Plant gas exchange is regulated by stomata, which coordinate leaf-level water loss with xylem transport. Stomatal opening responds to internal concentrations of CO 2 in the leaf, but changing CO 2 can also lead to changes in stomatal density that influence transpiration. Given that stomatal conductance increases under subambient concentrations of CO 2 and, con-versely, that plants lose less water at elevated concentrations, can downstream effects of atmospheric CO 2 be observed in xylem tissue? We approached this problem by evaluating leaf stomatal density, xylem transport, xylem anatomy and resistance to cavitation in Helianthus annuus plants grown under three CO 2 regimes ranging from pre-industrial to elevated concentrations. Xylem transport, conduit size and stomatal density all increased at 290 ppm relative to ambient and elevated CO 2 concentrations. The shoots of the 290-ppm-grown plants were most vulnerable to cavitation, whereas xylem cavitation resistance did not differ in 390-and 480-ppm-grown plants. Our data indicate that, even as an indirect driver of water loss, CO 2 can affect xylem struc-ture and water transport by coupling stomatal and xylem hydraulic functions during plant development. This plastic response has implications for plant water use under variable con-centrations of CO 2 , as well as the evolution of efficient xylem transport.