Accumulation rate of ABA in detached maize roots correlates with root water potential regardless of age and branching order

How much ABA can be supplied by the roots is a key issue for modelling the ABA-mediated influence of drought on shoot physiology. We quantified accumulation rates of ABA (S-ABA) in maize roots that were detached from well-watered plants and dehydrated to various extents by air-drying. S-ABA was estimated from changes in ABA content in root segments incubated at constant relative water content (RWC). Categories of root segments, differing in age and branching order, were compared (root branches, and nodal roots subdivided into root tips, subapical unbranched sections, and mature sections), All categories of roots accumulated ABA, including turgid and mature tissues containing no apex. S-ABA measured in turgid roots changed with root age and among root categories, This variability was largely accounted for by differences in water content among different categories of turgid roots. The response of S-ABA to changes in root water potential (Psi(root)) induced by dehydration was common to root tips, nodal roots and branches of several ages, while this was not the case if root dehydration was expressed in terms of RWC. Differences among root categories in the response of S-ABA to RWC were due to different RWC values among categories at a given Psi(root), and not to differences in the response of S-ABA to Psi(root).