Elevated CO₂ (eCO₂) is reported to accelerate the photosynthetic induction in response to sudden increases in photon flux density. However, it is still unclear which effect of eCO₂ contributes more to the acceleration, more CO₂ as substrate or photosynthetic acclimation? particularly in crop species. We grew two crop species, Oryza sativa L. (rice) and Triticum aestivum L. (wheat) in the open-top chambers at 400 and 600 μmol CO₂ mol^-1 air. We measured the time course of photosynthetic rate after a sudden PFD increase under 400 and 600 μmol CO₂ mol^-1air, respectively, for plants from both the growth CO₂ . The in situ measurements, i.e., measurements conducted under the same CO₂ as that during the growth, showed reduced induction time in leaves from the high CO₂ than the low CO₂. However, the instantaneous eCO₂, i.e., eCO₂ only during measurement, did not significantly changed the induction time in leaves from different growth CO₂. Moreover, in situ eCO₂ reduced stomatal opening time in wheat, while instantaneous eCO₂ tend to increase the stomatal limitation. These results suggest photosynthetic acclimation, rather than higher CO₂ substrate, would accelerate more the photosynthetic induction and the acceleration is mainly due to acclimation of stomatal response.