Physiology of abscisic acid (ABA) in roots under stress—a review of the relationship between root ABA and radial water and ABA flows

Abscisic acid (ABA), the universal plant stress hormone, is accumulated in roots subjected to a range of external stresses, including drought, salinity, and nutrient deficiencies. This accumulation is regulated by ABA-metabolism (biosynthesis and degradation), -recirculation, and -exudation. Stress ABA serves as a long-distance signal regulating the water relations of shoots (stomata, meristems) and roots (hydraulic conductivity, root development, desiccation tolerance). Endogenous ABA, radial water flows (JV), and radial ABA flows (JABA) are closely coupled. Here we described the relations between these processes that are crucial factors for the role of ABA as a stress hormone and a long-distance stress signal. Crop varieties with high ABA concentrations exhibit an intensified long distance ABA signalling that reduces water consumption and, in the case of grapevine, improves the quality of the berries.