Botanical Studies (2006) 47: 145-151.
*
Corresponding author: E-mail: sqzhang@ms.iswc.ac.cn;
Tel: +86-29-87010897 (O); Fax: +86-29-87092387.
Hydraulic conductivity of whole root system is better
than hydraulic conductivity of single root in correlation
with the leaf water status of maize
Zixin MU
1,3
, Suiqi ZHANG
2,3,
*, Linsheng ZHANG
1
, Aihua LIANG
1
, and Zongsuo LIANG
1,2
1
Life Science College of Northwest A&F University, Yangling Shaanxi 712100, P. R. China
2
Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Yangling
Shaanxi 712100, P. R. China
3
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Yangling Shaanxi 712100, P. R. China
(Received June 20, 2005; Accepted November 23, 2005)
ABSTRACT.
Under hydroponic culture conditions, we studied the relationship between two different types
of root hydraulic conductivity of maize (Zea mays L.) and its leaf water status. The results have proved the
inaccuracy of the single root hydraulic conductivity (Lp
sr
) to describe the ability of water uptake by maize
roots, which can be better described by the whole root systems hydraulic conductivity (Lp
wr
). Moreover, Lp
wr
can be measured easier. Although the whole roots surface area (WRA), which integrated all the root charac-
ters, such as root dry weight, volume and length, represent the interaction between root and soil (or water in
soil solutions) better, there is no significant relationship between WRA and Lp
wr
and leaf water potential
be-
cause part of the roots has no activity for water uptake. However, active root surface area (ARA) truly reflects
the level of root metabolic activity and root function efficiency, i.e., the ratio of the active roots to the whole
roots. Hence, ARA has a significant linear correlation with Lp
wr
. Because of the high plasticity of plant root
systems architecture and metabolism under changed water and nutrition conditions, the relationship between
single root Lp
sr
and whole root Lp
wr
and between WRA and ARA are not positively linear correlated. Results
demonstrate that the whole root Lp
wr
described by ARA can reflect more accurately both the water uptake by
plant roots and the leaf water status than the single root Lp
sr
.
Keywords: Active root surface area (ARA); Maize; Single root hydraulic conductivity (Lp
sr
); Water channels
(aquaporins); Whole root surface area (WRA); Whole root systems hydraulic conductivity (Lp
wr
).
INTRODUCTION
Root hydraulic conductivity (Lp
r
), is one of the major
parameters reflecting root water uptake ability. It has a
close correlation with plant water relations under both
normal and stressed conditions. The study of root water
uptake has been made progress recently from the ana-
tomical structure of the root to molecular level, i.e., water
channel (aquaporin) activity (Steudle, 2000a, b; 2001).
Moreover, many measurements are developed includ-
ing transpiration method, pressure chamber, potometers,
root pressure probe and cell pressure probe (Zwieniecki
and Boersma, 1997; Barrowclough et al., 2000; Steudle,
2000a). However, due to the high plasticity of plant root
systems both in architecture and metabolism (Gunse et al.,
1997; Liang et al., 1997; Joslin et al., 2000; Linkohr et al.,
2002; Lopez-Bucio et al., 2003), and the different proper-
ties among various measuring methods, the root Lp
r
were
highly variable even for the same plant species. These re-
sults in a meaningless comparison between different data,
and even results in confusion. For example, Lp
r
was usu-
ally used to describe plant root water uptake abilities, but
most of these results are obtained from root pressure probe
or cell pressure probe measurements, which mean most of
them are data of single root hydraulic conductivity (Lp
sr
)
or root cell hydraulic conductivity (Lp
cr
). Because of the
variety of single root development phase in root systems,
and even the tissue-specific hydraulic conductivity along a
single root could be different (Barrowclough et al., 2000;
North and Nobel, 1998, 2000), many questions remain to
be answered. Two of the key questions are (1) whether Lp
sr
or Lp
cr
should be used to describe the capability of whole
root system water uptake and water transport, and (2) what
is the contribution of whole roots surface area (WRA) to
the root water uptake. In this paper, we present results to
illustrate the relationship of two types of root hydraulic
PHYSIOLOGY
pg_0002
146
Botanical Studies, Vol. 47, 2006
conductivity and of two types of root surface area, respec-
tively, in hope to study their contributions to plant water
relations.
MATERIALS AND METHODS
Plant materials and growth conditions
Seeds of maize (Zea mays L. HD4) were sterilized for
20 min in 0.2% HgCl
2
solution. After washing several
times with distilled water, seeds were transferred to a cul-
ture medium mixed with vermiculite and quartz sand (v/v;
2/3) for 3 d. Temperature was kept at 25¢XC during the ger-
mination. When roots were 5-6 cm long, seedlings were
transferred to plastic barrels (depth: 20 cm and diameter:
18 cm; contained 2 seedlings each). Barrels were cov-
ered by double layer black plastic cloth to keep the roots
in dark. Barrels were placed in a climate chamber (KG-
206SHl-D, made in Japan). Growing conditions were light
intensity: 250-300 £gmol photons m
-2
s
-1
; day/night rhythm:
14/10 h; temperature 27/20oC; Relative humidity: 60-70%.
Everyday air pump was used to ventilate the solution 3-4
times, each time 1 h. The growing solution was replaced
every 2 d. At first, the barrels were filled with distilled wa-
ter, and then replaced by half-strength Hoagland nutrient
solution after seedlings have adapted for 1 d. There were
three nutrition treatments: control (half-strength Hoagland
nutrient solution); low N (N concentration was 1/3 of
the control, using 1.7 mol/l CaCl
2
and 1.7 mol/l K
2
SO
4
to complement the concentration of Ca
2+
and K
+
); low P
(P concentration was 1/3 of the control, using 0.85 mol/l
K
2
SO
4
to complement K
+
). In addition, there were two
water conditions: control (no water stress) and water stress
simulated by
adding
PEG-6000 to the growth medium
(water potential: -0.2 MPa). 15 d old seedlings (including
germination) were used in the experiments. Each treat-
ment has six repetitions.
Root hydraulic conductivity (Lp
r
) measurements
Root hydraulic conductivity was measured using the
pressure chamber method as described by Javot et al.
(2003) with some modifications. Root system or single
root (primary root) detached from maize seedlings were
inserted into the container of a pressure chamber filled
with growing solution. The cut stump was put carefully
through the soft plastic washer of the metal lid. The seal
was tightened using a low-viscosity dental paste. A bal-
ance pressure (P
0
), which was the ex-pressure when the
sap exuded initially, was determined at first. Pressure was
increased from P
0
(MPa) to P
0
+ 0.5 (MPa) at an interval
of 0.1 MPa. Under each pressure, when the flow rate was
stabilized (about 5 min), the exuded sap (V, m
3
) was col-
lected for 5 min. The collections were repeated at least
three times at a 1 min interval. The weight of the exuded
sap was determined using an analytical balance with an
accuracy of 0.01 mg. After experiments, root surface
area (S, m
2
) was measured. The flow rate Jv (m¡PS
-1
) was
calculated by Jv = V/ (S¡Ñt). Root hydraulic conductivity,
Lp
r
(m¡PS
-1
¡PMPa
-1
) was determined from the slope of the
regression line by plotting Jv against hydrostatic pressure
(P
0
- (P
0
+ 0.5) MPa), i.e., Lp
r
= Jv /
.
P.
Root surface area measurements
The whole roots surface area (WRA) was measured
using Root Image Analysis Software CID-400 (CID, Inc.
Vancouver, WA). Samples were washed and dyed by 0.5%
methylene blue for 10 h. Then the image of the roots was
scanned by a numerical scanner to analyze the WRA.
The determination of active root surface area (ARA)
was described by Zou (1995).
Methylene blue solution
was used at a concentration of 0.2 mM. The volume of
methylene blue solution used was about 10 times of the
root samples volume, and was divided into three beakers
marked by No. 1, 2, 3, respectively. The sample roots,
washed with distilled water and the residual water on its
surface removed with filter paper, were soaked into the
three beakers one after the other for 1.5 min in each bea-
ker. Then 0.5 ml solution was taken out from each beaker,
diluted by 20 times with distilled water, and the OD val-
ues were measured in 660 nm wavelengths. The residual
amount of methylene blue in three beakers were obtained
according to standard curve, and calculated the amount of
methylene blue absorbed by root samples in each beaker.
ARA (m
2
) = A
3
¡Ñ 1.1 m
2
(ARA is active root surface area; A
3
is the amount of
methylene blue absorbed by root samples in No. 3 beaker;
1.1 m
2
is the area of methylene blue when it is presented
in a single molecular layer).
Leaf water potential (
£Z
w
) document
Leaf £Z
w
was measured using pressure chamber under
excised conditions as described by Turner (1988). Com-
pletely expanded leaves were sampled from the upper part
of the maize seedlings.
Statistics
All data were analyzed by Microsoft Excel and SPSS
software.
RESULTS
There is different changing trends for both Lp
wr
and Lp
sr
under different water and nutrition
conditions
It can be seen that regardless of water conditions, both
N- and P-deficiency caused a decrease in Lp
r
of maize, not
only in single root but also in whole root system (Figure
1A, B). The difference is, compared with N-deficiency
treatments, the P-deficiency treatments had a higher Lp
wr
but a lower Lp
sr
. Except of N-deficiency treatment, water
has lower effect on whole root system than on single root,
because water stress made the Lp
wr
of Control and P-defi-
ciency treatments declined 36.2% and 22.7%, respectively,
pg_0003
MU et al. ¡X
Hydraulic conductivity of whole root system vs single root
147
and made the Lp
sr
of them declined 52.5% and 42.8%, re-
spectively. This meant that for the same plant, its Lp
wr
and
Lp
sr
response differently to both the same and different
environmental stress.
There are different changing trends for both
WRA and ARA under different water and
nutrition conditions
Table 1 indicated that regardless of water conditions,
the ratio of ARA/WRA varied between N- and P- defi-
ciency. Compared with N-deficiency treatments, the P-de-
ficiency treatments had a higher ARA but a lower WRA,
and therefore had a high ratio of ARA/WRA. Drought also
decreased both ARA and WRA, but the decreasing degree
are different among the three nutrient conditions. For
ARA, the decreasing degree for Control and N-deficiency
treatments are greater than for P-deficiency treatment
(They are 48.3%, 48.8% and 42.8% respectively), howev-
er, for WRA, the decreasing degree for Control and N-de-
ficiency treatments are lower than P-deficiency treatment
(They are 28.7%, 27.8% and 34.4% respectively), hence
the ratio of ARA/WRA for (Low P + PEG) treatment is
nearly equal (Control + PEG) treatment and significantly
greater than (Low N + PEG) treatment. In summary, it is
clear that, N-deficiency had effect on the absolute values
of ARA while P-deficiency affected the relative values, the
two nutrients function differently in root area served as
water uptake. This also indicated that ARA and WRA have
different response to environmental stress.
Relationships between the Two Types of Lp
r
and
leaf water potential (
£Z
w
)
The root system contains various single roots. Due to
different growing conditions and the spatial distributions
along root axis, the Lp
sr
differed significantly (Doussan
et al., 1998; North and Nobel, 1998, 2000; Jackson et al.,
2000). Although we selected the same position as well
as the same age of the seedlings, we found that there was
non-relevant or low relevant between single root Lp
sr
and
its leaf water potential (Figure 2B). However, as Figure
2A showed, there is a significant positive relation between
Lp
wr
and leaf water potential.
Table 1. Root surface area of maize under two water and three nutrient conditions.
Treatments
Control Control + PEG Low N Low N + PEG Low P
Low P + PEG
ARA (cm
2
)
110.61¡Ó10.45
a
57.24¡Ó3.54
b
50.43¡Ó2.75
bc
25.82¡Ó4.28
d
61.41¡Ó2.36
b
35.15¡Ó1.78
c
WRA (cm
2
)
127.00¡Ó8.21
a
90.50¡Ó5.32
c
101.51¡Ó12
b
73.51¡Ó3.85
cd
87.45¡Ó1.65
c
57.39¡Ó3.21
d
ARA/WRA (%)
87¡Ó8
a
63¡Ó4
bc
49¡Ó7
c
35¡Ó4
d
70¡Ó2
b
61¡Ó2
bc
Data (means ¡Ó SE) were compiled from individual measurements (n=20), and the experimental conditions are correspondence with
Lp
r
measurements. a: n=15; b: n=10. WRA: whole root surface area; ARA: active root surface area. Superscripted letters indicate
statistically different groups at (p < 0.05).
Figure 1. The single root hydraulic conductivity (Lp
sr
) (A) and whole root systems hydraulic conductivity (Lp
wr
) (B) under six treat-
ments. The seedling was 15d old, and treated 10d before measurement. Lp
r
was determined at the same time each day, and maintained
the environmental temperature held constant. Each bar is the means ¡Ó SE (for Lp
sr
, n=30; for Lp
wr
, n=10). Different letters are used to
indicate means that differ significantly (p < 0.05).
pg_0004
148
Botanical Studies, Vol. 47, 2006
Relationships between the two types of root
surface area and leaf water potential (
£Z
w
)
There was almost non-relevant between WRA and leaf
water potential by making regression analysis, while a sig-
nificant positive relevant between ARA and leaf water po-
tential had been found (Figure 3A, B). This indicated that
leaf water status is mainly correlated with root metabolism
activity (or ARA), but not with root bio-mass (or WRA).
Relationships between two types of root
surface area and whole root Lp
wr
It is well known that under normal conditions of both
water and nutrition, root surface area increased, the root
volume conducting water is increased too. This in turn,
increased the ability of root water uptake (Jackson et al.,
2000). However, with the changes of growing conditions,
especially in the case of water or nutrition deficient con-
ditions, the WRA is not always identical with the ARA
(Table 1). This may due to varieties of root systems in
architecture and metabolism activityzones. For roots, the
main part to absorb water is the ARA. Figure 4 showed a
positive linear relationship between ARA and whole root
Lp
wr
, but it is non-relevant between WRA and whole root
Lp
wr
.
DISCUSSIONS
The whole root Lp
wr
, is not equal single root
Lp
sr
, and correlates with the leaf water potential
better
In previous study, we have explored the difference be-
tween Lp
sr
and Lp
wr
among maize varieties under single
nutrition stress, and explained the phenomenon in term
of the plasticity of root architecture and of water channel
activity (Mu et al., 2003). In the present paper, from wa-
ter and nutrient double stresses, we found that phosphate
stress had stronger effect on Lp
sr
, but less effect on Lp
wr
than nitrate stress (Figure 1A, B).
Single root Lp
sr
and whole root Lp
wr
reflect root water
uptake from two different levels. The whole root Lp
wr
can
better reflect the indirect effect of both developmental
and environmental factors on the root water uptake and
represents an integrated effect among different single root
(Lopez-Bucio et al., 2003). In contrast, the single root Lp
sr
relatively reflects the effects of extra- and intro-factors
on the water transport resistance through single root cyl-
inder. It has been shown in the literatures that, depending
on transpiration requirements, roots are able to switch
between apoplastic pathway and cell-to-cell pathway (the
Figure 2. Relation between two root hydraulic conductivity (A: whole root systems hydraulic conductivity, Lp
wr
; B: single root hy-
draulic conductivity, Lp
sr
) and leaf water potential (£Z
w
). *Means significant at 0.05 level.
Figure 3. Relation between both root surface area (A: whole root surface area, WRA; B: active root surface area, ARA) and leaf water
potential (£Z
w
). **Means significant at 0.01 level.
pg_0005
MU et al. ¡X
Hydraulic conductivity of whole root system vs single root
149
composite transport model; Steudle, 2000a, b; 2001). It
has been shown too that the activity and abundance of
water channel proteins (or aquaporins) in the root plasma
membrane play a significant role in the cell-to-cell path-
way for maize (Maurel and Chrispeels, 2001; Chaumont et
al., 2001; Aroca et al., 2005). Interestingly plant aquapo-
rins were regulated (gated) by phosphorylation, pH, pCa,
osmotic pressure and salinity, heavy metals, temperature,
nutrient deprivation, or anoxia and oxidative stresses
(Luu and Maurel, 2005). This means the characteristics
of aquaporins represent a highly plasticity like plant
root architecture, which together result in a highly vari-
able root hydraulic conductivity under different external
environments. In a certain condition, such as a limited wa-
ter or nutrition deprivation, the decreased ability of single
root water uptake might be compensated for by the in-
crease of whole absorbing area of the root systems¡Xan in-
crease of whole root systems water uptake ability (Jackson
et al., 2000; Javot and Maurel, 2002; Mu et al., 2003). At
the same time, in the
circumstance of
a relative high single
Lp
sr
(such as water or nutrition localized addition)
¡A
the
whole root Lp
wr
may be not the highest due to the decrease
of whole root system area. Thus the relationship between
the single root Lp
sr
and the whole root Lp
wr
are not linear
(Mu et al., 2003), and the latter can better reflect leaf wa-
ter status (Figure 2).
The root active surface area ARA represents
better hydraulic conductance and leaf water
potential than WRA
The multitude of fine roots is the most active part of
the system in acquiring water and nutrients, with its own
multitude of root tips, sites of intense chemical activity
that strongly modify the soil around them, and mobilize re-
luctant ions (McCully, 1995, 1999). Since different water
or nutrients application bring out variable of the fine roots,
for example, in Arabidopsis, N- or P-nutrients deprivation
had a contrasting effect on its lateral roots and root hairs,
i.e., the former kept a constant lateral root density while
the latter increased it, and both of them accelerated lateral
root elongation (Linkohr et al., 2002). Besides, it has been
shown that phosphate stress significantly increased root
hair density and length (Bates and Lynch, 1996; Ma et
al., 2001). Under water stress conditions, especially in the
fields, the drought is preceding gradually from the upper
soil layers to the deeper soil layers. Often, the deeper roots
are active roots as compared with the upper ones. How-
ever, when water was re-supplied, the situation reversed
because more fine roots could be produced from the upper
roots (Liang et al., 1997; Jackson et al., 2000). Evidence
suggests that compared with N-deficiency, P-deficiency
greatly increased active roots area, as found (Table 1).
In addition to environmental conditions, root activ-
ity can also be regulated by the developmental status of
the roots (Schiefelbein and Benfey, 1991; Lynch, 1995;
Blum and Sullivan, 1997; Graham and Nobel, 1999). In
conclusion, plant root systems are plasticity in both archi-
tecture and metabolism activity resulting in the difference
between WRA and ARA. The former contributes to root
biomass, whereas the latter, which is the major functional
section of whole root systems and usually distributes in
the root hair region, contributes to root functional efficien-
cy (such as water uptake ability) (Figure 3B, Figure 4A).
The axial resistance in xylem does not be
ignored under stress conditions
Xylem vessels, composed of dead cells, well known
for their low resistance in water transport, were usually
ignored. However, under stress conditions, especially un-
der water deficient, cavitations occurred in xylem could
significantly increase the axial resistance (decrease the hy-
draulic conductivity) (Zwieniecki et al., 2001; Stiller et al.,
2003). Except for refilled after rewater, Zwieniecki et al.
(2001) found that increasing concentrations of ions flow-
ing through the xylem of plants produced rapid, substan-
tial, and reversible decrease in hydraulic resistance, and
this ion-dependent mechanism allow plants to compensate
for decrease in hydraulic conductivity induced by cavita-
tions. It is very likely that this is also the mechanism of
improving plant drought-resistance by nutrition addition.
Figure 4. Relation between two type root area (A: active root surface area, ARA; B: whole root surface area, WRA) and whole root
hydraulic conductivity (Lp
wr
). ** Means significant at 0.01 level.
pg_0006
150
Botanical Studies, Vol. 47, 2006
It is clear that root cell Lp
cr
cannot reflect root axial re-
sistance, while single root Lp
sr
can. However, because sin-
gle roots in different layers along root systems axis have
different radius and length of xylem conduits, their axial
resistance differed greatly (Jackson et al., 2000). Thus,
whether from radial hydraulic conductivity or from axial
hydraulic conductivity, can Lp
sr
only reflect the hydraulic
properties of one layer or of one site where the single root
located (Figure 2B). On the other hand, the axial resis-
tance in stem xylem also affects water transport under
stress conditions, and in turn affects plant water relations.
For the Lp
wr
measured in the present study, we retained
a segment of stem, so the data also partially reflect the
capacity of water transport in the stem. We conclude that
whole root Lp
wr
not only reflects plant root water uptake
ability, but also represents root water transport capacity,
due to it integrates both radial and axial resistances of the
root system (Figure 2A).
Acknowledgements. We are grateful for the support of
the Key Innovation Project of the Chinese Academy of
Science (KZCX3-SW-444), the Chinese National Natural
Science Foundation (30571127), the Innovation Project of
the Institute of Soil and Water Conservation, the Chinese
Academy of Sciences, the Ministry of Water Resources
(SW05101), and the Youth Science Foundation of
Northwest A&F University.
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