Impact of elevated atmospheric CO2 and O3 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 (O3) on the photosynthetic and gas exchange properties of leaves of differing age and position within the canopy. The observed effects were related to estimated ozone fluxes to individual leaves. Foliar chlorophyll content was unaffected by elevated CO2, but photosynthesis under saturating irradiances was increased by up to 100% at 68 micromoles mol-1 CO2 relative to the ambient CO2 control; instantaneous water use efficiency was improved by a combination of increased photosynthesis and reduced transpiration. Exposure to a seasonal mean O3 concentration (7 h d-1) of 84 nmol mol-1 under ambient CO2 accelerated leaf senescence following full expansion, at which time chlorophyll content was unaffected. Stomatal regulation of pollutant uptake was limited since estimated O3 fluxes to individual leaves were not reduced by elevated atmospheric CO2. A common feature of O3-treated leaves under ambient CO2 was an initial stimulation of photosynthesis and stomatal conductance for up to 4 d and 10 d, respectively, after full leaf expansion, but thereafter both variables declined rapidly. The O3-induced decline in chlorophyll content was less rapid under elevated CO2 and photosynthesis was increased relative to the ambient CO2 treatment. A/Ci analyses suggested that an increase in the amount of in vivo active RuBisCO may be involved in mitigating O3-induced damage to leaves. The results obtained suggest that elevated atmospheric CO2 has an important role in restricting the damaging effects of O3 on photosynthetic activity during the vegetative growth of spring wheat, and that additional direct effects on reproductive development were responsible for the substantial reductions in grain yield obtained at final harvest, against which elevated CO2 provided little or no protection.