Potentially high future carbon dioxide (CO sub(2)) levels that affect water use by wheat could have secondary effects on plant-water relations, plant physiological processes, growth, and net primary productivity. The effect of a rise in atmospheric CO sub(2) on pre-dawn, mid-day, and sunset total leaf water potential of wheat ( psi sub(W)) as the soil matric potential ( psi sub(M)) varied from -0.03 to -1.50 MPa was studied in an open field in Maricopa, AZ, in ambient and CO sub(2)-enriched air. Two soil moisture levels also were tested. Analysis of pressure chamber measurements of psi on upper canopy leaves at pre-dawn, mid-day, and sunset showed that as psi sub(M) rose, an increase in atmospheric CO sub(2) resulted in a curvilinear increase in drought stress alleviation (to 77% at pre-dawn, 67% at mid-day, and 79% at sunset). As CO sub(2) concentrations rose, drought avoidance and tolerance mechanisms were enhanced, resulting in improved water relations in wheat.