Although both DI and PRD are forms of deficit irrigation, in DI water is applied evenly to all parts of the root system while in PRD it is distributed unevenly such that part of the root system is irrigated while the remainder is allowed to dry the soil. Roots in contact with drying soil synthesise chemical signals such as abscisic acid (ABA), which are transported to the shoot to regulate water use. [X-ABA] was investigated in plants grown in split-pots with either DI or PRD. Tomato (Lycopersicon esculentum 'Ailsa Craig') plants grown with PRD maintained a higher [X-ABA] in the leaves than DI-grown plants, when compared at the same whole pot soil water content (pot). To investigate the contribution of different parts of the root system to [X-ABA], individual shoots were grafted onto the root system of two plants, so that the graft union had the appearance of an inverted 'Y'. Xylem sap was collected from each root system independently. Root [X-ABA] increased exponentially as soil water content () declined, with data from root systems of both DI- and PRD-grown plants fitting the one relationship. This relationship was used to model [X-ABA] as the xylem exits the root system of DI- or PRD-grown plants held at the same pot. If each root system contributes an equal proportion of total transpirational flux, PRD-grown plants are predicted to have a higher [X-ABA] than DI-grown plants, due to the non-linearity of the [X-ABA] versus relationship. If the dry side contributes proportionally less to total transpirational flux as the soil dries, DI-grown plants are predicted to have a higher [X-ABA] than PRD-grown plants, in opposition to observed data.