Tree Physiology 19, 619--624
© 1999 Heron Publishing----Victoria, Canada
Can differences in root responses to soil drying and compaction
explain differences in performance of trees growing on landfill sites?
JIANSHENG LIANG,1 JIANHUA ZHANG,2 GILBERT Y. S. CHAN3 and M. H. WONG2
1
College of BioSciences and Biotechnology, Yangzhou University, Jiangsu, Peoples Republic of China
2
Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong
3
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
Received August 18, 1998
Summary Two tropical woody species, Acacia confusa Merrill and Litsea glutinosa (Lour.) C.B. Robinson, were grown
under controlled conditions in PVC pipes filled with John Innes
No. 2 soil. To investigate root distribution, physiological characteristics and hydraulic conductivity, four soil treatments were
imposed----well-watered and noncompacted (control), wellwatered and compacted; unwatered and noncompacted, and
unwatered and compacted. In L. glutinosa, rooting depth and
root elongation were severely restricted when soil bulk density
increased from around 1.12 to 1.62 g cm −3, whereas soil compaction had little effect on these parameters in A. confusa. As
soil drying progressed, root water potential and osmotic potential declined more slowly in L. glutinosa than in A. confusa.
Both the soil drying and compaction treatments significantly
stimulated the accumulation of root abscisic acid (ABA) in
both species. Soil drying damaged the root cell membrane of
A. confusa, but had little influence on the root cell membrane
of L. glutinosa. Soil drying had a greater effect on root hydraulic conductivity (Lp) in L. glutinosa than in A. confusa, whereas
the effect of soil compaction on L p was less in L. glutinosa than
in A. confusa. Soil drying enhanced the effects of soil compaction on root Lp. We conclude that soil drying and compaction
have large species-specific effects on the distribution, growth
and physiology of roots. The relationships of these root properties to the species’ ability to tolerate unfavorable soil conditions were examined.
Keywords: abscisic acid, Acacia confusa, Litsea glutinosa,
root distribution, root hydraulic conductivity, root physiology,
soil compaction, soil drying.
Introduction
Difficulties in vegetating sanitary landfill sites, because of the
compaction of solid wastes and cover soil by large machines
during the disposal process, have been documented over the
last two decades in many places in the world (USA: Flower et
al.1978, Finland: Ettala et al. 1988, UK: Wong 1988, Hong
Kong: Wong and Yu 1989). One consequence of soil compaction is a high soil mechanical impedance to root growth and
function. The soil bulk density of landfill soils in Hong Kong
is between 1.6 and 1.8 g cm −3 (Liang et al. unpublished data).
High soil mechanical impedance seriously restricts rooting
depth and root growth, resulting in shallow root systems that
are often a limiting factor in crop growth and yield (Klepper
and Rickman 1991). Morphological changes, including reductions in root branching, increases in radial expansion, proliferation of lateral roots, and cellular distortion of roots,
frequently occur in plant roots grown in compacted soil (Goss
1977, Russell 1977, Hartung et al. 1994, Tsegaye and Mullins
1994). In maize, the total length of lateral roots per centimeter
of main axis decreases as soil mechanical resistance increases
(Boone and Veen 1982). Smucker and Atwell (1988) concluded that the reduction in root growth in mechanically impeded soil results from both a reduction in cell division and
proliferation and a decrease in the length of the fully extended
cells. In field-grown winter wheat, Masse et al. (1988) showed
that the depressive effect of soil compaction on rooting depth
and root spatial arrangement appears only at the beginning of
plant growth.
Root function is also affected by soil compaction. Soil
compaction generally reduces water and nutrient uptake
(Shierlaw and Alston 1984, Agnew and Carrow 1985), and it
also reduces water-use efficiency (Agnew and Carrow 1985).
Shallow root systems, as a result of high soil mechanical
impedance, render plants more susceptible to soil drought
(Chan et al. 1991). When roots are exposed to drying soils,
plant growth is usually inhibited (Saab et al. 1990, 1992).
There is a body of evidence indicating that the root is a sensing
organ that measures rhizospheric conditions and produces the
stress signal abscisic acid (ABA), which is transported through
the xylem transpiration stream to shoots where it regulates
physiological processes (Davies and Zhang 1991, Liang et al.
1996b, 1997a, 1997b). Soil strength usually increases during
dehydration; however, it is not clear whether the roots are
sensing soil water status or increased mechanical resistance, or
both.
In the present study, two tropical tree species, Acacia confusa Merrill and Litsea glutinosa (Lour.) C.B. Robinson, which
differ in their response to the landfill environment----with the
former species being tolerant to and the latter species being
susceptible to soil drying and soil compaction----were used to
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LIANG, ZHANG, CHAN AND WONG
study the effects of soil compaction and drought on root
growth and root functions. The objective was to explain why
some plants perform poorly on landfill sites and to provide
guidelines for the selection of suitable species or genotypes for
revegetating landfill sites.
Materials and methods
Plant materials
Seedlings of Acacia confusa and Litsea glutinosa were purchased from a local nursery. One hundred, 3-month-old seedlings, about 30 cm in height, were transplanted to PVC pipes
(12 cm in diameter, 50 cm in height) perforated at the bottom
for drainage and filled with John Innes No.2 soil. The pipes
were vertically bisected and the halves taped together. For the
soil compaction treatment, soil was compacted with a steel rod
layer by layer as described earlier (Liang et al. 1996b). For the
non-compacted treatment pipes were filled with loose soil
without compaction. The bulk densities were 1.62 g cm −3 and
1.12 g cm −3 for compacted and non-compacted treatments,
respectively. A hole (10 cm in depth, 3 cm in diameter) was left
in the middle of the soil column for seedling transplantation
and was then filled with the same soil after transplanting. The
trees were placed in a greenhouse at a temperature of 20 to
28 °C and minimum photosynthetic active radiation (PAR) of
400 µmol m −2 s −1 at plant height supplemented with high-pressure sodium lamps and watered daily. Twelve days later, when
trees were well established (manifested by the appearance of
new leaves), a soil drying treatment was imposed by withholding water from half the plants in the soil compaction and
non-compaction treatments for 28 days. The remaining plants
in each treatment were watered daily and denoted the well-watered treatment.
Soil sampling and determination of soil water content
Soil at a depth of 0 to 15 cm was sampled with a punch (1.0 cm
in diameter and 20 cm in length). Four samples were collected
per treatment. Fresh soil was weighed and then oven-dried at
80 °C to constant weight. Soil water content was calculated
based on soil dry weight. Soil water content decreased as soil
drying progressed in both the soil compaction and non-compaction treatments (Figure 1).
Observation of root distribution
Two months after transplanting, root samples in each 10-cm
layer of soil were harvested and separated from soil by hand.
The length and dry weight of roots were measured according
to the procedures described by Liang et al. (1996a). Four plants
per treatment were measured.
Measurement of root water relations
Three to four 4-mm long root tips were sampled from the
10--20 cm soil layer and sealed in thermocouple chambers that
were connected to a Wescor HR-33T dew-point microvoltmeter (Wescor Inc., Logan, UT). Samples were incubated for 2 h
at 25 °C before water potential was measured. Following
Figure 1. Changes in soil water content at 0--15 cm depth following
treatments. Symbols: Well-watered and non-compacted treatment (s);
well-watered and compacted treatment (h); unwatered and non-compacted treatment (n) unwatered and compacted treatment (,). Values
are means ± SD of four measurements.
measurement, the samples were wrapped with aluminum foil
and plunged in liquid nitrogen. Osmotic potential was then
measured by a procedure similar to that used for the determination of water potential.
ABA assay of root samples
For assay of ABA, root samples were harvested from four
plants per treatment. Roots were washed thoroughly with tap
water and 5-cm root tip segments were collected from roots
that had been located in the 10--20 cm soil layer. The sampled
root tips were immediately plunged into liquid nitrogen,
ground and stored in a desiccator until assayed for ABA.
Abscisic acid was assayed by the radioimmunoassay (RIA)
method as described by Liang et al. (1997a). Highly specific
monoclonal antibody was provided by Dr. S.A. Quarrie. About
100 mg of ground root samples was extracted by adding 4 cm3
of double-distilled water and shaking at 4 °C for at least 24 h.
Then 50 mm3 of aqueous extract was mixed with 200 mm3 of
phosphate-buffered saline (pH 6.0), 100 mm3 of 3H-ABA
(about 20,000 dpm) and 100 mm3 of diluted antibody solution.
Reaction mixtures were incubated at 4 °C for 45 min. The
3
H-ABA bound with the antibody was precipitated with 50%saturated (NH4)2SO4 solution. The radioactivity of the precipitates was measured by liquid scintillation spectrometry.
Measurement of root cell membrane integrity
Root tip segments about 5 cm long were collected and washed
with tap water 5--6 times and then cut into 1-cm long segments
with scissors. The root segments were again washed with
deionized water at least 3 times and placed in 10-ml beakers
containing 5 ml of de-ionized water, and incubated at room
temperature for 24 h. The electrical conductivity of the bathing
solution was measured at 25 °C with a digital conductivity
meter (PW9526, Philips Co., Eindhoven, The Netherlands).
Following the conductivity measurements, beakers with root
TREE PHYSIOLOGY VOLUME 19, 1999
ROOT RESPONSES TO SOIL DRYING AND COMPACTION
621
samples were incubated in a boiling water bath for 15 min and
immediately cooled to 25 °C in flowing water. The electrical
conductivity of the cooled bathing solution was remeasured at
25 °C. Root cell membrane integrity (CMI) was evaluated as:
CMI = (EC2 -- EC 1) / EC2 × 100 ,
where EC1 and EC2 are the electrical conductivities of the root
bathing solution before and after incubation in boiling water,
respectively.
Measurement of root hydraulic conductivity
Whole-root hydraulic conductivity was measured in seedlings
that had been grown in a special type of PVC pipe (5.5 cm in
diameter, 25 cm in height) that fitted into a pressure chamber
(Console 3000 Series, Soil Moisture Equipment Corp., Santa
Barbara, CA) so that hydraulic conductivity measurements
could be made in situ (cf. Zhang et al. 1995). Treatments were
similar to those described previously. Measurements were
made 7 days after the imposition of the treatments when the
soil water content was 0.1 g g −1 soil dry weight. Before
measurement, the pipes were immersed in a water bath at
25 °C for 20 min and then placed in the pressure chamber.
Preliminary tests showed that, after 20 min of immersion in
water, the soil air spaces inside the pipes were entirely filled
with water and the xylem sap flux showed no changes for about
1 h at a given pressure, indicating that the resistance between
the root surface and the surrounding soil particles was constant. The plants were de-topped with a sharp razor blade 5 cm
above the soil surface and the root system pressurized. The root
exudate was collected in 1.5 cm3 Eppendorf tubes every 7--10
min and the volume of exudate was determined by weighing.
Whole-root hydraulic conductivity was calculated based on
the slope of the volume flux versus pressure curve (Zhang et
al. 1995).
Figure 2. Root distribution (showing total root length) of Acacia
confusa (a) and Litsea glutinosa (b) in non-compacted soil (blank
bars) (soil bulky density: 1.12 g cm − 3) and compacted soil (hatched
bars) (soil bulk density: 1.62 g cm − 3). Values are means + SD of four
measurements.
A. confusa (data not shown).
Under well-watered conditions, soil compaction did not
significantly influence root water relations of either species.
However, under conditions of soil drying, the two species
showed a marked difference in root water relations in response
to soil compaction (Figure 3). As soil drying became severe,
root water potential and osmotic potential of A. confusa decreased steadily reaching values of − 1.7 and −2.1 MPa, respectively, at the end of the 28-day experiment in both the
compacted and non-compacted treatments (Figures 3a and 3c).
Results
The ability of roots to penetrate a compacted soil varies with
plant species. Figure 2 shows the root distribution of A. confusa and L. glutinosa in compacted and non-compacted soils.
When growing in non-compacted soil (soil bulk density = 1.12
g cm −3), the roots of both species penetrated to a depth of
40 cm during the experimental period. When soil bulk density
was increased to 1.62 g cm −3, root penetration of L. glutinosa
was largely inhibited and most parts of the root system were
localized within the 0--20 cm soil layer, whereas some roots of
A. confusa penetrated the deeper soil layers, indicating that the
roots of A. confusa had a greater relative ability to penetrate
compacted soil than roots of L. glutinosa.
Root morphological characteristics were also affected by the
high soil mechanical impedance. Compared with roots growing in non-compacted soil, roots of L. glutinosa growing in the
compacted soil showed an increased diameter, especially near
the root tips, resulting in the formation of deformed roots.
Furthermore, root surfaces became brown. There were no
obvious effects of soil compaction on root morphology of
Figure 3. Effects of soil drying and compaction on root water potentials and osmotic potentials of Acacia confusa (a, c) and Litsea glutinosa (b, d). Symbols: well-watered and non-compacted treatment (s);
well-watered and compacted treatment (h); unwatered and non-compacted treatment (n) unwatered and compacted treatment (,). Values
are means ± SD of four measurements.
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622
LIANG, ZHANG, CHAN AND WONG
Both root water potential and osmotic potential decreased
more slowly with soil drying in L. glutinosa than in A. confusa
and so values of both parameters were higher at the end of the
28-day experiment (Figures 3b and 3d). Moreover, the water
potential of L. glutinosa roots in the drying + compacted soil
treatment showed no further decrease after 20 days of treatment (Figure 3b).
There was a significant difference in root cell membrane
integrity (CMI) in response to soil drought and compaction
between the two species. No substantial treatment effects were
observed on root CMI of L. glutinosa (Figure 4b). In contrast,
root CMI of A. confusa displayed a steady decrease as the soil
dried, but no significant effect of soil compaction on root CMI
was detected (Figure 4a).
Compared with the well-watered and non-compacted soil
treatments, root ABA concentration of A. confusa in the soil
drying and soil compacted treatments showed no obvious
changes until Day 15 of the treatment period, when rapid and
significant increases in root ABA concentration were observed. After Day 20 of treatment, root ABA concentration
decreased rapidly to the control value (Figure 5a). In both the
compacted and non-compacted treatments, root ABA concentration of L. glutinosa increased significantly when subjected
to soil drying for 4 days, and remained high for the remainder
of the study (Figure 5b).
Changes in root hydraulic conductivity in response to soil
drying and soil compaction varied significantly with plant
Figure 5. Effects of soil drying and compaction on root ABA concentrations of Acacia confusa (a) and Litsea glutinosa (b). Symbols:
well-watered and non-compacted treatment (s); well-watered and
compacted treatment (h); unwatered and non-compacted treatment
(n) unwatered and compacted treatment (,). Values are means ± SD
of four measurements.
species (Table 1). Compared with the control treatment, root
hydraulic conductivity of A. confusa decreased by about 30
and 32% in the well-watered + compacted and soil drying +
non-compacted treatments, respectively, and a more than 50%
decrease in root hydraulic conductivity was detected in the soil
drying + compacted treatment. In contrast, the effects of soil
compaction on root hydraulic conductivity of L. glutinosa
were small (less than 9%), but the soil drying treatment reduced root hydraulic conductivity by more than 60% compared with that of control plants. The soil drying plus soil
compaction treatment had an additive effect on root hydraulic
conductivity of L. glutinosa (Table 1).
Discussion
The high mechanical impedance of compacted soil restricts the
Table 1. Effects of soil drought and compaction on root hydraulic
conductivity (× 10 −7 kg s − 1 MPa −1 plant −1). Values are means ± SD of
four plants. An asterisk indicates that the treatment differs significantly from the control at α = 0.01 (t-test).
Figure 4. Effects of soil drying and compaction on root cell membrane
integrity of Acacia confusa (a) and Litsea glutinosa (b). The root cell
membrane integrity was calculated based on the difference in electrical conductivity of root samples before and after incubation in boiling
water. Symbols: well-watered and non-compacted treatment (s);
well-watered and compacted treatment (h); unwatered and non-compacted treatment (n) unwatered and compacted treatment (,). Values
are means ± SD of four measurements.
Treatments
Acacia confusa
Litsea glutinosa
Non-compacted, watered soil
Compacted, watered soil
Non-compacted, dry soil
Compacted, dry soil
7.51 ± 1.94
5.12 ± 0.95*
5.31 ± 0.10*
3.62 ± 0.55*
7.47 ± 1.08
6.87 ± 0.75
2.76 ± 0.20*
1.96 ± 0.40*
TREE PHYSIOLOGY VOLUME 19, 1999
ROOT RESPONSES TO SOIL DRYING AND COMPACTION
depth of root penetration (Figure 2), resulting in decreased root
development and a shallow root system (Figure 2), which
severely restricts access to water and nutrients. The negative
effects of compacted soil on root growth and function can be
aggravated when associated with soil drought. The responses
to soil compaction and drought vary considerably among plant
species. Acacia confusa can grow well in compacted soil,
presumably reflecting its ability to penetrate compacted soil
(Figure 2). Such penetration ability enables roots to absorb
water from deep soil layers when the surface soil dries and may
explain why A. confusa is tolerant to landfill conditions, where
water deficit is a major factor limiting tree growth. Although
the ability of roots of L. glutinosa to penetrate a densely
compacted soil layer is less than that of A. confusa, L. glutinosa has evolved mechanisms to prevent water loss from the
root surface when exposed to compacted or drying soil. For
example, L. glutinosa plants were able to maintain a relatively
high root water potential when the soil water content was low
(Figure 3). This ability may be related to the suberization of
the root surface (Varade et al. 1970), which can prevent water
loss from roots. However, suberization may, in turn, affect root
hydraulic conductivity, which will reduce water uptake by root
systems when soil water is available (Table 1, Liang et al.
unpublished data).
When roots encounter mechanical impedance and drought,
complex metabolic processes are involved (Russell 1977, Goss
and Russell 1980, Voorhees 1992). When the root system was
exposed to soil compaction and drought, its respiratory rate
decreased, and the decline occurred earlier in L. glutinosa than
in A. confusa (data not shown). In contrast to the change in
respiratory rate, the accumulation of ABA in roots of L. glutinosa in the soil drying treatment occurred 8 days earlier than
in roots of A. confusa (Figure 5). This finding explains, in part,
the higher sensitivity of leaf conductance to soil drying and
compaction in L. glutinosa than in A. confusa (Liang et al.
1996b). Although it is not clear whether the accumulation of
ABA in roots when roots encounter unfavorable soil conditions has a positive role in root growth and development, it has
been shown that root ABA can increase root tolerance to soil
stresses by either increasing root cell membrane integrity
(Reynold and Bewley 1993) and stimulating biosynthesis of
some root proteins (Hasson and Poljakoff-Mayber 1983), or
maintaining root growth and increasing root hydraulic conductivity (Saab et al. 1990, Ober and Sharp 1994, Zhang et al.
1995).
Root cell membrane integrity and composition were affected by soil environmental conditions. Liljenberg and Kates
(1985) showed that the lipid:protein ratio of oat roots decreased with increasing drought stress and that lipid composition also changed, with the degree of unsaturation decreasing
as drought stress intensified. However, the effect of soil
drought on root cell membrane integrity varies greatly among
plant species. A steady decline in root cell membrane integrity
was observed in A. confusa, but not in L. glutinosa, which may
be related to the degree of suberization of its root surface. Soil
compaction had little influence on root cell membrane integrity of either species.
Root hydraulic conductivity (L p) varies with root age and
623
anatomical characteristics (Kruger and Sucoff 1989, North and
Nobel 1991, 1992, Cruz et al. 1992, Huang and Nobel 1992).
Decreases in root Lp in response to soil drying have been
observed in Sorghum bicolor L. (Cruz et al. 1992), Agave
deserti Engelm., Ferocactus acanthodes (Lem.) Britt. & Rose
and Opuntia ficus-indica (L.) Mill. (North and Nobel 1991,
1992, Huang and Nobel 1992), Citrus jambhiri Limon (L.)
Burm (Ramos and Kaufman 1979), Glycine max (L.) Merrill
(Blizzard and Boyer 1980). We also observed a decrease in L p
with soil drying in A. confusa and L. glutinosa; however, we
found a substantial difference in the response of L p to soil
drying between the two species (Table 1). Compared with the
control, L p of L. glutinosa decreased by more than 60% after
7 days of soil drying, whereas L p decreased by less than 30%
in A. confusa. In both species, the decline in root Lp was
enhanced when roots were subjected to both soil compaction
and soil drying, but especially for L. glutinosa, implying that
roots of A. confusa can maintain a lower resistance in the water
transport pathway enabling it to access available soil water
more readily than L. glutinosa.
In summary, soil compaction and drought are major factors
affecting revegetation of sanitary landfill sites. We observed
large differences in root responses to soil drying and compaction between the two plant species. The roots of A. confusa had
a greater ability to penetrate the compacted soil layers and to
absorb water from surrounding soil than roots of L. glutinosa,
which may partially explain why A. confusa outperforms
L. glutinosa on landfill sites.
Acknowledgments
The authors acknowledge financial support from RGC (Research
Grant Council) of UGC (The University Grant Council of Hong
Kong).
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