It was anticipated that wheat net photosynthesis would rise under elevated CO₂, and that this would alter the progress of senescence due to the unbalance of carbohydrates and nitrogen. Our study showed that ear carbon sink was limited, and sugar was accumulated, hexokinase activities and levels of phosphorylated sugar were increased within the flag leaves, grain nitrogen sink capacity was enhanced, and flag leaf senescence was accelerated under elevated CO₂. However, if the ear of the main stem was covered, these responses to elevated CO₂ were absent, and the senescence of flag leaf was not accelerated by elevated CO₂. Thus, it appeared that elevated CO₂ accelerated the rate of flag leaf senescence, depending on ear photosynthesis. The ears have far higher enhancement of net photosynthesis than flag leaves, and the role of the flag leaf relative to the ear was declined in supplying C assimilation to grain under elevated CO₂. This indicates that as CO₂ rises, the grain sink needs the N more than C assimilate from flag leaf, so the declining rates of N% and soluble proteins concentration were markedly accelerated under elevated CO₂ conditions. This suggests that, the large increase in ear net photosynthesis accelerated grain filling, accelerated remobilising N within flag leaf as the result of the greater grain nitrogen sink capacity. In addition, as the result of grain carbon sink limitation, it limited the export of flag leaf sucrose and enhanced sugar cycling, which was the signal to accelerate leaf senescence. Hence, elevated CO₂ subsequently accelerates senescence of flag leaf.