全 文 ：Received 22 Oct. 2003 Accepted 9 Jan. 2004
Supported by the National Natural Science Foundation of China (30170559) and the State Key Basic Research Development Plan of China
(G1999011708).
* Author for correspondence. E-mail: .
http://www.chineseplantscience.com
Acta Botanica Sinica
植 物 学 报 2004, 46 (5): 505-514
.Review.
Advances in the Studies on Water Uptake by Plant Roots
ZHAO Chang-Xing1, DENG Xi-Ping1*, ZHANG Sui-Qi1, YE Qing2, Ernst STEUDLE2, SHAN Lun1
(1. State Key Laboratory of Soil Erosion and Dryland Farming on Loess Plateau, Institute of Soil and Water Conservation,
The Chinese Academy of Sciences and Ministry of Water Resource, Yangling Shaanxi 712100, China;
2. Lehrstuhl Pflanzenökologie, Universität Bayreuth , Bayreuth, D-95440, Germany)
Abstract: In the past decade, our understanding of the mechanisms of water uptake by plant roots at the
cell, tissue, and whole-plant levels rapidly progressed due to the introduction of new techniques and
concepts. Some aspects of this work were reviewed, mainly including the composite structure of roots and
effects of the distribution of roots in the soil. The nature of water flow in plant roots was discussed. A link
was provided between root hydraulics and the expression and function of aquaporins. This was related to
the regulation of water transport and to the signaling between roots and shoots. The composite transport
mode l of root was mentioned which represents a physical model of water uptake. This is part of a
comprehensive analysis of recent findings of studies on water uptake by plant roots and contributes to our
current understanding of the basic mechanisms that govern the water uptake by roots.
Key words: aquaporins; water uptake; root anatomy; water channel; composite transport model; soil-
plant-atmosphere continuum
Water uptake by plan t roots is contro lled or is even
regu lated by d ifferen t physical and phys io log ical
processes. Water supplied to plants by its roots has a ma-
jor influence on the shoot water s tatus , and , in turn, on
plant growth and development. Water moves from the sur-
face of a root to the root xylem through a series of tissues.
Each of these tissues represents a hydraulic conductance
that changes during root development and in response to
environmental factors (Steudle, 1994; 2000; 2001). In differ-
ent root habitats, root anatomy has to adap t to specific
physical and water constraints. Water transport properties
of roots are adjusted to the phys iological demand of the
whole plant (Javot and Maurel, 2002).
In recent years, our understanding of water uptake and
transport within plant roots has been subs tant ially im-
proved by new tools, which operate at the molecular, cell,
tissue, organ, plant, and ecosystem levels. Techniques such
as cell and root pressure probes, stopped flow, the use of
trans genic plan ts, and o f stable isotopes p rovided a fast
progress in water transport research (Steud le, 1993; 2001;
Kramer and Boyer, 1995; Maurel, 1997; Steudle and Peterson,
1998; Tyerman et al., 1999; 2002;). A better understanding
of the mechanisms of water uptake by plant roo ts should
be v ital for improv ing water us e efficiency (W UE) in
agriculture. The present article reviews the progress in re-
cent years for people who want to update their understanding
of basic mechanisms of root hydraulics and plant water
relations.
1 Distribution of Root Systems in Soil
The spatial distribution of roots in soil determines the
ability of plants to take up soil water and nutrients in order
to sustain plant growth and development. A large number
of studies confirm that deeper root systems enable plants
to access water not available to shallow rooted plants, and
to balance rather high rates of transpiration during periods
of water deficit (Zhu et al., 2002). Usually, plants in drier
environment develop deeper root systems. A survey of glo-
bal patterns o f root distribution and depth was given by
Canadell et al. (1996) and Jackson et al. (1996). On a global
average, desert vegetation reaches a maximum root depth
of 13.4 m, with 31% of the total roo t biomass below 0.3 m
soil in depth. Temperate forests have a maximum rooting
depth of 3.7 m, with 35% of the root biomass below the 0.3
m soil in depth . The root systems of temperate grass land
reach a maximum depth of on ly 2.4 m, and root biomass
accounts for 17% of the total. Differences in rooting depths
and ability of plants to extract soil water are likely to influ-
ence survival and productivity in the natu ral or artificial
ecosystem. However, large varieties of root distribution in
soil p rofile prevented the es tablishment of any definitive
model fo r crops , irres pective o f the s oil env ironment


Acta Botanica Sinica 植物学报 Vol.46 No.5 2004508
transport model of the root (see below), the effect is due to
a switching of water flow between pathways.
In contrast to the apoplast, cell membranes allow to es-
tablish and maintain osmotic gradients along the cell-to-
cell path. In addition to hydrostatic forces, these gradients
will drive water flow (Steudle, 1994; Steudle and Peterson,
1998). In the past, the movement of water across cell mem-
branes in roots has been often described as an osmotic
process (Kramer and Boyer, 1995). Recen t ev idence,
however, indicated that th is v iew has to be modified
(Steudle, 2000). This is true, because the hydraulic conduc-
tivity of root should differ depending on specific conditions,
namely, in the presence or absence of hydrostatic pressure.
Water transport from root to shoot involves both axial
and radial hydraulic resistance in plants. Vascular p lants
have evolved two types of highly modified cells, tracheids
and vessel members, strands of which provide an axial path-
way with an exceedingly low resistance to water flow. The
importance of the removal of the protoplasts for reduction
of the axial resistance to water flow can be demons trated
both theoretically and experimentally (Steudle and Peterson,
1998). According to Poiseuille’s law, axial resis tances can
be worked out. They can be measured as well (Steud le,
2001; Tyree and Zimmermann, 2002). Many researches in-
dicated that effects of vessel maturation on the axial resis-
tance were enormous (Steudle and Peterson, 1998). Usually,
axial resistances of roots are much smaller than radial ones.
However, this may change at high rates o f transpirat ion
which may cause cavitations (embolism) of water in vessels.
Cavitations interrupt water flow and reduce axial hydraulic
conductivity of roots substantially (Peterson et al., 1993;
Holbrook et al., 1999).
In roots with mature vessels, it is the radial res istance
rather than the axial which limits water uptake (see above).
For the early metaxylem of young maize root, axial resis-
tance is smaller by 1-2 order(s) of magnitude than that for
the radial flow (Steudle and Peterson, 1998). Overall, radial
res istances res ult from a series of smaller resis tances in
plant roots. Experimental results obtained with young roots
of maize, indicated that, the major resistance to water flow
was evenly spread over the living tissues of the root, which
is in contrast to the usual concept that in roots the major
resistance to water flow is in the endodermis. However, the
situation may change during later development of roots
and in different s pecies (Miyamoto et al., 2001).To date,
there are only a few results of hydraulic properties of indi-
vidual parts in roots to water up take (s uch as in rice:
Ranathunge et al., 2003). Novel methods are required to
quantify the specific effects of different root tissue such as
the role of the exo- and endodermis (North and Nobel, 1996;
Zimmermann and Steudle, 1998; Zimmermann et al., 2000;
Miyamoto et al., 2001; Ranatunge et al., 2003). Changes in
the hydraulic resistance at the tissue level may be derived
from the propagation of changes in water potential across
the root cylinder, and during root growth and development
(Westgate and Steudle, 1985).
Water balance and movement in plants is driven by gra-
dients in water potential. Catenary models have been pro-
posed for the movement of water between soil and atmo-
sphere in analogy to the flow of electricity in electric cir-
cu its (soil-plant-air-con tinuum, SPAC: van den Honert,
1948). At steady water flow across a plant, we have in anal-
ogy to basic laws of electricity (Ohm’s law):
Water flow= (Ysoil -Yroot surface)/Rsoil
= (Yroot surface-Yxylem)/Rroot
= (Y root xylem-Y leafxylem)/Rxylem
= (Yleaf-Yair )/Rleaf.
Water flow equals to total water potential gradient/total
resistance (Tyree, 1997; Steudle and Peterson, 1998). In the
root cylinder, water potential will drop along the different
tissues which are arranged in series (epidermis, exodermis,
cortex, etc.). The analogy with electrical circuits holds both
on the “microscopic scale” (e.g. when comparing pathways
for water across cells within tissue), and at a macroscopic,
level (e.g. when comparing different zones along roo ts.
Steudle and Peterson, 1998).
4 Water Transport at the Level of Root Mem-
branes
The flow of water in and ou t of plant cells is largely
regulated by water channels or aquaporins sitting in plasma
membranes (Fig.3). Plant-water relations and water flow in
plant tissues have been well characterized, but the p res-
ence of aquaporins was not established until the early 1990s.
The first aquaporin was iden tified in human erythrocytes
by Peter Agre who received the Nobel Prize for the “dis-
covery of water channels” earlier this year (Preston et al.,
1992; Zeidel et al., 1992; Walz et al., 1994). Aquaporins
(water channel proteins) are major intrinsic proteins (MIPs)
of a molar weight of around 30 kD (Maurel and Chrispeels,
1994; Maurel, 1997; Murata et al., 2000). They are found in
vacuolar and plasma membranes of plants. Aquaporins fa-
cilitate the transport of water across cell membranes (Fig.
3), and 75%-95% of the water permeation of plasma mem-
branes is due to aquaporin activity (Steudle and Henzler,
1995).
Aquaporins are highly selective membrane-spanning
pores that facilitate the rapid, passive exchange of water

Acta Botanica Sinica 植物学报 Vol.46 No.5 2004510
aquaporins (by phosphorylat ion and dephosphorylation)
or regulate aquaporins indirect ly, s uch as by regulat ing
protein kinases and phosphatases，or gating by a cohe-
sion/tension mechanism (Johannson et al., 2000; Ye et al.,
2004). In cortical cells of corn, high rates of water flow across
aquaporins caused a reversible deformation of channel pro-
tein and a closure of channels. The intensity of water flow
affected the extent of deformation and the time required for
a relaxation back to the native (ground) state (Wan et al.,
2004). The effect was independent of the direction of the
change of turgor which induced the water flow. It has been
interpreted as an effect of the kinetic energy injected into
the pores which was largely transmitted to the aquaporin
(Wan et al., 2004). Despite this progress , the molecu lar
mechanis ms that link hormone action, or stress stimuli of
different kind to the activity of aquaporins in root cell mem-
branes are still poorly understood.
Fine regulation of the water balance may also include
alternatives such as osmotic adjustment, photosynthesis,
respiration, and changes of key gene expression. However,
these mechanisms do not necessarily imply a regulation of
water balance at the level of cell hydraulic conductivity.
Coarse regulation may co-exist with fine regulation. While
coarse regu lation is phys ical in nature, and s trongly de-
pends on root st ructure, water channel activity is under
metabo lic cont rol as well. W hen regu lated by physical
means, water channel activity may be rapidly adjus ted in
response to adverse conditions. This would tend to avoid
tissue dehydrat ion, while maintaining tissue water poten-
tial as high as possible, or by to lerating low tissue water
potential. Plant water status and uptake are associated with
a lot of adaptive traits. These coars e regu lations mainly
involve minimizing water loss and maximizing water uptake.
Water loss is minimized by clos ing stomata or s tomatal
limitat ion, by reducing ligh t absorbance through rolled
leaves (Ehleringer and Cooper, 1992), a dense trichome layer
increasing reflectance (Larcher, 2000), or by other means.
Water uptake, on the other hand, is maximized by adjusting
the allocation pattern, namely increasing investment in the
roots , enhancing root dep th or extending roo t s ystem
distribution.
In o rder to keep water in balance, plants can regu late
water transport in roots or shoots by sensing the water
status in plant o r in soil (Yang et al., 2001). Thus, roo t/
shoot communication is being increasingly studied at the
molecular level. The return of root pressure after water stress
was associated with the complete recovery of leaf diffusive
conductance, leaf-specific photos ynthetic rate, and soil-
leaf hydraulic conductance (Stiller et al., 2003). Altering
speed of changes of water status (potential) in plant or soil
may result in totally different responses in terms of a regu-
lation or adaptation to stress. The time is an important fac-
tor in shaping plant responses according to genotype and
environment. Root-to-shoot signaling requires that chemi-
cal or physical (hydraulic) signals travel through the plant
in response to stresses sensed in roots. The nature of the
primary mediators of cellular process— water status, turgor,
ratio of bound and free water, hormones, alteration in cell
membranes and others— are st ill under debate (Chaves et
al., 2003). After the first stress reorganization events, cell
to organ responses diverge in different pathways, such as
during the involvement of ABA (ABA-dependent and ABA-
independent pathways) (Zhu, 2002). ABA is a major regula-
tor of plant water balance. It acts as a stress hormone in-
volved in the adaptation to various environmental condi-
tions (Gazzarrini and McCourt , 2001). ABA affects the hy-
draulic conductivity of roots and cells, that is, ABA facili-
tates water uptake by roots and the cell-to-cell component
of trans port of water across the root cylinder when s oil
s tart s d ry ing , es pecially under non-t rans p irat ion
conditions, when the apoplastic path of water transport is
largely excluding (Hose et al., 2000). There is a cons ider-
able overlap between abiotic stress signaling pathways with
specificity to water conditions in or ou ts ide of p lan ts
present. For example, this occurs at the level of initial stress
perception (Knight and Knight, 2001). Signaling pathways
may contribute to a complex network, interconnected at
many different levels. A revived interest in hydraulic com-
ponents of signaling is apparen t. However, litt le is st ill
known as to how chemical and hydraulic signals are inte-
grated into the overall regulation of plant water.
6 Composite Transport Model of Root: Physi-
cal Model of Water Uptake
The composite transport model mentioned above (Fig.
2) explains the variable uptake of water by roots and the
response of root hydrau lics to different factors. In the
model, the cohesion-tension mechan ism of ascent of sap
plays an important role (Steudle, 2001). It is based on de-
tailed measurement of root hydraulics both at the level of
excis ed roots (root hydraulic conductivity , Lpr) and root
cells (membrane level, cell Lp) using press ure probes and
other techniques (Azaizeh et al., 1992; Melchior and Steudle,
1993). The composite transport model integrates apoplastic
and cellular components of radial water flow across the
root cylinder. It well explains why the hydraulic conductiv-
ity of roots changes in response to the nature and inten-
sity of water flow (Steudle, 2000). In fact, the compos ite
ZHAO Chang-Xing et al.: Advances in the Studies on Water Uptake by Plant Roots 511
transport model is based on the compos ite structure of
root. The variability of root hydraulic properties in terms of
changes in forces is used causing a switching between the
pathways. The model tends to optimize the water balance
by adjusting root hydraulics according to the demand from
the shoot. Thus, the composite transport model provides
some kind of integration of views to explain the variability
of water uptake.
The basic ass umption is that the force driving water
from the soil through the plant to the atmosphere (Fig.4) is
the gradient at the energy level of the water along th is
pathway. In the so il, this level is expressed by the water
potential of the soil around the root. The vapor pressure of
the air is a measure of the level of free energy of water in the
atmosphere which is almost always lower than that of the
water in the soil. Hence, the driving force is in the direction
from s oil towards the atmos phere. The pathway from the
soil up to the evaporating surfaces in the leaves is assumed
to consist of a continuum of liquid water where the strong
cohesive forces between water molecules provide a strong
chain that can sustain the strong driving forces (tensions).
Therefore, Philip (1966) proposed a more integrated physi-
cal model of the Soil Plant Atmosphere Continuum (SPAC).
The concept of SPAC resulted in the introduction of math-
ematical models of water up take by roo t systems. SPAC
involves some aspects of water movement in the soil such
as unsaturated hydraulic conductivity and water diffusibil-
ity as well as water uptake by plant roots, water movement
in p lant roo ts as described by hydraulic res istances and
capacities (Shao and Huang, 2000). The water t ransport
rate (Jv) can be calculated from the follow equation: Jv = Lp
× Dyw, where Jv is the volume flow of water across the
membrane per unit area of membrane and per unit time
(m3·m-2·s-1). Water movement through the root system is a
very important component of the SPAC. Dynamic hydrau-
lic properties of roots differ between species and between
different parts of the same roots (depending on age). To
some extent, mathematical models can be used to account
for a quantitative description of water uptake by roots. Such
models of water uptake by roots can be looked at as a basis
of agricultural moisture management of dryland farming used
to op timize irrigation sys tems. However, in the fu ture,
present physical models need to be extended to incorpo-
rate detailed plan t anatomy (such as apoplastic barriers),
root biochemistry and the gating of water channels.
7 Conclusions
A higher WUE at a given water uptake is a desirable trait
for plants. Improving WUE of plants has been always an
important goal during plant breeding and for cultivation
practices. The effective use of precipitation and optimiza-
tion of WUE are critical for promoting crop productivity of
dryland farming systems (Shan, 1998; Shan and Chen, 1998;
Shan, 2002). The focus should be on whole-plant processes
that enhance or maintain water uptake by roots, increase
the p lant’s capacity to retain water in a desiccat ing
environment, or maintain a positive carbon balance during
prolonged periods of water shortage. Avoidance of severe
water deficit requires coordination at the whole level be-
tween the control of water loss from transpiring shoots and
water absorption through root systems. Thus , it is impor-
tant to understand the regulation of water transport in plants
and the basic mechanisms of water uptake by roots. Only
in this way it is possible to combine the present knowledge
of plant adaptation and water use with available technol-
ogy to control the efficient use of limited water resources.
So far, relatively little is known about the processes that
govern or even regulate root water uptake. From transport
and anatomical studies it is clear that the composite struc-
ture of roots and the distribution conditions of roots play a
key role during the regulation of water up take by roo ts.
With new regulatory mechanisms to be discovered in plants,
the water transport propert ies of membranes now appear
as a new and important focus in modern plant physiology
and agronomy.
During variable periods of water supply, roots are opti-
mized in their ab ilities to us e water resources in the s oil.
Future work should concentrate on a more detailed map-
ping of hydraulic resistance in the root cylinder and how
Fig.4. Soil-Plant-Air-Continuum (SPAC).Water is transferred
from the soil through the plant to the atmosphere. The force
driving the water across the SPAC is the gradient in water poten-
tial which is in the direction from soil towards the atmosphere.
Acta Botanica Sinica 植物学报 Vol.46 No.5 2004512
this would change in response to water and other stresses.
There are only a few alternat ives to the composite trans-
port model to explain the finding of variable root water up-
take (Fiscus, 1975; Wheatherley, 1982; Boyer, 1995), which
is crucial fo r our unders tanding of overall p lant water
relations. A lthough a link has been established between
root hydraulics and expression and function of aquaporins,
the knowledge of cell-specific expression, specified loca-
tion and funct ion of root aquaporins is largely lacking,
mainly due to the high divers ity of aquaporin isoforms in
plants. In the future, analysis of single knock-out aquaporin
mutants will hopefully provide evidence for the mult iple
functions of aquaporins in the growth and development of
plants and in their adaptive response to st resses. Further
investigat ions are necessary to determine the molecu lar
structure of the water pores and the mechanisms of its se-
lectivity and gating. There are, to date, no novel approaches
to estimate the relative contribution of the three water trans-
port pathways to the overall uptake on hydraulic conduc-
tivity o f roots. With respect to water stress signaling, we
are still far from having a clear pictu re. Models of water
uptake by root systems are still only semiquantitative and
requ ire completion. In order to set up good quantitative
(physical) models, we must combine mathematical principles
and computer technology with biological principles, includ-
ing some biophysics and biochemistry. Studies o f root
physiology and of key genes that regulate water transport,
shou ld complement traditional studies of shoot phys iol-
ogy and stomatal control of water loss, are fundamental for
the understanding of plant water balance.
Many researchers have cont ributed a large amount of
valid work in the mechanisms of water uptake by roots. Yet
there are still a lot of aspects, which need perfection and
improvement. Intense research is under way in different
laboratory to clarify mechanisms. Therefore, the future prom-
ises to see a much clearer picture of the basic mechanisms
for water uptake by plant roots.
Acknowledgements: We thank Mrs. Libuse Badewitz,
University of Bayreuth, Germany, for preparing some of
the drawings for the manuscript.
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