and gas exchange properties of leaves of differing age an d position within the canopy. The observed effects were related to est imated ozone fluxes to individual leaves. Foliar chlorophyll content w as unaffected by elevated CO2, but photosynthesis under saturating irr adiances was increased by up to 100% at 680 mu mol mol(-1) CO2 relativ e to the ambient CO2 control; instantaneous water use efficiency was i mproved by a combination of increased photosynthesis and reduced trans piration. Exposure to a seasonal mean O-3 concentration (7 h d(-1)) of 84 nmol mol(-1) under ambient CO2 accelerated leaf senescence followi ng full expansion, at which time chlorophyll content was unaffected. S tomatal regulation of pollutant uptake was limited since estimated O-3 fluxes to individual leaves were not reduced by elevated atmospheric CO2. A common feature of O-3-treated leaves under ambient CO2 was an i nitial stimulation of photosynthesis and stomatal conductance for up t o 4 d and 10 d, respectively, after full leaf expansion, but thereafte r both variables declined rapidly. The O-3-induced decline in chloroph yll content was less rapid under elevated CO2 and photosynthesis was i ncreased relative to the ambient CO2 treatment. A/C-i analyses suggest ed that an increase in the amount of in vivo active RuBisCO may be inv olved in mitigating O-3-induced damage to leaves. The results obtained suggest that elevated atmospheric CO2 has an important role in restri cting the damaging effects of O-3 on photosynthetic activity during th e vegetative growth of spring wheat, and that additional direct effect s on reproductive development were responsible for the substantial red uctions in grain yield obtained at final harvest, against which elevat ed CO2 provided little or no protection.