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Anchorage Mechanics of the Tap Root System of Winter-sown Oilseed Rape (Brassica napus L.)
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 dierent mechanisms for providing adequate anchorage against toppling, and the mechanics of several dierent 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 9=2tDL2
1
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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
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(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 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 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 L3 dF=dY=48
E 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 stier 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
Maximum anchorage moment Shoot weight centre of gravity sin y
and the factor of safety against stem lodging, FSS , is given by: FSS
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
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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 dierence (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 73.7 %, P 5 0.001, n 13) and at 40 mm down the tap root (r2 65.3 %, P 5 0.001, n 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 77.4 %, P 5 0.001, n 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 13 for 0±18 degrees and n 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 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 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 73.7 %, P 5 0.001, n 13) and 40 mm down (r2 65.3 %, P 5 0.001, n 13) and the diameter of the stem at the base (r2 77.4 %, P 5 0.001, n 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
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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 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 dierent 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 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 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 suered 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 eects 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 eect on resistance: thin tubes were almost as dicult 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 eectively clamped below some critical depth, it does enable the contribution of the tap root to the anchorage moment to be assessed. The stiness 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 stier 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. L I T E R AT U R E C I T E D Armstrong EL, Nicol HI. 1991. Reducing height and lodging in rapeseed with growth regulators. Australian Journal of Experimental Agriculture 31: 245±250. Baylis AD, Wright ITJ. 1990. The eects of lodging and paclobutrazol±chlormequat chloride mixture on the yield and quality of oilseed rape. Annals of Applied Biology 116: 287±295. Broms BB. 1964. Lateral resistance of piles in cohesive soils. Journal of Soil Mechanics and Foundations Division: Proceedings of the American Society of Civil Engineers SM2 90: 27±63. Coutts MP. 1983. Root architecture and tree stability. Plant and Soil 71: 171±188. Coutts MP. 1986. Components of tree stability in Sitka spruce on peaty gley soil. Forestry 59: 173±197. Crook MJ, Ennos AR. 1993. The mechanics of root lodging in winter wheat Triticum aestivum L. Journal of Experimental Botany 44: 1219±1224. Crook MJ, Ennos AR. 1994. Stem and root characteristics associated with lodging resistance in four winter wheat varieties. Journal of Agricultural Science 123: 167±174. Crook MJ, Ennos AR. 1996. The anchorage mechanics of mature larch Larix europea x L. japonica. Journal of Experimental Botany 47: 1509±1517. Crook MJ, Ennos AR. 1997. Scaling of anchorage in the tap rooted tree Mallotus wrayi. In: Jeronmidis G, Vincent JFV, eds. Plant biomechanics: conference proceedings I. Reading, UK: Centre for Biomimetics, The University of Reading, 31±36. Crook MJ, Ennos AR, Banks JR. 1997. The function of buttress roots: a comparative study of the anchorage systems of buttressed (Aglaia and Nephileum ramboutan species) and non-buttressed (Mallotus wrayi) tropical trees. Journal of Experimental Botany 48: 1703±1716. Ennos AR. 1993. The scaling of root anchorage. Journal of Theoretical Biology 161: 61±75. Ennos AR, Fitter AH. 1992. Comparative functional morphology of the anchorage systems of annual dicots. Functional Ecology 6: 71±78. Ennos AR, Crook MJ, Grimshaw C. 1993a. A comparative study of the anchorage systems of himalayan balsam Impatiens glandulifera and mature sun¯ower Helianthus annuus. Journal of Experimental Botany 44: 133±146. Ennos AR, Crook MJ, Grimshaw C. 1993b. The anchorage mechanics of maize Zea mays. Journal of Experimental Botany 44: 147±153. Goodman AM, Ennos AR. 1997. The response of ®eld-grown sun¯ower and maize to mechanical support. Annals of Botany 79: 703±711.
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Gordon JE. 1978. Structures: or why things don't fall down. London: Penguin Books. Islam N. 1988. Eects of nitrogen fertiliser on the growth and yield of oilseed rape (B. napus L.). PhD Thesis, University of Newcastle upon Tyne, UK. Lancashire PD, Bleiholder H, Van Den Boom T, Langeluddeke P, Stauss R, Weber E, Witzenberger A. 1991. A uniform decimal code for growth-stages of crops and weeds. Annals of Applied Biology 119: 561±601. Rowell DL. 1994. Soil science: methods and applications. Harlow: Longman Group. Scott RK, Ogunremi EA, Ivins JD, Mendham NJ. 1973. The eect of fertilisers and harvest date on growth and yield of oilseed rape
sown in autumn and spring. Journal of Agricultural Science 81: 287±293. Sokal RR, Rohlf FJ. 1995. Biometry: the principles and practice of statistics in biological research, 3rd edn. New York: W.H. Freeman and Company. Spoor G. 1973. Fundamental aspects of cultivations. MAFF Technical Bulletin 29: Soil physical conditions and crop production. Stokes A. 1999. Strain distribution during anchorage failure of Pinus pinaster Ait. at dierent ages and tree growth response to windinduced root movement. Plant and Soil 217: 17±27. Vincent JFV. 1992. Biomechanics-materials: a practical approach. Oxford: 165-IRL Press at Oxford University Press.
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 dierent mechanisms for providing adequate anchorage against toppling, and the mechanics of several dierent 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 9=2tDL2
1
# 2001 Annals of Botany Company
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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
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(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 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 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 L3 dF=dY=48
E 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 stier 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
Maximum anchorage moment Shoot weight centre of gravity sin y
and the factor of safety against stem lodging, FSS , is given by: FSS
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
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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 dierence (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 73.7 %, P 5 0.001, n 13) and at 40 mm down the tap root (r2 65.3 %, P 5 0.001, n 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 77.4 %, P 5 0.001, n 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 13 for 0±18 degrees and n 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 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 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 73.7 %, P 5 0.001, n 13) and 40 mm down (r2 65.3 %, P 5 0.001, n 13) and the diameter of the stem at the base (r2 77.4 %, P 5 0.001, n 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
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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 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 dierent 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 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 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 suered 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 eects 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 eect on resistance: thin tubes were almost as dicult 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 eectively clamped below some critical depth, it does enable the contribution of the tap root to the anchorage moment to be assessed. The stiness 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
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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 stier 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. L I T E R AT U R E C I T E D Armstrong EL, Nicol HI. 1991. Reducing height and lodging in rapeseed with growth regulators. Australian Journal of Experimental Agriculture 31: 245±250. Baylis AD, Wright ITJ. 1990. The eects of lodging and paclobutrazol±chlormequat chloride mixture on the yield and quality of oilseed rape. Annals of Applied Biology 116: 287±295. Broms BB. 1964. Lateral resistance of piles in cohesive soils. Journal of Soil Mechanics and Foundations Division: Proceedings of the American Society of Civil Engineers SM2 90: 27±63. Coutts MP. 1983. Root architecture and tree stability. Plant and Soil 71: 171±188. Coutts MP. 1986. Components of tree stability in Sitka spruce on peaty gley soil. Forestry 59: 173±197. Crook MJ, Ennos AR. 1993. The mechanics of root lodging in winter wheat Triticum aestivum L. Journal of Experimental Botany 44: 1219±1224. Crook MJ, Ennos AR. 1994. Stem and root characteristics associated with lodging resistance in four winter wheat varieties. Journal of Agricultural Science 123: 167±174. Crook MJ, Ennos AR. 1996. The anchorage mechanics of mature larch Larix europea x L. japonica. Journal of Experimental Botany 47: 1509±1517. Crook MJ, Ennos AR. 1997. Scaling of anchorage in the tap rooted tree Mallotus wrayi. In: Jeronmidis G, Vincent JFV, eds. Plant biomechanics: conference proceedings I. Reading, UK: Centre for Biomimetics, The University of Reading, 31±36. Crook MJ, Ennos AR, Banks JR. 1997. The function of buttress roots: a comparative study of the anchorage systems of buttressed (Aglaia and Nephileum ramboutan species) and non-buttressed (Mallotus wrayi) tropical trees. Journal of Experimental Botany 48: 1703±1716. Ennos AR. 1993. The scaling of root anchorage. Journal of Theoretical Biology 161: 61±75. Ennos AR, Fitter AH. 1992. Comparative functional morphology of the anchorage systems of annual dicots. Functional Ecology 6: 71±78. Ennos AR, Crook MJ, Grimshaw C. 1993a. A comparative study of the anchorage systems of himalayan balsam Impatiens glandulifera and mature sun¯ower Helianthus annuus. Journal of Experimental Botany 44: 133±146. Ennos AR, Crook MJ, Grimshaw C. 1993b. The anchorage mechanics of maize Zea mays. Journal of Experimental Botany 44: 147±153. Goodman AM, Ennos AR. 1997. The response of ®eld-grown sun¯ower and maize to mechanical support. Annals of Botany 79: 703±711.
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