Annals of Botany 87: 397±404, 2001
doi:10.1006/anbo.2000.1347, available online at http://www.idealibrary.com on
Anchorage Mechanics of the Tap Root System of Winter-sown Oilseed Rape
(Brassica napus L.)
A . M . G O O D M A N *{, M. J . C RO O K {{ and A . R . E N N O S{
{School of Biological Sciences, 3.614 Stopford Building, University of Manchester, Oxford Road, Manchester
M13 9PT, UK
Received: 7 August 2000
Returned for revision: 11 October 2000 Accepted: 26 November 2000 Published electronically: 26 January 2001
The anchorage mechanics of mature winter-sown oilseed rape (`Envol') were investigated by combining a morphological and mechanical study of the root system with anchorage tests on real and model plants. Oilseed rape plants
were anchored by a rigid tap root; the few laterals all emerged below the centre of rotation of the root system (approx.
30 mm below the soil surface). When plants were pulled over, the tap root bent and the top 30 mm moved in the soil
towards the direction of pull, creating a crevice on the opposite side. The maximum anchorage moment was
2.9 + 0.36 N m. Two main components of anchorage were identi®ed: the bending resistance of the tap root and the
resistance of the soil on the near side to compression. The relative importance of these components was determined by
measuring both the bending resistance of the tap root, and the resistance of metal tubes of varying diameter, inserted
to various depths in the soil, to being pulled over. These tests showed that the tap root bending moment at failure
could account for around 40 % of anchorage moment, while soil resistance could account for around 60 %. The
model tests on the tubes also help to shed light on the way in which the dimensions of tap roots will in¯uence their
# 2001 Annals of Botany Company
anchorage capability.
Key words: Anchorage, lodging, root bending resistance, mechanical properties, oilseed rape, Brassica napus L.
I N T RO D U C T I O N
Plants have developed a number of di€erent mechanisms for
providing adequate anchorage against toppling, and
the mechanics of several di€erent types of anchorage
systems have recently been investigated. These include the
plate systems of temperate trees (Coutts, 1983, 1986; Crook
and Ennos, 1996; Stokes, 1999); the plate and tap root
systems of buttressed and unbuttressed tropical trees (Crook
et al., 1997; Crook and Ennos, 1997); the much branched
tap and root ball systems of herbaceous dicots such as
sun¯owers (Helianthus annuus L.) and Himalayan balsam
(Impatiens glandulifera Royle) (Ennos et al., 1993a); and the
adventitious `coronal' root systems of cereal grasses such as
wheat (Triticum aestivum L.) (Crook and Ennos, 1993) and
maize (Zea mays L.) (Ennos et al., 1993b).
The anchorage mechanics of those herbaceous plants
whose root systems are dominated by a single tap root
(Ennos and Fitter, 1992) have never been examined. This is
an important omission for two reasons: ®rst because such
systems are fairly common in families such as Brassicaceae;
second, many of these species are important crop plants,
prone to anchorage failure (or `root lodging') which can
cause large yield losses (Scott et al., 1973; Islam, 1988;
Baylis and Wright, 1990; Armstrong and Nicol, 1991).
Baylis and Wright (1990) arti®cially induced lodging in
oilseed rape (Brassica napus L.) and recorded yield losses in
* For correspondence at: De Montfort University Lincoln, School
of Agriculture, Lindsey Centre, Riseholme, Lincoln, LN2 2LG, UK.
Fax ‡44 (0) 1522 545436, e-mail [email protected]
{ Present address: Harper Adams Agricultural College, Newport,
Shropshire, TF10 8NB, UK.
0305-7364/01/030397+08 $35.00/00
excess of 50 %. However, Islam (1988) recorded a yield
reduction of 13±20 % as a result of a natural lodging event
in the ®eld. Yield losses resulting from lodging in oilseed
rape are related to losses incurred both at maturation (due
to reduced assimilate supply and poor grain ®ll) and during
the harvest operation as a result of pod shatter (Armstrong
and Nicol, 1991).
This study examines the anchorage mechanics of oilseed
rape and, as in previous studies (Ennos et al., 1993a; Crook
and Ennos, 1996), a morphological and mechanical
approach was used. Preliminary observations and comparison with work on the unbuttressed rainforest tree
Mallotus wrayi (Crook et al., 1997), which is also anchored
by a tap root, suggest that there are two possible components for anchorage of the plant: the resistance of the
tap root to bending below the soil surface and the resistance of the soil to lateral compression. This study therefore
aims to identify and determine the magnitude of both of
these components in oilseed rape. The ®rst step simply
involves bending tests on the tap root. The second involves
model tests in which metal tubes are rotated through the
soil. Such tests are needed because the resistance of
agricultural soils to lateral root movement has previously
only been estimated (Ennos and Fitter, 1992; Ennos, 1993)
based on the engineering theory of the resistance of piles to
lateral loads (Broms, 1964). Assuming that a tap root is a
rigid rod of length L and diameter D, and that it rotates
about its base, engineering theory predicts that the
maximum resistance (Rmax) to lateral loading is given by:
Rmax ˆ 9=2tDL2
…1†
# 2001 Annals of Botany Company
398
Goodman et al.ÐAnchorage Mechanics in Oilseed Rape
where t is the shear strength of the soil. The tests will
therefore also determine whether the theory is appropriate,
and whether the prediction of the model, that the maximum
anchorage moment is proportional to the diameter of the
root and the square of its length, is correct.
The practical work of this study therefore involved three
stages: quantitative anchorage tests on ®eld-grown plants;
morphological study and mechanical testing of plants in the
laboratory; and anchorage tests on metal tubes in the ®eld.
M AT E R I A L S A N D M E T H O D S
Winter oilseed rape (`Envol') was grown in a sandy loam
soil at the University of Manchester's Experimental
grounds, Jodrell Bank, Cheshire, UK. In early September
1995, a ®eld plot (16 18 m) was drilled using a
Wintersteiger precision plot drill (Wintersteiger G.m.b.H.,
Ried im Innkreis, Austria). Seeds were sown at a density of
116 seeds m ÿ2 and the drill set at a row spacing of 0.10 m.
The ®eld site was maintained using herbicides and
fungicides, Butisan S (BASF plc. Cheadle, UK) was applied
pre-emergence at 1 l ha ÿ1 in September, and Punch C
(DuPont (UK) Ltd, Stevenage, UK) at 0.4 l ha ÿ1
in February. Nitrogen was applied as a split application
of 40 kg ha ÿ1 in November and 190 kg ha ÿ1 in March in
the form of urea.
Preliminary tests: root movement during uprooting
Before any quantitative measurements were carried out,
movements of the root systems were examined qualitatively
during uprooting. Five oilseed rape plants were pruned to a
height of 0.8 m and a trench (0.3 m deep 0.3 m wide and
extending 0.3 m on either side of the stem) was dug
alongside the base of the stem parallel to the direction in
which the stem was to be pulled over. The stem was then
pushed over at a height of 0.5 m and an approximate rate of
rotation of 1.58 s ÿ1. During the test, particular attention
was paid to the movements of the roots, the soil and the
depth below the soil surface of the centre of rotation of the
plant.
Anchorage tests in the ®eld
In mid-July 1996, a series of anchorage tests was carried
out in the ®eld. By this time the pods had developed and
most seeds had turned green (growth stage 80) (Lancashire
et al., 1991).
Anchorage mechanics were investigated quantitatively
using a method devised by Ennos et al. (1993a). Plants in a
6 6 m area in the centre of the plot were pruned to a
height of 0.8 m. To ensure that failure occurred in the root
system rather than in the stem, and to mimic conditions in
which root lodging might occur, a quarter of the area
(3 3 m) was brought approximately to ®eld capacity by
applying a total of approx. 1660 mm of water over a 2 week
period using buckets. The plot was then left to drain for
48 h. To determine the mechanical properties of the soil,
the shear strength was measured using a 33 mm diameter
shear vane (Pilcon DR 2645; Pilcon Engineering Ltd,
Basingstoke, UK) pressed into the soil to a depth of 50 mm
(to the top of the vane) and was slowly rotated in 23 places,
at random, across the trial area. Readings of shear strength
were indicated on a dial.
The stems of 13 randomly selected plants were then
pruned with a razor to remove the lower petioles and
axillary racemes and a pulling force was applied, perpendicular to the axis of the stem at a height of 0.5 m, using a
Mecmesin portable force indicator (Mecmesin Ltd, Broadbridge Heath, West Sussex, UK). The resulting inclination
of the stem base was measured by attaching a cane of length
0.8 m and measuring its horizontal movement along a ruler
placed 0.6 m above the soil. Readings of force were
measured at lateral intervals of 2, 4, 6, 8, 10, 12, 16, 20,
24, 28, 32, 36, 40, 44, 48, 52, 56 and 60 cm. The
approximate rate of rotation was 1.58 s ÿ1. The mechanism
of failure was noted along with any cracking noises, and the
readings used to produce a curve of restoring moment
( force perpendicular height), which we have called
anchorage moment, vs. angular displacement for each
plant. In calculating the anchorage moment, the length of
the lever arm was taken as the distance from the point of
applied force to the pivot point at 30 mm below the soil
surface.
Following tests, the number of ®rst-order lateral roots
was counted and the diameters of the bases of the stem and
tap root of each plant were also measured.
Laboratory tests
A further 15 plants were selected at random from outside
the anchorage test area and taken to the laboratory for
morphological examination and mechanical testing.
Shoot morphology. The height and degree of taper of each
stem was measured by taking diameter measurements at the
soil level and at heights of 0.5 and 1.0 m above the soil
surface. To calculate the safety factor of lodging, the centre
of gravity of the shoot was measured by balancing the entire
shoot perpendicularly on a pivot and the distance from the
balance point to the base of the stem was measured with a
ruler. The total fresh weight of the shoot system of each
plant was also measured.
Root system morphology. The root systems were carefully
excavated using a spade to remove a root soil ball of
approx. 0.02 m3. Soil was washed away with a hose and the
root systems were placed in plastic bags to prevent
desiccation; care was taken not to damage any of the
®rst-order laterals or the ®rst 200 mm of tap root. Root
systems were transferred to buckets of water and placed in a
cold room at 58C overnight before morphological examination and mechanical testing.
The ®rst-order lateral roots were then removed at the
base using a razor blade. The total number of structural
roots (de®ned as ®rst-order lateral roots which had a basal
diameter greater than 2 mm) was counted for each plant.
The diameters of the tap roots were also measured at the
base of the shoot and 40 mm from the base. The length of
the tap root which showed noticeable rigidity in bending
Goodman et al.ÐAnchorage Mechanics in Oilseed Rape
399
(termed `length of rigid tap root') was also measured from
the base down the root to the point at which the root no
longer resisted bending.
The bending rigidity, R, of a uniform beam is the resistance
of that beam to curvature and is given by:
Mechanical tests
where dF/dY is the initial slope of the force displacement
curve. The bending modulus, E, is given by:
Taproot. The bending moment of the tap roots was
measured to determine the extent to which resistance to
bending of the tap root contributed to anchorage. To do
this, the tap root was clamped ®rmly at the top and a force
was applied between 55 and 90 mm from the top, depending
on the size of the tap root, using a Mecmesin portable force
indicator (Mecmesin Ltd, Broadbridge Heath, West Sussex,
UK). The root was bent perpendicular to its length by
moving the force indicator at a rate of approx. 7.5 mm s ÿ1
until the root broke. The peak force, Fmax , required to break
the root was recorded. After the test the distance from the
break to the point where the force was applied, d, was
measured using a ruler. The maximum bending moment, Sr ,
of the root was then calculated using the equation:
Sr ˆ Fmax d
…2†
Stems. Three-point bending tests were carried out on the
bottom 220 mm of the stems using a universal testing
machine (Instron, model 4301). The diameter of each stem
sample was measured at the mid-point using callipers. Stem
samples were placed between two supports which were set
apart a distance approx. 15-times the mid-point diameter of
the sample to avoid problems with shear (Vincent, 1992). A
pushing probe of radius 20 mm was attached to the load
cell and lowered until it just touched the mid-point of the
sample. The crosshead was then lowered at a rate of
20 mm min ÿ1, bending the sample until it eventually
buckled. A computer with an interface to the testing
machine was used to produce a graph of force vs.
displacement, permitting calculation of the mechanical
properties of the sample (Ennos et al., 1993b).
Using the data collected from the test, an interfaced
computer calculated three mechanical properties: the
maximum bending moment, Sb [eqn (3)], and bending
rigidity, EI [eqn (4)], of the stem; and the bending modulus,
E [eqn (5)], of the material of which it was composed. In the
analysis it was assumed that there was little taper. The
errors due to this assumption are small and it was felt that
this was an acceptable approximation (Ennos et al., 1993b).
Analysis of bending tests
The mechanical properties of the samples were calculated
using known equations (Gordon, 1978). Maximum bending
moment is given by the expression:
Sb ˆ Fmax L=4
…3†
where Fmax is the maximum force a sample will withstand
before it fails and L is the distance between the supports.
R ˆ L3 …dF=dY†=48
E ˆ R=I
…4†
…5†
where R is the rigidity of the sample and I is the second
moment of area. Oilseed rape shoots were approximately
cylindrical in cross-section and the second moment of area
was calculated for a solid cylinder using pr4/4 where r is the
radius. A high modulus indicates a sti€er material. Both the
bending rigidity and the maximum bending moment
depend on sample geometry whereas the bending modulus
is a property of the material only.
Factors of safety
Both the stem and anchorage system of a plant must
remain structurally intact to resist the overturning moments
generated by the wind and by the weight of the plant. A
`factor of safety' against self-weight moment can be
calculated for crop plants (Crook and Ennos, 1994). The
factor of safety against root lodging, FSR , is given by the
expression:
FSR ˆ
Maximum anchorage moment
Shoot weight centre of gravity sin …y†
and the factor of safety against stem lodging, FSS , is given
by:
FSS ˆ
Maximum stem bending moment
Shoot weight centre of gravity sin …y†
where y is the angle from the vertical in degrees where the
maximum anchorage moment was recorded. Values of the
safety factor were calculated for a stem inclination from the
vertical of 188. Although these simple factors of safety can
be criticized because they omit the importance of wind
loading, they still provide a means of comparing the
mechanical ability of a plant to withstand physical damage.
Model tests: anchorage tests on metal tubes
To determine the importance of the resistance of soil to
lateral movement, and to test the hypothesis that tap roots
of oilseed rape in loam soils behave like engineering piles in
clay soils, metal tubes embedded in the soil were pulled over
in the ®eld.
Three 0.8 m long metal tubes of diameters 9.5, 15 and
19 mm were submerged at depths of 30, 60 and 90 mm.
First, a narrow trench of width 10±20 mm with a vertical
face 200 mm wide and 30, 60 or 90 mm deep was dug into
the damp (moisture content approx. 12 %), but undisturbed
soil using a spade. The tube was placed vertically in the
trench, touching the face of the soil, and a 2 mm thick steel
plate (dimensions 250 110 mm) was placed on one side of
400
Goodman et al.ÐAnchorage Mechanics in Oilseed Rape
the tube to ensure that the tube would rotate about its lowest
point. The tubes were then pulled over in the opposite
direction to the plate and perpendicular to the trench at a
height of 0.6 m using a digital force gauge. Ten tests were
carried out for each tube diameter at each soil depth.
The results could be used to calculate the restoring
moment supplied by the lateral resistance of the earth, just
as in the anchorage tests on the plants. However, because the
metal tubes were much heavier than the plant stems, they
themselves would contribute a `self-weight moment' causing
them to continue to rotate and fall over. This moment was
calculated for each angle by measuring the force required to
hold tubes up in the laboratory at each angle, and prevent
them rotating about their unconstrained base.
Direction of
pulling
Movement of tap
root into the soil
C.O.R
Soil properties
Standard methods of sedimentation and sieving were
used to classify soil type (Rowell, 1994). Soil samples were
collected using cores of diameter 48 mm and length 30 mm.
For each sample a shallow trench was dug and a core taken
at a depth of 50 mm from the undisturbed face of the
trench. Shear strength was measured with a 33 mm shear
vane as previously described.
Statistical analysis
A Kolmogorov-Smirnov Test (Sokal and Rohlf, 1995)
was used to test the normality and similarity of the shapes
of underlying distributions before proceeding with correlation, regression analysis and analysis of variance. All values
in the text are means + s.e.
R E S U LT S
Root movement during uprooting
Five oilseed rape plants, of basal diameter ranging from
15±17 mm, all showed essentially similar root system
morphology, and their behaviour was also similar when
they were pulled over in saturated soil. Each plant was
anchored by a single rigid tap root that had only a few
laterals. The laterals emerged well below the region which
showed movement during anchorage failure, the upper
29 + 2.5 mm of the tap root.
When plants were pulled over the tap root bent and the
top 30 mm moved in the direction of pull, compressing the
soil on this side, and leaving a crevice on the opposite side
of the plant (Fig. 1). During the test no obvious sounds of
root or stem failure were noted; anchorage failure was
largely a result of compression of the soil on the near side
and bending of the tap root. On release of the force, the
plants did not return totally to an upright position but it
was observed (but not quanti®ed) that they continued to
lean by several degrees in the crevice.
Soil properties
The soil was classi®ed as a sandy clay loam with the
following particle size distribution: 0.656 kg kg ÿ1 sand
F I G . 1. Stem and root movements during anchorage failure of winter
oilseed rape. As the plant is pulled over the plant rotates and bends at a
point approx. 30 mm below the soil surface (C.O.R., centre of
rotation). The top 30 mm of the tap root moved in the soil towards
the direction of pull, creating a crevice on the opposite side. This
suggests that there are two components of anchorage: the resistance of
the tap root to bending and the resistance of the top 30 mm of soil to
lateral compression.
(463 mm), 0.144 kg kg ÿ1 silt and 0.200 kg kg ÿ1 clay.
There was no signi®cant di€erence (P 4 0.05) between
soil shear strength of the model tests (48.4 + 2.4 kPa) and
that in the anchorage tests on the real plants
(44.4 + 2.9 kPa). As a result the anchorage moment,
which is dependent on the soil shear strength, could be
compared between the real and model plants.
Anchorage tests
In all the tests, the anchorage moment rose at ®rst as the
plants were pulled sideways, reaching a maximum moment
at around 188 (Fig. 2). The maximum anchorage moment
for the plants which had a tap root diameter at the top of
15.6 + 0.79 mm and 40 mm down of 10.7 + 0.72 mm, was
2.9 + 0.36 N m.
The maximum anchorage moment was strongly positively correlated both with the diameter at the top of the tap
root (r2 ˆ 73.7 %, P 5 0.001, n ˆ 13) and at 40 mm down
the tap root (r2 ˆ 65.3 %, P 5 0.001, n ˆ 13); the thicker
the tap root the greater the resistance to overturning
(Fig. 3A and B). The diameter of the stem at the base was
also signi®cantly correlated with anchorage moment
(r2 ˆ 77.4 %, P 5 0.001, n ˆ 13) (Fig. 3C).
Anchorage moment (N m)
Anchorage moment (N m)
Goodman et al.ÐAnchorage Mechanics in Oilseed Rape
3
2
1
0
0
5
10
15
y = 0.38x –3.0
r2 = 73.7%
5
4
3
2
1
0
10
12
T A B L E 1. Morphology of the shoots and roots of mature
®eld-grown winter-sown oilseed rape
14
16
18
20
22
Tap root diameter at the top (mm)
Anchorage moment (N m)
F I G . 2. The results of anchorage tests on 13 winter oilseed rape plants.
The anchorage moment rose at ®rst reaching a maximum moment at
around 188. The vertical bars indicate + s.e.m (n ˆ 13 for 0±18 degrees
and n ˆ 10 for 22 degrees).
Measurement
7
B
6
5
y = 0.40x –1.3
r2 = 65.3%
4
3
2
1
0
6
8
10
12
14
16
Tap root diameter (mm) 40 mm down
14.8 + 0.61
8.7 + 0.39
3.8 + 0.30
Root morphology
Lateral root diameter (mm) (n ˆ 12)
Base
Base ‡ 20 mm
Base ‡ 40 mm
2.8 + 0.22
1.6 + 0.19
1.0 + 0.14
Tap root diameter (mm) (n ˆ 14)
Top
20 mm down
40 mm down
16.5 + 0.69
14.4 + 0.76
10.4 + 0.52
Values are means of 15 plants + s.e.m.
Morphology
Oilseed rape plants had single upright stems and their
shoot height ranged from 1.31±1.56 m and basal diameters
ranged from 14±17 mm. The basal axillary raceme was
situated approx. 1 m above the soil surface and the depth of
canopy (de®ned as the distance from the basal axillary
raceme to the tip of the terminal raceme) was
0.42 + 0.015 m. This produced a centre of gravity of the
shoot at 0.86 + 0.015 m from the soil surface. The shoot
fresh weight was 254 + 29 g. The stems tapered gradually
from the base upwards (Table 1).
The largest single root in these plants was the tapering
tap root, which had a top diameter of 11±20 mm and a
mean rigid length of 123 + 5.8 mm. There were only a few
rigid ®rst-order lateral roots (averaging between four and
®ve roots per plant) which mostly emerged further than
30 mm from the top of the tap root. The laterals ranged in
basal diameter from 1.7±4 mm but tapered rapidly to less
Anchorage moment (N m)
Shoot morphology
Stem diameter (mm)
Base
Base ‡ 0.5 m
Base ‡ 1 m
A
6
20
Angle from vertical in degrees
Property
7
401
7
6
C
5
y = 1.11x –14.1
r2 = 77.4%
4
3
2
1
0
12
13
14
15
16
17
18
Stem base diameter (mm)
F I G . 3. Plots of maximum anchorage moment against the diameter of
the tap root at the top (A), 40 mm down (B) and the stem base
diameter (C) of plants pulled over with a force gauge. Anchorage
moment was strongly positively correlated both with the diameter of
the tap root at the top (r2 ˆ 73.7 %, P 5 0.001, n ˆ 13) and 40 mm
down (r2 ˆ 65.3 %, P 5 0.001, n ˆ 13) and the diameter of the stem at
the base (r2 ˆ 77.4 %, P 5 0.001, n ˆ 13).
than 1 mm in diameter by 40 mm from the base (Table 1)
and consequently would serve little anchorage function.
Mechanical properties
The mechanical properties of the stems and tap roots are
summarized in Table 2. The stems were fairly rigid and the
material of which they were composed had a bending
modulus of between 1300 and 2900 MPa (Table 2), similar
to that of ®eld-grown sun¯ower (Goodman and Ennos,
1997). The maximum bending moment of the stem before
failure was slightly below the maximum anchorage moment
of the plants. The tap roots had a maximum bending
402
Goodman et al.ÐAnchorage Mechanics in Oilseed Rape
T A B L E 2. Mechanical properties of the roots and stems
(basal section: length 220 mm, midpoint diameter
11.9 + 0.28 mm) of mature winter-sown oilseed rape
Property
Measurement
Mechanical properties of the shoot
Rigidity (N m2)
Bending moment (N m)
Bending modulus (MPa)
1.9 + 0.19
2.4 + 0.22
1840 + 109
Mechanical properties of the tap root (n ˆ 12)
Tap root bending moment (N m)
9.5
15
19
1.1 + 0.14
moment of somewhat less than half that of the stems
(Table 2).
Anchorage tests on metal tubes
All the tests worked well apart from the 19 mm tube in a
30 mm depth of soil. In this case the self-weight moment of
the tube pulled it over even without adding a load. In many
ways the behaviour of the metal tubes during the anchorage
tests was similar to that of the plants. As the tube was
pulled over the restoring moment rose at ®rst, before
starting to level o€, although it tended to increase even
beyond inclinations of 188 (Fig. 4). Qualitatively similar
results were obtained for the 9.5 and 19 mm tubes.
The maximum moments needed to pull the tubes over at
di€erent depths are summarized in Table 3. Initially the
restoring moment increased rapidly: restoring moments of
the tubes at a depth of 90 mm continued to rise,
approximately linearly after levelling o€ (Table 3, Fig. 4).
Therefore, to standardize results, the maximum moments
were analysed up to an angle of 258; at this angle all the
restoring moments had started to level o€ (i.e. for tubes at
30 and 60 mm depth) or the tubes were starting to rotate
and fall over under their own `self-weight moment'. Two
trends were clear. The maximum moment rose rapidly with
increasing depth, even faster than the predicted value of the
length squared [eqn (1)] (Table 3). In contrast there was a
35
Moment N m
30
25
20
15
10
5
0
5
10
15
20
Angle from vertical degrees
Restoring moment (N m)
Tube diameter
(mm)
Values are means of 15 plants + s.e.m., except for the tap root data
where n ˆ 12.
0
T A B L E 3. Results of the model anchorage tests. Maximum
restoring moments (up to an angular displacement of 258) for
the anchorage tests on metal tubes of varying diameter
25
F I G . 4. The results of ten model anchorage tests on 15 mm diameter
tubes at a depth of 30 mm (j), 60 mm (m) and 90 mm (h). Vertical
bars indicate + s.e.m (n ˆ 10).
30
Depth (mm)
60
90
1.4 + 0.14
1.5 + 0.13
Ð
6.1 + 0.55
6.8 + 0.70
9.2 + 0.62
24 + 1.1
25 + 2.2
23 + 1.7
Values are means of ten tests + s.e.m.
much lower increase in maximum moment with tube
diameter (Table 3) than predicted by eqn (1).
Perhaps the most biologically relevant result was that of
the 15 mm diameter tube at a depth of 30 mm (approximately the dimensions of the moving part of the tap root).
It can be seen that the overturning moment required was
1.5 + 0.13 N m (Table 3).
Factors of safety
The factor of safety against root lodging, FSR , was 4.41,
and the factor of safety against stem lodging FSS was 3.69.
DISCUSSION
The quantitative results from these experiments support the
model of anchorage developed in the introduction, and
provide evidence about the relative importance of the two
components. The anchorage moment (2.9 + 0.36 N m) was
similar to the sum of the tap root bending moment
(1.1 + 0.14 N m) and the resistance of the soil to rotation
of the 15 mm tube at a depth of 30 mm (1.5 + 0.13 N m),
suggesting that both components act as suggested. These
results also imply that the resistance of the soil provided a
slightly larger component of anchorage: around 60 %
compared with about 40 % for the tap root bending
moment. Of course, these values are only approximations
and the data could be improved by testing a larger number
of plants. However, the major role of both components, at
least in loamy soil, is clear.
A comparison of the anchorage moment with stem and
the self-weight moment of the plants (Crook and Ennos,
1994) allows one to assess the likelihood of stem and root
lodging. Surprisingly, the maximum stem moment
(2.4 + 0.22 N m) calculated from bending tests was slightly
lower than the maximum anchorage moment of the plants
in the ®eld, despite the fact that no ®eld plants su€ered stem
breakage. The discrepancy in the results probably occurred
because the bending tests actually measure the stem
bending moment 0.11 m above its base where the stem
was thinner (11.9 + 0.28 mm).
In this loamy soil, the anchorage moment and stem
bending moment at failure were similar, suggesting that
even in wet conditions plants have a very similar resistance
Goodman et al.ÐAnchorage Mechanics in Oilseed Rape
to root and stem lodging. However, the chances of either
occurring were probably very low. The `factors of safety'
against self-weight moment for FSR of 4.41, and for FSS of
3.69, were similar to those of lodging-resistant varieties of
winter wheat (Crook and Ennos, 1994) such as `Hereward'.
The height and relative ¯exibility of the stem might cause
the shoots of leaning plants to bend over further in wet and
windy conditions, increasing the self-weight moment, and
lowering the safety factor. However, the safety factors do
not take into account the forces generated by the wind, the
additional weight of wet foliage and also the e€ects of the
interaction of the foliage with adjacent plants which may
provide a degree of protection against lodging.
As might be expected, larger plants with thicker and
longer tap roots were better anchored. This is not surprising
because both the bending rigidity and the resistance to
lateral movement through the soil would be greater.
However, the moment required to rotate the metal tubes
scaled in a way that was not predicted by Broms (1964). The
maximum resistance rose even faster than the square of the
length of the tube as predicted by the model of anchorage
(Ennos and Fitter, 1992). An even greater surprise was that
tube diameter had little e€ect on resistance: thin tubes were
almost as dicult to move as thick ones. However, this
®nding may perhaps be explicable because the behaviour of
tubes in soil will be like that of narrow tines which have
been the subject of study by agricultural engineers (Spoor,
1973). The behaviour of agricultural soil when a narrow
tine is moved through it depends more on the tine's depth
than its width (Spoor, 1973). Shallow areas of soil will fail
in a brittle manner, cracks appearing on the side to which
the tine is moved, while deeper soil fails in compression and
is smeared. The depth to which brittle failure takes place
increases with decreasing moisture and compaction of the
soil, and decreased forward inclination of the tine.
These ®ndings have important implications for the
`design' of these root systems. The main implication is
that to maximize anchorage for minimum investment in
structural material, the best design is long and thin, rather
than short and wide. Therefore a plant should produce as
long a tap root as possible. Of course, there is a limit to this
process. Above a critical length the tap root will fail in
bending before the soil strength can be mobilized. Although
the mechanical role of the tap root is simpli®ed by assuming
that the root is e€ectively clamped below some critical
depth, it does enable the contribution of the tap root to the
anchorage moment to be assessed. The sti€ness of the tap
root will also play a role: if the root is too elastic there may
be some plastic deformation of the soil near the soil surface.
It may be that the tap roots of herbs such as rape are
composed of such a solid mass of xylem, as this will give a
strong, buckling resistant length of root.
Considering the major role lateral roots in the upper
reaches of the soil could make to anchorage (Ennos et al.,
1993a) it seems surprising that they do not develop in rape
plants. Perhaps the single root system of oilseed rape
principally evolved as a storage organ, to facilitate a
biennial growth habit, and less as a means of anchoring the
plant. It is clear that root systems are multifunctional
(Ennos and Fitter, 1992) and that this simple root system is
403
adequate to provide for most of the needs of the oilseed
rape plant. Indeed it is not surprising that modern varieties,
which have been selected for increased seed yield but not
for decreased shoot height, would have a greater self-weight
moment and therefore be more prone to anchorage failure.
In reality, a plant's resistance to overturning is dependent
not only on its genetic characteristics (Crook and Ennos,
1994) but also its ability to respond, during growth, to
physical stimuli (such as wind loading) by producing a
greater number of thicker, stronger roots that are composed
of a sti€er material (Goodman and Ennos, 1997). Clearly,
much more work needs to be carried out measuring the
behaviour of real and model roots in agricultural soils
before we can explain the design of even this simplest of
anchorage systems.
AC K N OW L E D G E M E N T S
We would like to thank Sue Challinor for technical
assistance. The work was carried out with funding from
the BBSRC.
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