Quantifying sap fluxes and ABA concentrations of xylem saps collected from different parts of the root system may assist in modelling leaf xylem ABA concentration ([X-ABA]leaf) of plants grown with heterogeneous soil moisture distribution. To investigate the contribution of different parts of the root system to total sap flow and [X-ABA]leaf , individual shoots were grafted onto the root systems of two plants grown in two separate compartments and sap flow sensors placed on each hypocotyl below the graft union. Two different irrigation regimes were imposed: conventional deficit irrigation (DI) where both pots received the same volumes of water, and partial rootzone drying (PRD) where only one pot (designated “wet”) was watered and other (designated “dry”) dried the substrate. In plants grown in 3 L pots that were vertically divided into two watertight compartments, soil water content () and sap flow were continuously monitored. In plants grown in 2 appressed 0.43 L pots, xylem sap was collected independently from each root system (after severing the shoot below the graft union) to measure root xylem ABA concentration. During PRD, sap flow from roots in drying soil decreased: this relationship was described by a threshold dry (at which sap flow from roots in drying soil started to decrease) and sensitivity (the rate of decline in sap flow). The threshold dry varied with both wet and soil volume. As wet increased, the threshold dry increased but the sensitivity of sap flow to dry was independent of wet. At a similar wet, as soil volume increased, the threshold dry decreased. Root xylem ABA concentration increased as declined in both DI and PRD plants. A simple model that weighted the ABA contributions of wet and dry root systems to [X-ABA]leaf according to the sap flow from each, showed that with moderate soil drying, simulated [X-ABA]leaf was higher under PRD than DI.