Water is about to become increasingly limited for crop production, which jeopardizes the whole maize sector. Potassium (K) nutrition has been proposed to mitigate water deficit in plants, but field-scale studies involving grain yield components are scarce. In this study, we aimed at analyzing the effect of K nutrition on grain yield, vegetative growth and physiological parameters of maize subjected to water deficit. The effect of K nutrition on water use efficiency was also calculated at the whole-crop level (WUE) and through leaf gas exchange measurements (WUEi). A large-scale field experiment was designed, combining three levels of K fertilization (low, normal and high K) and two water supply scenarios (normal and 30% deficit based on a water balance model). Water deficit induced a strong decrease in leaf area, which was essentially due to a lower leaf elongation rate. The grain yield of the water-stressed plants was 25% lower than that of the well-irrigated ones. Grain yield was even worse when K deficiency was superimposed on water deficit, with a specific effect of K on grain filling. The optimal K fertilization helped the plants mitigate the effect of water deficit, through a better WUE (+ 30%), which was related to lower leaf evapotranspiration (ET). Moreover, under water deficit, leaf rolling was more pronounced when K was added, which also prevented water losses. Leaf water potential measurements suggested that the isohydric behaviour (maintenance of close stomata during water stress) of the maize was made possible thanks to K fertilization. When calculated at field scale, ET was higher with K fertilization, due to its positive effect on leaf area, in spite of a better stomata control and better WUE. We concluded that K addition, in K deficient soils, can help maize to cope with droughts and could be used as a new management option.Read less <