Yield vs. quality trade-offs for wheat in response to carbon dioxide and ozone

Although it is established that there exist potential trade-offs between grain yield and grain quality in wheat exposed to elevated carbon dioxide (CO₂ ) and ozone (O₃ ), their underlying causes remain poorly explored. To investigate the processes affecting grain quality under altered CO₂ and O₃ , we analysed 57 experiments with CO₂ or O₃ exposure in different exposure systems. The study covered 24 cultivars studied in 112 experimental treatments from 11 countries. A significant growth dilution effect on grain protein was found: a change in grain yield of 10% by O₃ was associated with a change in grain protein yield of 8.1% (R² =0.96), while a change in yield effect of 10% by CO₂ was linked to a change in grain protein yield effect of 7.5% (R² =0.74). Superimposed on this effect, elevated CO₂ , but not O₃ , had a significant negative effect on grain protein yield also in the absence of effects on grain yield, indicating that there exists a process by which CO₂ restricts grain protein accumulation, which is absent for O₃ . Grain mass, another quality trait, was more strongly affected by O₃ than grain number, while the opposite was true for CO₂ . Harvest index was strongly and negatively influenced by O₃ , but was unaffected by CO₂ . We conclude that yield vs. protein trade-offs for wheat in response to CO₂ and O₃ are constrained by close relationships between effects on grain biomass and less than proportional effects on grain protein. An important and novel finding was that elevated CO₂ has a direct negative effect on grain protein accumulation independent of the yield effect, supporting recent evidence of CO₂ -induced impairment of nitrate uptake/assimilation. Finally, our results demonstrated that processes underlying responses of grain yield vs. quality trade-offs are very different in wheat exposed to elevated O₃ compared to elevated CO₂ .