Novel glasshouse method separates direct and indirect effects of CO₂ on crop growth.

Novel modelling technique scales CO₂ effects from glasshouse to field environments.

Reduced transpiration under elevated CO₂ accounts for sugarcane responses.

Direct effects of elevated CO₂ on sugarcane photosynthesis, if any, are small.

Effect of CO₂ on transpiration of field crops declines as canopy develops.