To determine how root to shoot abscisic (ABA) acid signalling is regulated by vertical soil moisture gradients, root ABA concentration ( [ABA]root ), the fraction of root water uptake from, and root water potential of different parts of the root-zone, along with bulk root water potential, were measured to test various predictive models of root xylem ABA concentration [RX-ABA]sap. Beans (Phaseolus vulgaris L. cv. Nassau) were grown in soil columns and received different irrigation treatments (top and basal watering, and withholding water for varying lengths of time) to induce different vertical soil moisture gradients. Root water uptake was measured at 4 positions within the column by continuously recording volumetric soil water content (θv). Average θv was inversely related to bulk root water potential (root). In turn, root was correlated with both average [ABA]root and [RX-ABA]sap. Despite large gradients in θv, [ABA]root and root water potential was homogenous within the root zone. Consequently, unlike some split-root studies, root water uptake fraction from layers with different soil moisture did not influence xylem sap [ABA]. This suggests two different patterns of ABA signalling, depending on how soil moisture heterogeneity is distributed within the root-zone, which might have implications for implementing water-saving irrigation techniques.