As plants experience drought, transpiration is regulated by decreases in stomatal conductance (gs) that can reduce carbon assimilation, biomass production and yield. The plant water potential (Ψ) provides an estimate of the plant water status, and the relationship between predawn (Ψpd) and midday (Ψmd) water potential (i.e. the water potential curve) could help determine when plants transition from water-consumptive (higher gs) to water-conservative (lower gs) behaviour. In this study, we apply the water potential curve framework (WP curve; i.e. Ψpd ~ Ψmd relationship) to an annual crop (Cucumis melo). The WP curve was evaluated over several dry-down experiments in both greenhouse (GH) and field conditions. Leaf gas exchange and Ψ measurements were taken on the same days. Overall, the WP curve differed between environments and the shift from higher to lower gs occurred earlier (higher Ψpd) under GH conditions, likely driven by a smaller root system, reduced access to soil water availability and a more rapid onset of drought. The WP curve exhibited two phases divided by a breakpoint (Θ) at −0.5 MPa (GH) and −0.72 MPa (field) of Ψpd that coincided with a gs reduction of 55% and 85% respectively. During phase I, plants reduced gs as the drought intensified without significantly compromising carbon assimilation (Pn). Yet, at Θ, Pn decreased by 57% and 61% under GH and field conditions respectively. During phase II, leaves reached the turgor loss point (TLP) at a Ψmd of −0.83 MPa (GH) and −1.3 MPa (field) that were similar to the TLP estimated from bench-top leaf pressure curves. Our results suggest that the WP curve in melons identifies the transition from water-consumptive to water-conservative behaviour and sets a boundary at which plants substantially reduce leaf gas exchange. Hence, the WP curve could be used to select crop varieties able to endure longer periods of drought with minimal impact on carbon assimilation, and better manage irrigation based on estimates of Ψpd to support effective use of water without a yield decrease.