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Water is one of the key resources challenging the sustainability of modern agriculture. In developing countries, potato production is increasing because of its ability to provide nutritious food in a short season. However, the potato crop requires profuse irrigation. Yields of commercial potato varieties are often restricted by water availability. Their root systems are generally sparse and shallow, and they close their stomata, preventing photosynthetic carbon assimilation, whilst water is still available in the soil. To enable breeding of drought tolerant varieties, we are developing phenotypic screens that will allow us to explore the genetic basis of key traits for water use efficiency (WUE). From field trials of a genetic mapping population, ten genotypes with contrasting transpiration efficiencies based on leaf δ13C values were cultivated under controlled glasshouse conditions. After emergence, plants were grown for 30 days in soils watered to field capacity (30 % volumetric content,-5 kPa water potential) before being divided into three groups irrigated to 30%, 20 % (-300kPa, slight stress) and 12 % (-1500kPa, wilting point) volumetric content. As WUE is a complex phenomenon, we evaluated a number of associated physiological and morphological traits. Tissue samples were also collected at different time points to determine differentially expressed genes at these moisture levels. Response to water stress not only includes closing stomata but also reducing the density of stomata during leaf development. Preliminary data from our experiments shows that transpiration-efficient genotypes, as indicated by low leaf δ13C values, have consistently lower stomatal conductance at 12 % volumetric soil moisture than transpiration-inefficient genotypes. Thus transpiration-inefficient genotypes transpire more water at lower soil water content