ABSTRACT. The reduced pool of the abscisic acid (ABA) precursors, ?-? carotenoids, in roots does not account for the substantial increase of ABA content in response to water stress (WS) conditions, suggesting that ABA could be transported from other organs. Basipetal transport was interrupted by stem-girdling and ABA levels were determined in roots after two cycles of WS induced by transplanting plants to dry perlite. Leaf applications of isotope-labelled ABA and reciprocal grafting of ABA-deficient tomato mutants were used to confirm the involvement of aerial organs on root ABA accumulation. Disruption of basipetal transport reduced ABA accumulation in roots and this decrease was more severe after two consecutive WS periods. This effect was linked to a sharp decrease in the ?-? carotenoid pool in roots in response to water deficit. Significant levels of isotope-labelled ABA were transported from leaves to roots, mainly in plants subjected to water dehydration. Furthermore, the use of different ABA-deficient tomato mutants in reciprocal grafting combinations with wild-type genotypes confirmed the involvement of aerial organs in the ABA accumulation in roots. In conclusion, accumulation of ABA in roots after long-term WS periods largely relies on the aerial organs suggesting a reduced ability of the roots to synthesize ABA from carotenoids. Furthermore, plants are able to basipetally transport ABA to sustain high hormone levels in roots. © The Author 2015. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved.