Journal of Experimental Botany, Vol. 54, No. 390, pp. 2105±2109, September 2003
DOI: 10.1093/jxb/erg226
RESEARCH PAPER
Root cap removal increases root penetration resistance in
maize (Zea mays L.)
Morio Iijima1,*, Toshifumi Higuchi1, Peter W. Barlow2 and A. Glyn Bengough3
1
Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan
2
School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK
Scottish Crop Research Institute, Dundee DD2 5DA, UK
3
Received 28 February 2003; Accepted 29 May 2003
Introduction
2000). However, the degree to which the mechanical
impedance to root growth is reduced by the presence of the
root cap is not known.
Those plant species with a closed-type meristem, such
as maize (see Fig. 3A in Bengough et al., 2001), have a
clearly demarcated boundary between the root cap and the
root proper. One consequence of this is that the cap can be
easily removed from the root tip. To date, removal of the
root cap (decapping) has been used to investigate the
regeneration process of the root cap (Barlow, 1974;
Barlow and Sargent, 1978; Barlow and Hines, 1982), the
reactivation of cells in the quiescent centre (Grundwag and
Barlow, 1973; MuÈller et al., 1994), aluminium toxicity
(Scho®eld et al., 1998), and the role of the root cap as a
sensory organ for perceiving mechanical (Goss and
Russell, 1980) or gravitational stimuli (Pilet, 1971;
Barlow, 1974; Stinemetz, 1995). This decapping technique
could potentially be used for testing various other roles of
the root cap: for example, whether or not the root cap
reduces root penetration resistance. Root penetration
resistance is the reaction force exerted by the soil on the
penetrating root per unit root cross-sectional area
(Bengough and Mullins, 1990). Root penetration resistance has been directly measured in a number of studies
(Stolzy and Barley, 1968; Whiteley et al., 1981; Bengough
and Mullins, 1991). In this paper, a comparison of the root
penetration resistance of decapped and intact roots was
used to evaluate the role of the root cap in alleviating soil
mechanical impedance.
The root cap protects the root meristem from abrasion by
soil particles. It has also been suggested that it reduces soil
mechanical impedance by means of its secretion of slimy
mucilage and by the sloughing of border cells (Iijima and
Kono, 1992; Bengough and McKenzie, 1997; Iijima et al.,
Seed germination and soil preparation
Maize (Zea mays L. cv. Mephisto) seedlings were used for the
experiments. This variety of maize has a cap which is easily
Abstract
The root cap assists the passage of the root through
soil by means of its slimy mucilage secretion and by
the sloughing of its outer cells. The root penetration
resistance of decapped primary roots of maize (Zea
mays L. cv. Mephisto) was compared with that of
intact roots in loose (dry bulk density 1.0 g cm±3;
penetration resistance 0.06 MPa) and compact soil
(1.4 g cm±3; penetration resistance 1.0 MPa), to evaluate the contribution of the cap to decreasing the
impedance to root growth. Root elongation rate and
diameter were the same for decapped and intact
roots when the plants were grown in loose soil. In
compacted soil, however, the elongation rate of decapped roots was only about half that of intact roots,
whilst the diameter was 30% larger. Root penetration
resistances of intact and decapped seminal axis were
0.31 and 0.52 MPa, respectively, when the roots were
grown in compacted soil. These results indicated that
the presence of a root cap alleviates much of the
mechanical impedance to root penetration, and
enables roots to grow faster in compacted soils.
Key words: Border cell, decapping, mucilage, root cap, root
growth pressure, soil compaction.
Materials and methods
* To whom correspondence should be addressed. Fax: +81 52 789 4012. E-mail: [email protected]
Journal of Experimental Botany, Vol. 54, No. 390, ã Society for Experimental Biology 2003; all rights reserved
2106 Iijima et al.
Fig. 1. An SEM sequence showing the regeneration of the maize root cap following its removal (Zea mays cv. GH390F grown in vermiculite and
prepared as in Barlow and Hines (1982). (A) Intact root tip and cap (360). (B) The tip of the root and portions of cap immediately after removal
of the major portion of the root cap. The walls seen lying on the surface of the tip are the broken longitudinal walls of the cap meristem cells
(3340). (C) The tip of the decapped root, 8 h after decapping. The exposed tip has begun to protrude through the surrounding ring of cap cells
that have remained on the ¯ank of the root. There are some adhering fragments of vermiculite on the tip (3340). (D) Root tip 24 h after cap
removal. The tip now begins to show signs of recovering a cap-like surface. The old remaining cap is still just visible at the base of this new cap,
with a few pieces of vermiculite adhering to the new cap (3110).
removed, and has been used previously in root penetration resistance
experiments (Iijima et al., 2000). Maize caryopses were surfacesterilized by immersion in a saturated solution of calcium
hypochlorite for 5 min. They were then washed several times with
distilled water and allowed to germinate on blotting paper moistened
with distilled water in a Petri dish, in darkness, at 23 °C for 72 h.
Seedlings with a straight seminal root, 20±35 mm long, were used in
all experiments.
Sandy loam soil (sand 56%, silt 36%, clay 8%) was sieved through
a 2 mm mesh, and then wetted to a water content of 23.6 g water per
100 g soil (matric potential approximately ±5.4 kPa). The soil was
kept in a plastic bag prior to packing.
Experiment 1: effects of decapping and soil compaction on
root elongation and diameter
First, the effect of decapping upon root growth in length and
diameter was tested. The wetted soil was packed in layers, using a
metal plunger and compressor, into plastic tubes (100 mm height,
60 mm diameter) at soil bulk densities of 0.8 Mg m±3 or 1.4 Mg m±3
(air-®lled porosity 0.14 cm3 cm±3), and here termed `loose' and
`compact', respectively. A sharp scalpel blade was used to decap the
roots, following the method used by Barlow and Hines (1982), and
those seedlings from which the root cap came off cleanly were
selected (Fig. 1). Four seedlings, with known root length, were
transplanted into each plastic cylinder. After 24 h growth in darkness
at 23 °C, the roots were excavated gently and their lengths measured.
Diameters at 1±4 mm behind the root apex were measured using a
stereo microscope with an eyepiece graticule. In total, 9±12 replicate
plants were grown in the two bulk densities and decapping or intact
root treatments.
Experiment 2: root penetration resistances
Initial root diameters were measured 2, 3, 4, and 5 mm behind the
apex of both decapped and intact seminal roots under the
stereomicroscope. After this the plants were transferred to the
seedling holder (50 mm height, 30 mm diameter) illustrated in Fig. 2.
The seminal root axis was held vertically and anchored rigidly
behind the zone of elongation. The air gap between the root holder
and the soil core surface was made as small as possible to minimize
the chance of the root buckling and any drying of the root surface.
The distal 1 mm of root tip was inserted into a narrowly tapered hole
(2 mm deep) in the surface of the soil core. Compact cores (50 mm
Root penetration resistance in maize 2107
Table 1. Root elongation rate, diameter, root penetration resistance, and penetrometer resistance for intact and decapped roots
Loose
Compact
Intact
Experiment 1
Elongation rate (mm d±1)
Penetrometer resistance (MPa)
Diameter (mm)
Experiment 2
Initial root diameter (mm)
Estimated root diameterb (mm)
Final root diameter (mm)
Force (mN)
Root penetration resistance (MPa)
Penetrometer resistance (MPa)
a
b
41.162.1 aa
1.0660.01 a
0.0660.00
Decapped
Intact
42.161.6 a
22.961.1 b
1.0660.03 a
1.2460.03 b
1.1060.02
1.1160.02
1.1660.03
281618
0.3160.02
Decapped
1.0660.09
0.9660.05
12.460.5 c
1.6160.05 c
1.1160.02
1.2160.02**c
1.5360.05**
584642**
0.5260.03**
Values with the same letter are not signi®cantly different at the P <0.05 level of probability by the Duncan's multiple range test.
Estimated root diameter corresponding to when the root was between 3±6 mm deep in the soil core. Values are mean 6SE of 9±12 replicates.
Indicates signi®cant difference between intact and decapped treatments at P <0.01 level, tested by ANOVA.
c **
Fig. 2. Apparatus for measuring root penetration resistance. The force
exerted on the soil core by the root was recorded on the digital
balance and logged using a personal computer.
height, 50 mm diameter) were used with the same bulk densities as in
experiment 1. The resistance experienced by the root tip in the loose
soil type was considered to be too small to evaluate the difference
between decapped and intact roots accurately. Hence, only the root
penetration resistances obtained by penetration into the compact soil
are reported here. The root was ®xed in position with a small
quantity of plaster of Paris, and the whole seedling was covered
loosely with the wetted soil. One seedling was transplanted into each
root holder and allowed to grow for a further 20±22 h at 23 °C in
darkness. In total, 10 replicate plants were grown in the decapping
and intact root treatments. The soil core was set on an electric
balance so that the pushing force of the root into the soil core was
recorded as the `root force'. Balance readings (accurate to within
0.01 g) were taken automatically every 10 or 30 min during the 20±
22 h growth period by means of a personal computer interfaced to
the balance output. After this, the root was excavated from the soil
core, and the ®nal root diameters were measured at the locations
mentioned, and compared with the initial diameters. The root
penetration resistance (Q) was calculated as
F
Qˆ
…1†
A
where F is the root force and A is the average cross-sectional area of
the root at 2, 3, 4, and 5 mm behind the apex.
Penetrometer resistance
Penetrometer resistance of the soil core was measured using a 0.98
mm diameter probe with a 30° cone angle. The probe was pushed
into the soil core at a rate of 1 mm min±1 from the soil surface to
Fig. 3. Penetrometer resistance (mean 6SE) as a function of depth
and a typical example of root penetration resistance as a function of
estimated depth. Root penetration resistance was calculated assuming
a constant elongation rate (as in Bengough and Mullins, 1991) and a
constant rate of diameter increase with time. Root penetration
resistance was evaluated based on the average values along the arrow.
depths of 10 mm or 20 mm in the cases of compact or loose soils,
respectively. The penetrometer resistance was calculated according
to equation 1, substituting penetrometer force for root force, and the
maximum cone cross-sectional area for root cross-sectional area.
Three such penetrometer tests were made in the loose and compact
treatments of experiment 1, and for the compact treatment in
experiment 2.
Statistical analysis
In experiment 1, differences among the treatments were subjected to
a two-way analysis of variance (2-way ANOVA), then Duncan's
multiple range test was applied to compare means among the four
treatments. In experiment 2, differences between the two treatments
were subjected to a one-way analysis of variance (1-way ANOVA).
Results
Experiment 1: effects of decapping and soil
compaction on root elongation and diameter
Penetrometer resistance of the soil used in experiment 1
was 0.06 MPa for loose, and 1.06 MPa for compact
treatments, respectively. Decapping did not alter the
elongation rate nor the root diameter in the loose treatment
(Table 1). In this experiment, using intact roots, the
2108 Iijima et al.
compact soil treatment reduced root elongation by 44%
and increased root diameter by 17% (Table 1). By contrast,
elongation rates of decapped roots in compact soil showed
71% lower elongation rates and 52% thicker diameters
than those of the loose treatment. In both root elongation
rates and diameter, interactions between the decapping and
compaction treatments were signi®cant at P <0.01, therefore, treatment means were subjected to the Duncan's
multiple range tests. The decapped roots were signi®cantly
more sensitive than intact roots to the effect of mechanical
impedance.
Experiment 2: root penetration resistances
Penetrometer resistance increased initially during the ®rst
3 mm of penetration, and then plateaued (Fig. 3). Root
elongation rate was assumed constant in the calculation (as
in Bengough and Mullins, 1991), and root diameter was
assumed to increase linearly with time. Root penetration
resistance increased to a depth of 6 mm and then plateaued
for the decapped roots, with a continued gradual increase
for the intact roots. The root penetration resistance was
evaluated when the root tip was estimated to be between 3
mm and 6 mm below the soil surface. This was suf®cient to
eliminate soil surface deformation effects, but before the
root became anchored by root hairs. Root force doubled,
and root penetration resistance was increased by 68% in
roots that had their caps removed (Table 1). Thus,
decapped roots experienced greater mechanical impedance
than intact roots.
Discussion
Removal of the root cap increased the root force and root
penetration resistance substantially (Table 1). This increased mechanical impedance to root growth resulted in
slower root elongation (12 mm d±1 in decapped roots, 22
mm d±1 in intact roots), and increased thickening in the
decapped roots (1.61 mm diameter compared with 1.06
mm). The lack of any effect of decapping on elongation
rate and diameter in the loose soil suggests that decapping
per se did not cause these changes. In other words, the
mechanical impedance of loose soil, 0.06 MPa, used in this
experiment was too small to slow root elongation growth.
Generally speaking, ®eld soils exhibit larger mechanical
impedance than this, unless roots are growing in continuous pores or a newly-tilled seed bed at near ®eld capacity.
The root cap is thus generally important to reduce soil
mechanical impedance except in the case of such loose soil
conditions.
Root penetration resistance is the sum of the frictional
resistance to root penetration, plus the pressure required to
form a cavity in the soil (Greacen et al., 1968; Bengough
and Mullins, 1990, 1991). The cause of the increase in root
penetration resistance may, therefore, be due to changes in
the frictional resistance to root penetration or changes in
the pressure required to expand a cavity in the soil
resulting from a change in the shape of the root tip. The
decapping experiments performed cannot differentiate
between these two components of penetration resistance,
although it is possible to consider their relative importance.
A possible reason for the increase in root penetration
resistance on decapping is the decrease in root cap border
cell production and the exudation of mucilage. In
compacted sand, Iijima et al. (2000) estimated that the
whole surface of the root cap might be covered in detached
border cells. Exudation of mucilage may also be increased
by soil compaction (Barber and Gunn, 1974; BoeufTremblay et al., 1995; Iijima and Kono, 1992; Iijima et al.,
2000). By removing the bulk of the root cap, including the
cap meristem, border cell and mucilage production is
effectively restricted to the small portion of the remaining
lateral cap (see Fig. 3A of Bengough et al., 2001). This
means that friction will occur directly between soil
particles and cells of the root proper, with a much
diminished lubricating layer of mucilage or border cellmucilage sandwich. By 24 h after decapping, mucilage
production would begin again in the outer cells of the
decapped apex (Barlow, 1974; Barlow and Sargen, 1978),
but this is beyond the timescale of these experiments.
It is also possible that the blunter shape of the decapped
root will increase root penetration resistance. In the case of
metal probes with cone-angles of 30°±60°, however, the
penetration resistance was relatively unaffected by coneangle (Gill, 1968). Probes with narrow cone-angles tend to
deform the soil cylindrically, whilst blunter probes cause
more spherical soil deformation (Greacen et al., 1968;
Bengough et al., 1997). The reality for roots probably lies
somewhere between the cylindrical and spherical extremes, and it is likely that the decapped blunter roots will
tend further towards the less ef®cient spherical mode of
deformation.
Root penetration resistance was smaller than penetrometer resistance, even in the case of the decapped roots. This
may be due to a combination of the root±soil friction being
smaller than the soil±metal friction, but also to the faster
rate of penetration of the penetrometer probe (some 65±
120 times faster). Although penetration resistance is not
strongly dependent on rate, this may well account for some
of the difference between root and penetrometer resistance.
In conclusion, this study has provided new direct
evidence that root caps facilitate root penetration, enabling
faster root elongation in compact soils. The mechanism for
this probably involves border cell and mucilage lubrication
of the root±soil interface, coupled with the tapering shape
of the root cap.
Acknowledgements
We thank the Japanese Society of Promotion of Science (B212460010) and the Royal Society for funding the visit to SCRI and
Root penetration resistance in maize 2109
IACR Long Ashton by Dr Iijima and Mr Higuchi. The Scottish
Of®ce Agriculture, Environment and Fisheries Department provide
grant-in-aid to SCRI. Thanks to Blair McKenzie for helpful
discussions.
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