Abstract
Although it is established that there exist potential trade‐offs between grain yield and grain quality in wheat exposed to elevated carbon dioxide ( CO 2 ) and ozone ( O 3 ), their underlying causes remain poorly explored. To investigate the processes affecting grain quality under altered CO 2 and O 3 , we analysed 57 experiments with CO 2 or O 3 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 O 3 was associated with a change in grain protein yield of 8.1% ( R 2 = 0.96), whereas a change in yield effect of 10% by CO 2 was linked to a change in grain protein yield effect of 7.5% ( R 2 = 0.74). Superimposed on this effect, elevated CO 2 , but not O 3 , 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 CO 2 restricts grain protein accumulation, which is absent for O 3 . Grain mass, another quality trait, was more strongly affected by O 3 than grain number, whereas the opposite was true for CO 2 . Harvest index was strongly and negatively influenced by O 3 , but was unaffected by CO 2 . We conclude that yield vs. protein trade‐offs for wheat in response to CO 2 and O 3 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 CO 2 has a direct negative effect on grain protein accumulation independent of the yield effect, supporting recent evidence of CO 2 ‐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 O 3 compared with elevated CO 2 .