This study examined the effects of elevated ozone (O-3) and/or carbon dioxide (CO2) on the diel allocation of photosynthetically fixed carbon in fully expanded leaves of young (growth stages 4-5) spring wheat (Triticum aestivum L. cv. Hanno). Plants were grown in controlled environment chambers and exposed to two O-3 regimes ['non-polluted' air (CF), < 5 nmol mol(-1); 'polluted' air, CF + 75 nmol mol(-1) 7 h d(-1)] and two CO2 treatments ('ambient', 354 mu mol mol(-1); 'elevated', 700 mu mol mol(-1)) over a 30 d period. Neutral sugars (predominantly sucrose) were found to be the most abundant form of carbohydrate accumulated by leaves during the day, but significant quantities of starch and high degree of polymerization (d.p.) fructans were also present. Elevated concentrations of O-3 and/or CO2 were found to have marked effects on diel patterns of export, storage and respiration, whilst the proportions of fixed carbon allocated to each of these processes were broadly similar. O-3 depressed the rate of net CO2 assimilation (-20%) and reduced stomatal conductance (-19%). This was reflected in a reduced amount of carbohydrate accumulated in, and exported by, source tissue during the day. Effects of O-3 on the rate of CO2 fixation were aggravated by an increased demand for carbon by dark respiratory processes. In contrast, doubling the atmospheric concentration of CO2 enhanced the rate of net CO2 assimilation (+ 47%) and reduced the proportion of fixed carbon retained in the leaf blade, increasing the rate of export. The favourable carbon balance of CO2 enriched leaves was further enhanced by a decrease in the cost of maintenance respiration, whilst simultaneous measurements of CO2 efflux and O-2 uptake at night suggested a shift in the substrates metabolized at high CO2. Effects of elevated CO2 and O-3 on the carbon balance of individual leaf blades over a single 24 h light/dark cycle were entirely consistent with the cumulative effects of the gases on plant growth over a 30 d period. O-3 reduced the rate of plant growth (-10%), but there were differential effects of O-3 on the growth of root and shoot which exacerbated the decrease in assimilate availability induced by O-3. In contrast the favourable effects of CO2 enrichment on the carbon balance of individual source leaves was reflected in the enhanced accumulation of dry matter in existing sinks, and the initiation of new sinks (i.e. increased tillering). In the combined treatment (elevated CO2 + O-3), O-3 counteracted the favourable effects of CO2 enrichment on the carbon balance of individual leaves, and the combined effects of the individual gases on the diel partitioning of photosynthetically fixed carbon in fully expanded leaf blades was reflected in a decreased rate of plant growth at elevated CO2, a situation further exacerbated by O-3-induced shifts in the relative partitioning of carbon between root and shoot. There was no evidence that CO2 enrichment afforded additional protection against O-3 damage: the extent of the O-3-induced reduction in photosynthesis, carbohydrate availability and growth observed at elevated CO2 was similar to that induced by O-3 in ambient air, despite additive effects of the gases on stomatal conductance that would reduce the effective dose of O-3 by approximate to 30%. The wider ecological significance of interactions between elevated CO2 and O-3 is discussed in the light of other recent findings.
