As atmospheric CO₂ increases globally, plants increase the rate of photosynthesis. Still, leaf–gas exchange can be downregulated by the plant. Here we tested the limits of these plant responses in a fruit tree species under very high CO₂ levels relevant to the future Earth and to contemporary Mars. Plant water use decreased at 1600 ppm CO₂ and remained low at 6000 ppm. Photosynthesis significantly increased at 6000 ppm. In summary, ultra-high CO₂ may partly compensate for limited water availability.