Selection for drought tolerance entails prioritizing plant traits that integrate critical physiological processes occurring during crop growth. Discrimination against 13C ({increment}) in leaflets ({increment}leaflet) and tubers ({increment}tuber) was compared under two water regimes in two potato-improved varieties selected to maintain yield under drought conditions (Unica and Sarnav) and one drought susceptible European cultivar (Désirée). In the control treatment, soil water content was kept at field capacity over the whole growth cycle, while in the drought treatment water supply was restricted after tuber initiation (50 % of field capacity). Gas exchange and N content per unit leaf area (Narea) as well as {increment} were assessed at different stages. Sarnav showed the highest tuber yield in both water conditions, suggesting that yield in the water restriction treatment was largely driven by yield potential in this genotype. Higher stomatal conductance (gs) and Narea and lower {increment}leaflet in well-watered Sarnav suggested higher photosynthetic capacity. Under water restriction, Sarnav maintained higher gs indicating that carbon diffusion was a key factor for biomass accumulation under water restriction. Our results suggest the use of {increment} determined after tuber initiation as an indirect selection indicator for tuber yield under both well-watered and restricted soil water availability conditions. Copyright