Long‐distance abscisic acid signalling under different vertical soil moisture gradients depends on bulk root water potential and average soil water content in the root zone

To determine how root‐to‐shoot abscisic acid (ABA) 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 four positions within the column by continuously recording volumetric soil water content (θv). Average θv was inversely related to bulk root water potential (Ψroot). In turn, Ψroot 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. Plant roots exposed to drying soil synthesise chemical signals such as ABA, which move from the roots to the shoots to limit transpiration, thereby increasing leaf water use efficiency. Our work tests alternative hypotheses as to how vertical soil moisture gradients quantitatively affect xylem ABA concentration. Unlike split‐root experiments where xylem ABA concentration depended on both the soil water content of each compartment and the fractional water uptake from each (Dodd et al. 2008; Plant Cell Env. 31, 1263–1274), when plants are exposed to dry upper soil and moist soil at depth (as is typical of many field conditions), xylem ABA concentration was best predicted by average soil water content in the entire root‐zone, because root ABA concentration was homogeneous across the root‐zone and related to bulk root water potential. These results will help understand the physiological effects of different water‐saving irrigation techniques applied in the field, where vertical gradients are an important source of soil moisture heterogeneity.