Journal of Experimental Botany, Vol 49, 775-788, Copyright © 1998 by Oxford University Press
E Steudle and C Peterson
On the basis of recent results with young primary maize roots, a model is
proposed for the movement of water across roots. It is shown how the
complex, 'composite anatomical structure' of roots results in a 'composite
transport' of both water and solutes. Parallel apoplastic, symplastic and
transcellular pathways play an important role during the passage of water
across the different tissues. These are arranged in series within the root
cylinder (epidermis, exodermis, central cortex, endodermis, pericycle
stelar parenchyma, and tracheary elements). The contribution of these
structures to the root's overall radial hydraulic resistance is examined.
It is shown that as soon as early metaxylem vessels mature, the axial
(longitudinal) hydraulic resistance within the xylem is usually not
rate-limiting. According to the model, there is a rapid exchange of water
between parallel radial pathways because, in contrast to solutes such as
nutrient ions, water permeates cell membranes readily. The roles of
apoplastic barriers (Casparian bands and suberin lamellae) in the root's
endo- and exodermis are discussed. The model allows for special
characteristics of roots such as a high hydraulic conductivity (water
permeability) in the presence of a low permeability of nutrient ions once
taken up into the stele by active processes. Low root reflection
coefficients indicate some apoplastic by-passes for water within the root
cylinder. For a given root, the model explains the large variability in the
hydraulic resistance in terms of a dependence of hydraulic conductivity on
the nature and intensity of the driving forces involved to move water. By
switching the apoplastic path on or off, the model allows for a regulation
of water uptake according to the demands from the shoot. At high rates of
transpiration, the apoplastic path will be partially used and the hydraulic
resistance of the root will be low, allowing for a rapid uptake of water.
On the contrary, at low rates of transpiration such as during the night or
during stress conditions (drought, high salinity, nutrient deprivation),
the apoplastic path will be less used and the hydraulic resistance will be
high. The role of water channels (aquaporins) in the transcellular path is
in the fine adjustment of water flow or in the regulation of uptake in
older, suberized parts of plant roots lacking a substantial apoplastic
component. The composite transport model explains how plants are designed
to optimize water uptake according to demands from the shoot and how
external factors may influence water passage across
roots.Keywords: Composite transport model, endodermis,
exodermis, hydraulic conductivity, reflection coefficient, root, water,
water channels.
ARTICLES
Review article. How does water get through roots?
Lehrstuhl fur Pflanzenokologie, Universitat Bayreuth, D-95440 Bayreuth, Germany; Department of Biology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1; Corresponding author; e-mail: ernst.steudle@uni-bayreuth.de
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