The Effects of Soil Bulk Density on the Morphology and Anchorage Mechanics of the Root Systems of Sunflower and Maize

Annals of Botany 83 : 293–302, 1999 Article No. anbo.1998.0822, available online at http:\\www.idealibrary.com on

The Effects of Soil Bulk Density on the Morphology and Anchorage Mechanics of the Root Systems of Sunflower and Maize A. M. G O O D M A N* and A. R. E N N O S School of Biological Sciences, 3.614 Stopford Building, UniŠersity of Manchester, Oxford Road, Manchester M13 9PT, UK Received : 4 September 1998

Returned for revision : 23 October 1998

Accepted : 22 November 1998

The effects of soil bulk density and hence strength on two contrasting species of herbaceous annuals, the dicot sunflower (Helianthus annuus L.) and the monocot maize (Zea mays L.), were investigated by comparing the morphology and mechanics of field-grown plants in soil with a low and high bulk density. Soil with a low bulk density had a significantly lower penetration resistance (118p4n4 kPa) than the high bulk density soil (325p12n2 kPa ; P 0n0001). Soil strength affected shoot and root systems of both species but had no significant effect on shoot height. In both species roots were thicker closer to the stem base in strong soil compared to those in weaker soil. Sunflower tap-roots growing in strong soil tapered more rapidly than those in weak soil. Only in maize, however, were roots growing in weak soil stiffer than those in strong soil. Despite only small absolute differences in the penetration resistance of the soil both species growing in strong soil had greater anchorage strength than those in weak soil. As a consequence more plants in weak soil lodged compared with those growing in strong soil. This study shows that plants can, to a small extent, respond to changes in soil strength, but that changes do not appear to compensate fully for alterations in soil conditions. Furthermore it may be possible, by manipulating soil strength, to control lodging. # 1999 Annals of Botany Company Key words : Roots, compaction, soil strength, anchorage mechanics, bulk density, thigmomorphogenesis, lodging, Helianthus annuus L., Zea mays L.

I N T R O D U C T I ON It is well known that soil bulk density and strength are important factors affecting both shoot and root growth of plants. Areas of compact soil with a high shear strength can be caused by agricultural machinery, and consequently numerous studies have investigated the effects of soil bulk density and strength on plant shoot and root growth (Barley, 1963 ; Barley and Greacen, 1967 ; Goss, 1977 ; Masle and Passioura, 1987 ; Atwell, 1988 ; Assaeed et al., 1990 ; Materechera, Dexter and Alston, 1991). Previous studies on the effects of strong soil on shoot growth have produced conflicting results. In many studies, both the height and weight of shoots were reduced in strong soils when compared to those grown in weak soils (Chaudhary and Prihar, 1974 ; Masle and Passioura, 1987 ; Atwell, 1990 ; Lowery and Schuler, 1991). However, in other studies shoot growth was not affected by soil strength (Kirkegaard, So and Troedson, 1992 ; Oussible, Crookston and Larson, 1992) and was even promoted in strong soils compared with weak soils (Iijima et al., 1991). The results of investigations into the influence of soil strength on root growth are more consistent. In strong soils there is a reduction in the elongation rate of roots (Barley, 1962, 1963 ; Goss, 1977). Barley (1962) and Goss (1977) simulated high soil strength by applying radial pressure to roots growing in ballotini ; the elongation rate fell sharply * Present address : De Montfort University Lincoln, School of Agriculture, Lindsey Centre, Riseholme, Lincoln LN2 2LG.

0305-7364\99\030293j10 $30.00\0

when mechanical impedance was increased. Bengough and Young (1993) showed that the daily elongation rate of pea roots which were growing through a high bulk density soil (1n4 Mg m−$) was only about 65 % of that of roots which were growing through the weaker low bulk density soil (0n85 Mg m−$). Not only is there a reduction in elongation growth of roots but this is accompanied by an increase in the diameter of roots (Barley, 1965 ; Atwell, 1988) and also changes in the pattern of lateral root initiation (Russell, 1977 ; Tsegaye and Mullins, 1994). An increase in the diameter of roots in response to high soil strength is well documented in many species (Materechera et al., 1991) including maize (Barley, 1963), cotton (Gossypium hirsutum L.), peanuts (Arachis hypogea L.) (Taylor and Ratliff, 1969) and barley (Hordeum Šulgare L.) (Wilson and Robards, 1977). In lupins (Lupinus angustifolius), the thickening of roots is caused by an increase in the diameter of each cell and is associated with a small reduction in cell length (Atwell, 1988). Roots growing in soil experience mechanical stress to varying extents. When roots grow through pores of insufficient diameter the root tip deforms the soil and hence experiences mechanical stress. Indeed, Bengough, Mackenzie and Elangwe (1994) showed that when a compressive force is applied to a seedling pea (Pisum satiŠum L.) root there is a stress response. Root elongation rate decreased by 50 % within 30 min, a smaller increase in growth rate occurred when the force was removed. Root growth does not return to normal as soon as the stress is removed and there appears to be a lag-phase. In the roots of # 1999 Annals of Botany Company

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Goodman and Ennos—Soil Strength and Root Mechanics

pea seedlings growing out of a strong layer of soil into weak soil, the growth rate of roots did not fully recover until 4–10 d later (Bengough and Young, 1993). Similar effects were found by Goss and Russell (1980) who studied the effects of releasing the confining pressure from whole root systems of barley grown in ballotini. This suggests that changes in root morphology may be due to increases in the mechanical stresses each root experiences. This delay in the recovery of elongation rates is a similar response to that found in stems of mechanically-stimulated beans (Phaseolus Šulgaris L.) by Jaffe (1973). Despite the wide coverage of the effects of high soil strength on plant growth in the literature, most studies have concentrated on the primary growth of the root system (Pearson, Ratliff and Taylor, 1970 ; Goss, 1977 ; Kirkegaard et al., 1992 ; Kaspar, Logsdon and Prieksat, 1995). Only one study has investigated the effects of soil strength on anchorage mechanics of seedlings (Ennos, 1990), and one the effects on the mechanics of the anchorage systems of mature plants (Crook, 1994). Ennos (1990) suggested that roots of leek seedlings (Allium porrum L.) grown in a weaker soil should provide less anchorage force per unit length than those grown in a stronger soil. In contrast, Crook (1994) showed that coronal root development in wheat was unaffected by soil strength ; the length, number and bending strength were the same regardless of whether the seedbed had been loosened or compacted. Recent work which has investigated the effects of mechanical stress on the roots of sunflower and maize showed that not only are root systems able to respond locally to mechanical stimulation but that this leads to an overall increase in anchorage strength (Goodman and Ennos, 1996, 1997 a, b), though different species responded in rather different ways. Materechera et al. (1991) examined the effects of increased soil strength on 22 monocot and dicot species ; the dicots showed less effect of increased soil strength on elongation rates of roots than the monocots. In weak soil anchorage strength is reduced and stresses might be expected to be transmitted further down the roots. These stresses might stimulate more secondary growth of roots further from the stem, a response which would increase the anchorage strength and so, to some extent, compensate for the weakness of the soil. This study examines the effect of altering soil strength by changing its bulk density on the morphology and mechanics of the major anchorage roots and hence on the anchorage strength of plants. These results may help determine the extent to which plants compensate for changes in soil strength and how changes in the cultivation of crops might reduce losses due to uprooting or lodging.

the peat, gravel and sandy loam were thoroughly mixed to produce a total weight of 600 kg of soil. This was mixed with 1n4 kg of John Innes base and 0n3 kg of lime and left to equilibrate for 2 d in covered dustbins. Fifteen soil samples with a fresh weight of approx. 45 g each were randomly collected at a depth of 20–30 cm. The moisture content was calculated by taking the difference in sample weight before and after oven drying for 4 d at 80 mC. Pots were packed to a wet bulk density of 1n0 Mg m−$ for weak, and 1n4 Mg m−$ for strong soil at a gravimetric moisture content of 14n1p0n34 %. Sunflowers were grown in 7n5 l (top diameter 25 cm, depth 21 cm) tapered pots, and maize was grown in 5 l (top diameter 21 cm, depth 19 cm) tapered pots. Soils of low strength were prepared by adding a fraction of soil (weighed to the nearest gram) and then tamping the pot once on a hard surface creating a single 4 cm layer of soil. This was repeated until there were six layers in the 7n5 l pots and five layers in the 5 l pots. For the strong soil treatment each layer was compacted to 4 cm by releasing a 5 kg ram of diameter equal to the internal basal diameter of the pot from a height of 30 cm onto the soil ten times. In addition 20 pots without plants growing in them were placed at random in the trial ; half the pots contained weak soil and half strong soil. These pots were split equally between the two trials ; ten pots were placed at random in the sunflower area and ten in the maize area. Plant establishment Six hundred seeds of Helianthus annuus L. (‘ Vincent ’) were germinated in moist vermiculite in July 1996 in a glasshouse. After 3 d 64 seedlings were transplanted singly into 7n5 l pots, half into pots containing weak soil and half into pots containing strong soil. Early transfer ensured that the tap root would be unrestricted and undergo normal growth (Ennos, Crook and Grimshaw, 1993 b). The sunflower radicle, approx. 1n5 cm long, was placed in a 2 cm deep tapered hole (8 mm maximum diameter) made centrally in the top layer of soil. The same number of Zea mays L. (‘ Lg 20–80 ’) seeds were germinated in moist Fisons F2 compost and, after 4 d, 64 seedlings were transplanted singly into 5 l pots, half into pots containing weak soil and half into pots containing strong soil. Maize seedlings were planted in a similar way to sunflower seedlings, but using a 20 mm diameter hole. Loose soil was then placed around the seedling. All pots were placed on saucers in a glasshouse at the University of Manchester’s experimental grounds. The saucers were filled daily with water and a 16 h photoperiod was maintained with sodium lights supplementing natural daylight as required.

MATERIALS AND METHODS Soil characteristics and core preparation

Field site and experimental design

A John Innes No. 1 growth medium was prepared using sandy loam topsoil (0–10 cm), peat and sand ( 2 mm diameter) in a ratio of 7 : 3 : 2 (v\v\v). The sandy loam was sieved through a 10 mm sieve, heated for 75 min to a maximum temperature of 100 mC, and left to cool. After 1 d

An area of bare loam soil at the University of Manchester Firs experimental ground was prepared in April 1996 by excavating holes 0n5 m apart and approx. 25 cm diameteri 25 cm deep. The site was sheltered with a prevailing south westerly wind.

Goodman and Ennos—Soil Strength and Root Mechanics After 10 d, when the seedlings were at the two leaf stage, pots from the glasshouse were randomly placed into holes in the trial area with the top of the pot level with the soil surface. Both trials were arranged in a randomized twin block design of 64 plants (excluding guard plants). Each trial was protected by sowing two guard rows of the same species around the perimeter.

295

first three nodes and four internodes was removed for mechanical testing. Fresh and dry weights of the shoot systems of each plant were also measured : the stem, leaves and reproductive parts being weighed separately for both species. Root morphology

Trail management Metaldehyde slug pellets were broadcast at 15 kg ha−" across the whole trial to control slugs and snails. Weeds were controlled as required by hand, and the area was irrigated during dry spells with a rotary sprayer to prevent water stress. Sunflower and maize plants were randomly harvested 15 and 16 weeks, respectively, after planting [maize kernels at the milk stage (GS 73 ; Tottman and Broad, 1987) ; sunflower achenes fully expanded]. Half the plants were randomly selected for morphological examination. Measurement at harŠest Soil strength measurements. Before testing the strength of the soil it was brought to approx. field capacity by watering to saturation over 3 d, and allowing it to drain under gravity for 48 h. Two penetrometer and two shear tests were carried out at random points within a zone at least 3 cm from the edge of the core. Shear strength was measured using a 33 mm (Pilcon DR 2645 ; Pilcon Engineering Ltd) shear vane. The vane was pressed into the soil to a depth of 5 cm and was slowly rotated. Readings of shear strength in kPa were indicated on a dial. The soil in each unplanted pot was also tested twice with a penetrometer using a 6 mm diameter 60m semi-angle probe attached to a Mecmesin portable force indicator (Mecmesin Ltd, Broadbridge Heath, West Sussex, UK). This was pushed into the soil to a depth of 10 cm and the maximum force recorded in Newtons (unfortunately the force could not be recorded as a function of depth). Penetrometer resistance, Q, is defined in eqn (1), where F is the force required to push the penetrometer probe through the soil, and A is the cross-sectional area of the penetrometer cone (Bengough and Mullins, 1990). Q l F\A

(1)

Stem morphology At harvest, the height and degree of taper of each stem was measured by taking diameter measurements at the soil level, at heights of 50 and 100 cm, and at the top of the stem just under the flower in sunflower, or the tassel of each plant in maize. Shoot height was taken as the distance from the surface of the soil to the point at which the flower head in sunflower and the tassel in maize joined the stem. Shoots were then cut off at the base, just above the topmost adventitious roots, and a length of 25 cm which included the

The root systems were stored in a cold room at 5 mC for up to 14 d until all of the roots had been measured and mechanically tested. Each was then carefully washed and all the fine roots collected using a sieve. The soil under each pot to a depth of 20 cm was carefully sieved to collect any roots not contained by the pot. The method of determining the angle of spread of the root system, adapted from Pinthus (1967), involved measurements (to the nearest 5m) in two planes by placing the system on a paper protractor and reading the maximum angle of the whole root system ; the roots were then rotated by 90m and the maximum angle again measured. The mean of these two measurements gave the spreading angle of the root system. For maize, the number of roots was counted at each node and each was classified either as entering the soil or not. The first-order lateral roots of each species were then removed at the base using a hacksaw and transverse basal sections were cut and stained with phloroglucinol to reveal the extent of lignification. The total number of structural roots (defined as firstorder lateral roots which had a basal diameter greater than 2 mm) was counted for each plant. Three first-order lateral roots were sampled randomly from each plant for measurement of the degree of root taper : root diameters at the base, the base plus 4 cm and the base plus 8 cm were measured using callipers. All of the roots were placed on moist sponges to prevent alteration of the mechanical properties by desiccation. The dimensions of the tap roots of sunflowers were also recorded by measuring their diameter at 7i2 cm intervals from the base. The length of sunflower first-order lateral roots which showed noticeable rigidity in bending (termed ‘ rigid root length ’) was also measured from the base down to the point at which the root no longer resisted bending. Fresh and dry weights of roots Root systems were divided into three different components : rigid first-order lateral roots which have a primary anchorage role ; central anchorage element (the tap root in the case of sunflower, and the basal internode in the case of maize) ; and fine lateral roots. Samples were weighed before and after being oven dried at 70 mC for 5 d. To investigate the partitioning of biomass between the shoot and root system, values of total root weight and total shoot weight were used to calculate the root : shoot ratio. Mechanical tests Three-point bending tests were carried out both on the stems and on all first-order lateral roots (
2 mm base

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Goodman and Ennos—Soil Strength and Root Mechanics

diameter) of both sunflower and maize, using a universal testing machine (Instron, model 4301).

where dF\dY is the initial slope of the force displacement curve. The bending modulus, E, is given by : E l R\I

Stems The diameter of each stem sample was measured at the mid-point using callipers ; samples included the first three nodes and four internodes. Stem samples were placed between two supports which were set apart a distance of 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−", bending the sample until it eventually buckled. A computer with an interface to the testing machine was used to produce a graph of force Šs. displacement, permitting calculation of the mechanical properties of the sample (Ennos, Crook and Grimshaw, 1993 a).

Roots The basal 60 mm of each root was cut where it joined the tap-root in sunflower or the basal nodes in maize, stripped of fine roots using a razor, placed between two sponges before testing, and the diameter was measured at the midpoint. The sample was placed between two supports (set apart a distance of approx. 15 times the diameter of the sample) and a pushing probe of radius 2 mm was lowered until it just touched the sample. During the test the crosshead was lowered at a rate of 10 mm min−", bending the sample until it failed. Using data collected from the test an interfaced computer calculated three mechanical properties : the bending strength, S [eqn (2)], and the rigidity, EI [eqn (3)], of each root ; and the bending modulus, E [eqn (4)], of the material of which they were composed. In the analysis performed using the Instron it was assumed that there was no taper. The errors due to this assumption are small ; for a beam of circular cross section, where the angle between the top edge of the beam and the horizontal is less than 20m the errors are less than 10 % (Gere and Timoshenko, 1991). In both species, the roots of flexed and control plants showed a low degree of root taper and this angle was no more than 2m.

Analysis of bending tests The mechanical properties of samples were calculated using well-known equations (Gordon, 1978). Bending strength is given by the expression : S l Fmax L\4

(2)

where Fmax is the maximum force a sample will withstand before it fails and L is the distance between the supports. The bending rigidity, R, of a uniform beam is the resistance of that beam to curvature and is given by : R l L$(dF\dY )\48

(3)

(4)

where R is the rigidity of the sample [eqn (3)] and I is the second moment of area. In sunflowers, which are cylindrical in cross-section, the second moment of area was calculated for a solid cylinder using πr%\4 where r is the radius. For maize, which is elliptical in cross-section, the second moment of area was calculated for an ellipse using π ba$\4, where a and b are the radii of the major and minor axes of the ellipse. A high modulus indicates a stiffer material. Lodging assessment Lodging (the permanent displacement of the stem from the vertical ; Pinthus, 1973) occurred in late September. The extent and type of lodging was recorded by scoring the severity of inclination of the stem base from the vertical. The number of plants which had lodged (stem base inclined
20m from the vertical) growing in the high bulk density soil was then compared with the number of lodged plants growing in the low bulk density soil. Statistical analysis A Kolmogorov-Smirnov test was used to test the normality and the similarity of the shape of the underlying distributions before proceeding with analysis of variance. The count data, before analysis, were normalized using a square root transformation. Two-way analysis of variance was used whenever possible to identify differences between treatments and account for block effects. The soil shear strength and penetrometer data were analysed using a repeated measure analysis of variance. Chi-squared tests were used to detect differences in the incidence of lodging between plants which were growing in a weak and strong soil. All values in the text are meansps.e. RESULTS EnŠironmental effects In both sunflowers and maize there was a significant difference in shoot height between blocks (P 0n05) ; this was most probably due to sheltering from nearby buildings. One of the sunflowers growing in the weak soil was badly damaged by slugs and was excluded from the analysis. Soil strength There were significant differences between the soil shear strength of the high (1n4 Mg m−$) and low (1n0 Mg m−$) bulk density treatments (P 0n01). Soils with a low bulk density had a significantly lower shear strength (4n7p0n21 kPa) than those with a high bulk density (11n9p0n61 kPa). Penetrometer readings followed the same pattern ; the maximum penetration resistance of low bulk density soil (118 p4n4 kPa) was significantly lower than the high bulk density soil (325p12n2 kPa ; P 0n0001). There were no

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Goodman and Ennos—Soil Strength and Root Mechanics

T     1. Morphology and mechanical properties of the mature stems (basal sample) of sunflower and maize growing in weak and strong soils Sunflower

Maize

Property

Weak soil

Strong soil

P

Weak soil

Strong soil

P

Shoot height (cm) Stem diameter (mm) Base Basej50 cm Basej100 cm Top Mechanical properties Rigidity (Nm#) Bending strength (Nm) Bending modulus (MPa)

113p1n6

112p1n7

NS

110p2n7

116p2n2

NS

16n3p0n57 15n1p0n63 13n4p0n50 27p1n7

17n5p0n56 16n9p0n63 14n6p0n38 29p1n9

NS * NS NS

17n9p0n31 13n4p0n43 6n8p0n37 5n1p0n10

18n1p0n29 14n0p0n50 9n0p0n81 5n0p0n15

NS NS * NS

6n2p0n73 7n2p0n71 1820p96

8n5p0n85 9n5p0n81 1670p71

* * NS

2n6p0n24 3n5p0n33 760p37

3n1p0n16 4n7p0n28 820p76

NS * NS

Results were analysed using two-way ANOVA. Only maize showed a significant block effect (P 0n05). Values are means of 16 plantsps.e.m. (except for mechanical properties where n l 15). ** P 0n01 ; * P 0n05 ; NS, not significant, P
0n05.

T     2. Fresh and dry weights of shoots of mature sunflower and maize growing in weak and strong soils Sunflower

Fresh weight (g) Stem Leaves Reproductive Basal node Total Dry weight (g) Stem Leaves Reproductive Basal node Total

Maize

Weak soil

Strong soil

P

Weak soil

Strong soil

P

148p13 72p6n9 140p11

192p16 105p12 205p15

* * **

360p28

502p41

**

110p4n6 71p2n5 42p4n6 4n5p0n3 228p9n6

126p3n6 69p2n1 55p3n9 5n5p0n25 256p6n0

* NS NS * NS

29p2n2 16p1n5 12p0n88

35p2n3 22p1n8 18p1n4

NS * **

57p4n4

75p5n7

*

16n3p0n77 16n1p0n50 6n4p0n50 1n2p0n08 40p1n6

18n8p0n55 17n3p0n37 8n2p0n51 1n2p0n04 46p1n0

* NS * NS **

Results were analysed using two-way ANOVA and there was a distinct block effect in maize (P ** P 0n01 ; * P 0n05 ; NS, not significant, P
0n05.

significant differences in the shear strength or penetrometer readings between sunflower and maize trials (P
0n05). Shoots Morphology. Soil strength had a small effect on the shoots of both species. There was no significant difference in the height of sunflower or maize grown in strong soils compared with those grown in weak soils (Table 1). Neither did plants show any significant difference in basal diameter between treatments, although sunflowers grown in strong soil were thicker at 50 cm, and maize at 100 cm, than those grown in weak soil. However, there were differences in fresh and dry weights of shoots between treatments. Sunflowers grown in strong soil had a greater stem, leaf, reproductive and total shoot fresh weight than those growing in weak soil. There was no significant difference in the dry weight of sunflower stems between treatments. Maize in strong soil only showed significant increases in the fresh weight of the stem and basal node, and increased dry weight of the stem and reproductive parts (Table 2).

0n05). Values are means of 16 plantsps.e.m.

Mechanics. Both species showed significant differences between treatments in the mechanical properties of stems. Sunflowers grown in strong soil had more rigid and stronger stems than those grown in weak soil. However, maize plants grown in strong soils had stronger, but not more rigid stems than those in weak soil (Table 1). Roots Morphology. The root systems of sunflower and maize showed a greater response to soil strength. There was no significant difference in the number or weight of first-order laterals of sunflower or maize growing in weak compared to strong soil (Tables 3 and 4). Neither was there a visible difference between treatments in the degree of lignification of roots of either species. However, there were other differences in root morphology. In sunflower, the root system of plants grown in strong soil had a greater angle of spread than those grown in weak soil (Table 3). The basal diameter of first-order lateral roots growing in strong soil was also significantly thicker than that of plants grown in weak soil (Fig. 1). Sunflower tap-roots growing in strong

298

Goodman and Ennos—Soil Strength and Root Mechanics T     3. Root morphology of mature sunflower and maize growing in weak and strong soils Morphological characteristics Sunflower Root angle of spread (degrees) Root lateral number (basal diameter
2 mm) Length of rigid root (cm) Maize Root angle of spread (degrees) Root lateral number (basal diameter
2 mm) First internode length (cm)

Weak soil

Strong soil

P

108p3n2 35p1n5 65p2n1

121p1n8 35p1n4 67p1n7

** NS NS

82p3n7 18n1p0n88 2n2p0n09

82p1n9 19n6p0n94 2n5p0n10

NS NS *

Results were analysed using two-way ANOVA. Values are means of 16 plantsps.e.m. (except for sunflower root angle where n l 15). ** P 0n01 ; * P 0n05 ; NS, not significant, P
0n05.

T     4. Fresh and dry weights of the first-order lateral roots of mature sunflower and maize growing in weak and strong soils Sunflower

Fresh weight (g) First-order laterals Tap root Fine roots Total Dry weight (g) First-order laterals Tap root Fine roots Total

Maize

Weak soil

Strong soil

P

Weak soil

Strong soil

P

21p1n9

27p2n5

NS

20p1n1

22p1n1

NS

20p2n0 22p2n2 63p4n5

20p1n7 23p3n1 70p5n4

NS NS NS

68p2n9 88p2n4

63p2n2 85p2n5

NS NS

3n4p0n33

4n3p0n42

NS

3n0p0n14

3n2p0n14

NS

4n9p0n51 2n3p0n18 10n6p0n86

4n8p0n47 2n4p0n27 11n5p0n88

NS NS NS

5n9p0n28 8n9p0n24

5n7p0n22 8n9p0n28

NS NS

Results were analysed using two-way ANOVA. Only maize showed a significant block effect (P ** P 0n01 ; * P 0n05 ; NS, not significant, P
0n05.

0n05). Values are means of 16 plantsps.e.m.

7

6

Diameter (mm)

5

4

3

2

1

0

4 Distance from base (cm)

8

F. 1. Diameter of first order lateral roots of sunflower (=, >) and maize ( , ) grown in weak (=, ) and strong soil (>, ). Mean diameter was taken at 4 cm intervals from the base of the root, and the results were analysed using two-way ANOVA (n l 16), showing significant differences (P 0n05) at the base for both species and 4 cm from the base in maize only. There was no significant block effect (P
0n05). Vertical bars indicateps.e.m.

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Goodman and Ennos—Soil Strength and Root Mechanics 30

Diameter (mm)

25

20

15

10

5

0

2

4

6 8 Distance from base (cm)

10

12

14

F. 2. Diameter of the tap roots of sunflower plants grown in weak (=) and strong soil (>). Mean diameter was taken at 2 cm intervals from the base of the stem, and results were analysed using two-way ANOVA (n l 16) showing significant differences (P 0n05) at the base, 12 and 14 cm from the base. There was no significant block effect (P
0n05). Vertical bars indicateps.e.m.

0·45 0·40

Mean root to shoot ratios

0·35 0·30 0·25 0·20 0·15 0·10 0·05 0·00

Dry weight Fresh weight Sunflower

Fresh weight

Dry weight Maize

F. 3. The effects of soil strength on the root : shoot ratio of sunflower and maize grown in weak ( ) and strong soil (). Only sunflowers showed a significant difference between the fresh and dry weight root : shoot ratios of plants grown in weak soil compared to those grown in strong soil (P 0n01). The ratios were analysed using two-way ANOVA (n l 16). Vertical bars indicateps.e.m.

soil tapered more rapidly than those in weak soils (Fig. 2). In contrast, there was no significant difference in the angle of spread of the root system of maize between treatments (Table 3). However, the first 4 cm of the first-order lateral roots were thicker in strong than in weak soil (Fig. 1). There were also differences in the root : shoot ratio between treatments. In sunflowers, plants grown in strong soil had significantly lower root : shoot ratios than those grown in weak soil (Fig. 3). Mechanics. There was only a small effect of soil strength

on the mechanical properties of first-order lateral roots. In sunflower, there were no significant differences between roots growing in strong compared to weak soil. In maize, roots growing in weak soil were stiffer than those in strong soil (Table 5). Lodging and anchorage failure In late September both sunflower and maize plants lodged ; failure occurred in the roots with no buckling of

300

Goodman and Ennos—Soil Strength and Root Mechanics T     5. Mechanical properties of the roots of sunflower and maize growing in weak and strong soils Sunflower

Maize

Mechanical property

Weak soil

Strong soil

P

Weak soil

Strong soil

P

Rigidity (Nm#i10−$) Bending strength (Nmi10−#) Bending modulus (MPa)

1n5p0n25 1n8p0n22 221p31

1n8p0n37 2n1p0n28 206p23

NS NS NS

5n0p0n53 4n9p0n43 958p74

5n0p0n38 4n7p0n29 709p45

NS NS **

Results were analysed using two-way ANOVA. Neither species showed a significant block effect (P
0n05). Values are means of 16 plantsps.e.m. ** P 0n01 ; * P 0n05 ; NS, not significant, P
0n05.

stems. In sunflower, there was a significant difference between the number of plants lodged in the different soil density treatments (χ# l 6n2, d.f. l 1, P 0n05) ; from a total of 64 sunflowers, 11 plants from the weak soil treatment lodged compared with two in the strong soil treatment. In maize there was also a similar significant effect (χ# l 24n1, d.f. l 1, P 0n01) ; 22 plants from the weak soil treatment lodged compared with two in the strong soil treatment.

D I S C U S S I ON This study shows that despite differences in soil bulk density, and hence strength, there were only small effects on the morphology and mechanical properties of roots of sunflower and maize. The effects of soil strength on the thickness of roots in this study are, however, consistent with those found in the literature (Barley, 1965 ; Atwell, 1988 ; Materechera et al., 1991) ; both species showed an increase in the diameter of first-order lateral roots in the stronger soil. In the stronger soil, roots were thicker closer to the base of the stem probably because in stronger soil they would sway less in the wind and would therefore experience higher peak stresses. However, stresses would be transmitted faster from the roots to the ground in the strong soil (Ennos and Fitter, 1992), so roots would not be stressed so far away from the base of the stem. Therefore it is not surprising that the taper of the tap-roots of sunflower was greater in strong soils. However, despite the increase in the thickness of roots, there was no change in root weight, possibly because the shorter root axes are often thicker (Atwell, 1993). Surprisingly, we found few differences in the mechanical properties of first-order lateral roots between treatments. Earlier work has shown that large differences can occur in the mechanical properties of roots in response to mechanical stimulation (Goodman and Ennos, 1996, 1997 a, b) ; the roots of mechanically-stimulated plants were thicker, stronger and composed of a stiffer material than those of untreated plants. In this study there were few effects of mechanical impedance on the mechanical properties of roots. We might have expected differences in root strength and stiffness closer to the stem base between plants grown in strong soil compared to those grown in weak soil because of the increase in stress at that point. The effects of soil strength on the mechanical properties of roots may have been greater if the difference in soil strength between

treatments was larger. Soil strength measurements in this study were small compared to the strength required to stop root growth in the field. It is usually considered that soil strength is a problem for field grown crops if soil penetrometer resistance exceeds 2 MPa (Materechera et al., 1991). Soil strength also had noticeable effects on the anchorage mechanics of sunflower and maize. Despite comparatively small absolute differences in soil penetration resistance and root morphology there were large differences in the susceptibility of sunflower and maize to lodging. Sunflowers grown in strong soil were more stable (i.e. lodged less) than those grown in weak soil. A similar effect was seen in maize with plants growing in weak soil being more likely to lodge than those growing in strong soil. Significant differences between lodging indicate that any compensatory changes in root growth in weak soil are inadequate to restore the same anchorage as in strong soil. This study suggests that it may be possible to reduce lodging in crops grown in very loose soil by compacting soil, without a significant reduction in shoot height. Oussible et al. (1992) showed that in compacted soils, even at penetration resistances of 1n5-4 MPa, compaction had no consistent effect on shoot height and no effect on shoot dry weight. Crook (1994) showed that the resistance to overturning was lowest in loosely cultivated seedbeds and greatest in winter wheat plants grown in the most compact seedbeds. In contrast, other studies have shown that shoot fresh and dry weight is lower in plants grown in compacted soil with a similar penetration resistance (1n4-4 MPa) (Atwell, 1990 ; Cook et al., 1996). Atwell (1990) found that the root : shoot ratio in winter wheat was smaller when roots were growing in stronger soils than in weak soils because soil compaction consistently inhibited the elongation of seminal root axes. A possible explanation for the contradictory evidence existing in the literature may be that in some studies plants could have been affected by periods of water stress. In dry environments growth rate might be improved in stronger soils because of increased water availability in the root zone (Masle and Passioura, 1987). This could explain why, in this study, shoot growth was unchanged or increased in the stronger soils. Alternatively, nutrient deficiency may develop as a consequence of restricted root growth and, in the long-term, limit shoot growth in mature plants (Masle and Passioura, 1987). There were differences in the way in which sunflower and maize responded to soil strength. Sunflowers showed

Goodman and Ennos—Soil Strength and Root Mechanics increases in the angle of spread of the root system and the thickness of roots, but maize only showed differences in the thickness of lateral roots. These results are consistent with those of Materechera et al. (1991) who also showed a contrast in the way in which monocot and dicot species respond to changes in soil strength. Their results showed that roots of plant species differed considerably in their ability to thicken under stress. Dicot species were also better at penetrating compacted soil layers than monocots ; generally dicots had more roots penetrating to a greater depth in both the compact and deep tilled soils (Materechera et al., 1993). This study has shown that plants are, to some extent, able to adapt their roots in response to soil strength, but that changes in root growth do not appear to fully compensate for alterations in soil conditions. Furthermore it has been shown that there are differences in the way in which sunflower and maize respond to high soil strength and that even small increases in soil strength can reduce the likelihood of anchorage failure. A C K N O W L E D G E M E N TS We thank Sue Challinor, Thurston Heaton and David Newton for technical assistance. The work was funded by the Biotechnology and Biological Sciences Research Council of the United Kingdom. L I T E R A T U R E C I T ED Assaeed AM, McGowan M, Hebblethwaite PD, Brereton JC. 1990. Effect of soil compaction on growth, yield and light interception of selected crops. Annals of Applied Biology 117 : 653–666. Atwell BJ. 1988. Physiological responses of lupin roots to soil compaction. Plant and Soil 111 : 277–281. Atwell BJ. 1990. The effect of soil compaction on wheat during early tillering. I. Growth, development and root structure. New Phytologist 115 : 29–35. Atwell BJ. 1993. Response of roots to mechanical impedance. EnŠironmental and Experimental Botany 33 : 27–40. Barley KP. 1962. The effect of mechanical stress on the growth of roots. Journal of Experimental Botany 13 : 95–110. Barley KP. 1963. The influence of soil strength on the growth of roots. Soil Science 96 : 175–180. Barley KP. 1965. The effect of localized pressure on the growth of the maize radicle. Australian Journal of Biological Science 18 : 499–503. Barley KP, Greacen EL. 1967. Mechanical resistance as a soil factor influencing the growth of roots and underground shoots. AdŠances in Agronomy 19 : 1–43. Bengough AG, Mullins CE. 1990. Mechanical impedance to root growth : a review of experimental techniques and root growth responses. Journal of Soil Science 41 : 341–358. Bengough AG, Young IM. 1993. Root elongation of seedling peas through layered soil of different penetration resistances. Plant and Soil 149 : 129–139. Bengough AG, Mackenzie CJ, Elangwe HE. 1994. Biophysics of the growth responses of pea roots to changes in penetration resistance. Plant and Soil 167 : 135–141. Chaudhary MR, Prihar SS. 1974. Root development and growth response of corn following mulching, cultivation or inter-row compaction. Agronomy 66 : 350–355. Cook A, Marriott CA, Seel W, Mullins CE. 1996. Effects of soil mechanical impedance on root and shoot growth of Lolium perenne L., Agrostis capillaris and Trifolium repens L. Journal of Experimental Botany 47 : 1075–1084.

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