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V. Effects of Mechanical Stress on the Growth of Roots and Shoots

V. Effects of Mechanical Stress on the Growth of Roots and Shoots

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MECHANICAL RESISTANCE OF SOIL



25



to the regulatory development of woody tissues in stems. Unfortunately

from our present point of view the work concerned was conducted

entirely with stems grown in the light. Although good evidence was

obtained showing that the tensile strength of certain stems increased

when grown under tension (see, for example, Bordner, 1909) results

were often contradictory, The literature on the topic has been reviewed

by Schwarz (1930). Secondly, following proposals of Heyn (1931) that

the rate of cell elongation was limited by the plasticity of the wall

material, considerable attention was given to the behavior of cellulose

fibers and samples of cell wall material under tension. For example, it

has been shown that, above a certain yield stress, strips of Nitella cell

wall creep at a rate that is roughly proportional to the applied stress

(Probine and Preston, 1962). Obviously, these studies need to be supplemented by experiments with living shoots, but, as any applied stress

disturbs the turgor relations and tissue stress initially present in a shoot,

results are difficult to interpret. Recently, Lockhart et al. (1964) avoided

this problem by working with sections of pea hypocotyl incubated in a

slightly hypotonic solution, and found that the living sections underwent

irreversible extension in response to tensions greater than 50 g.wt. ( u zz

-2 bar). Such studies are of considerable interest in relation to growth

processes, but they are of less interest in relation to emergence as the

emerging shoot is subject to axial compression rather than tension.

Before proceeding to examine the effects of compression, it is worth

noting that roots are subject to simple tension in many plants, as part of

the root proximal to the zone of elongation tends to shorten, sometimes

to a considerable degree. For example, de Vries (1879) measured extenin the primary roots of red clover, TrifoEium prutense L., as

sions (d/E)

large as -0.25 over a period of several weeks. This process helps to

anchor the plant to the ground, and young seedlings can sometimes be

drawn further into the soil.

The influence of a steady push, in the opposite sense to growth, on

the elongation of etiolated shoots has been described by Sedgley and

Barley (1963), who found that this slowed elongation. In their experiment, a load of 35 g.wt. ( uz = 0.5 bar) was applied to the top of the

plumular hook of etiolated epicotyls of tick bean. The reduction in elongation rate that followed was due to a change in shape, epicotyls grown

under axial compression being wider than controls. The rate of volumetric

enlargement was unchanged. As the epicotyl of tick bean lacks an intercalary meristem, the growth response observed in this particular experiment cannot have been due to any change in cell division.

In general it is known that, where internal controls are not overriding, as in poorly differentiated dividing tissues, the direction of cell



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K. P. BARLEY AND E. L. CREACEN



division can be influenced by an applied stress. For example, Kny (1896)

showed that in the periderm of cut slices of tuber of the potato, SoZunum

tuberosum L., the plane of cell division became oriented normal to an

applied tension, and parallel to an applied pressure. Clearly, such

changes could influence the form of growth, at least in simple tissues.

2. Plane Stress ( a1 = 0,



uX,u,



# 0)



The state of plane stress is found in nature when roots or rhizomes

tend to enlarge radially against the resistance offered by a strong soil.

Underground organs may also be compressed radially by the swelling

action of wetting clay. In an isotropic soil the stresses uZ = uy can be

replaced by the radial stress, ur.

Roots sometimes grow through compact layers of soil when the soil is

moist, but if the soil subsequently dries its increasing strength may

prevent an increase in girth, Tabenhaus et al. (1931) described field

situations where lengths of the taproot of cotton had been constricted in

this way; and Taylor et al. (1963) showed that gross constrictions could

lead to a reduced yield of tops. In earlier work Newcombe (1894)

studied the influence of radial confinement on the development of stems

of a number of species, and noted that halving the diameter of a short

length of stem reduced transpiration at high but not at moderate rates of

transpiration.

A recent report suggests that translocation toward the tip of the root

can be reduced by radial compression of proximal tissues. Barley (1965)

found that, when a pressure of 1 bar was applied to a proximal length

of corn radicle, the apical part gained weight less rapidly, even though

it received a plentiful ambient supply of water and oxygen. The experiment also showed that, while the application of pressures > 3 bar

damaged cells, radicles that had developed under pressure showed no

signs of cell damage. Perhaps compression has little effect on translocation within tissues that have developed under stress. In general the

response of a growing tissue to stress is likely to be much influenced by

the rate at which the stress builds up, as this determines the degree to

which the stress may be accommodated by changes in the pattern of

growth.



3. Triaxial Stress ( 0 1 , (I=, U~ # 0)

Root tips or underground shoots experience stress in each of the three

principal directions when penetrating the soil. In an isotropic soil ox =

q,, = ur, but, due to skin friction and shape factors (see Section 111, A, 3 ) ,

u1



>



UP



Using closely related techniques Gill and Miller (1956) and Barley



MECHANICAL RESISTANCE OF SOIL



27



(1962, 1965) measured the effect of triaxial compression on root growth.

Small corn seedlings were grown between a rigid plate and a flexible

diaphragm, gas pressure being applied to the diaphragm to compress

the seedlings. In the experiment of Gill and Miller the seedlings grew in

a thin bed of 50 p. beads between an impermeable plate and the diaphragm. In Barley’s experiments beads were omitted and the seedlings

were grown between a porous plate and a thin diaphragm. The stresses

operating on the root were difficult to ascertain, particularly when beads

were present. Without the beads the radial pressure, u ~ ,acting on the

root only slightly exceeded the gas pressure on the thin diaphragm, but

uZ exceeded u+.at the proximal end of the zone of elongation because of

the force needed to overcome skin friction. Whether or not beads were

present, the rate of root elongation decreased continuously as gas

pressure on the diaphragm increased, until elongation almost ceased at

gas pressures of 4 to 5 bar. The first increment of gas pressure reduced

elongation more than later increments, but the large initial effect vanished

when elongation was plotted against estimated values of V Z (Barley,

1962); elongation then decreased steadily almost ceasing at U Z = 7 bar.

When the applied stress is not isotropic, the apparent growth response

may be largely due to a change in shape. Data on cell shape obtained in

one of the above experiments (Barley, 1965) at one pressure, u+.= 1

bar, show that the decrease in length of the cortical cells (-68 percent),

compared with the control, accounted for most of the observed reduction

in radicle elongation (-80 percent). Setting aside the change in shape,

a genuine reduction in growth rate may well have been caused by the

influence of compression on the internal aeration of the tissues (Section

VI, B, 1 ) . Gessner (1961) points out that, as gas-filled intercellular

spaces are always present in the tissues or higher plants, their compressibility is high.

From the physiological point of view data on the effects of isotropic

compression would be particularly informative. As meristematic cells do

not fall below a certain size, the rate of cell division declines rapidly after

an organ has been completeIy confined (Hallbauer, 1909). But this does

not tell us what will happen when an organ enlarges against a steady

ambient pressure.

The ideal method of compressing an organ isotropically is to elevate

the pressure of an ambient fluid. However, a clear distinction should be

made between experiments with permeating and nonpermeating fluids.

In particular, if a permeable plant organ is compressed by raising the

pressure of an ambient aqueous solution, the original turgor, T,defined

as the pressure difference between the intracellular liquid and the

ambient solution, is restored when osmotic equilibrium is regained. This



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K. P. BARLEY AND E. L. GREACEN



is not the case when the organ is compressed by the reaction of the solid

phase of the soil, as the pressure of the intercellular water remains in or

near equilibrium with the pore water pressure, u, [see Eq. (9)]. Similarly, the intercellular air pressure is in equilibrium with the pore air

pressure, u,. Thus, a nonpermeating fluid has to be used to produce

compression of the kind experienced by organs growing through the soil.

Although it may not be easy to ensure an adequate internal supply of

water and air to the growing region, this approach appears to be promising for, say, short lengths of root or stem connected proximally to a

supply of air and water at datum pressure. Surprisingly, no experiments

of this kind have yet been reported.

The peculiar response of the root cap to mechanical “wear” warrants

special mention. Generally the peripheral cells of the cap are sheared

off rather easily, being only loosely cohesive. When the root grows in resistant media, increased cell destruction at the periphery of the cap is

accompanied by more rapid cell division at the base of the cap (Stalfelt,

1920). As the mean length of cells in the cap is reduced little if at all, the

rate at which the cells elongate must also be increased so that cells are

transferred more rapidly to the periphery. This response does not appear

to have any counterpart in the main body of the root tip.



4. Bending Moments

In coarsely structured soils less resistance is encountered by root tips

than by probes, as the growing tip tends to bend when it meets an

obstruction. Emerging shoots also avoid local obstructions in this way,

but bending can reduce emergence if shoots encounter a strong, continuous crust.

Root tips are bent by very small moments, but older parts of the

root are often less flexible, The prop roots of corn, that brace the

tops of the plant to the ground, are well adapted to resist bending; they

contain two concentric rings of fibers, the outer subepidermal ring consisting of fibers with very thick walls (Haberlandt, 1914). Turgid

etiolated stems can withstand small bending moments, but many are

bent markedly when moments greater than 5 g.wt.cm. are applied

suddenly. Generally, shoots are weakest in the region where elongation

is proceeding most rapidly. When bending occurs, the degree of bending

increases as turgor decreases ( see Fig. 5, Lockhart, 1959).

If bending moments are increased gradually or applied intermittently

over extended periods of time, shoots often become more resistant to

bending. In the course of a classical paper on translocation in stems,

Knight (1803) noted that frequent bending of the stem accelerated

secondary thickening in the plane of bending. Rasdorsky (1925)



MECHANICAL RESISTANCE OF SOIL



29



reviewed earlier work, and provided a number of examples of strengthening by changes in growth associated with bending, However, the effects

on growth that he reports need not have been caused directly by

mechanical stress, as changes in orientation with respect to gravity and

light during bending could equally well have been responsible.



B. PERTURBATION

OF STRESS

In Section IV, C, 1 we noted that, after root tips or seedling shoots

had been confined mechanically for a few days, elastic strain disappeared

from the walls of cells in the meristem. The resulting loss of wall pressure,

W, enabled the turgor, T , to act on the surroundings.

Pfeffer (1893) attributed the loss of strain to continued surface

extension of the cell wall during the period of confinement. He noted

that, when a growing organ met an obstruction, the force exerted increased gradually over several days to reach a maximum. Because he

believed that wall growth was an essential part of this process, he concluded that, if an organ suddenly encountered a resistance, no further

elongation could occur until wall growth during an “induction period

allowed the force to increase sufficiently to overcome the resistance.

Lengthy induction periods observed by Pfeffer may, however, have been

more apparent than real. In his experiments the plant organs, particularly roots, tended to widen within the thin gap that separated the fixed

block from the movable block (Section IV, B). This would have led

to a continuing increase in the force measured at large times, without

there necessarily being any increase in the pressure applied by the organ.

In fact turgor may be mobilized rather rapidly to act against an

external resistance. When a root or shoot meets an obstruction, as soon

as wall growth or relaxation permits a directed force to act externally,

stresses arise within the organ that tend to distort its cells. It is known

that even small shearing stresses can considerably distort young growing

organs over periods of the order of minutes (Sedgley and Barley, 1963).

The change in shape greatly magnifies the initial effect of growth or

relaxation on wall pressure, W. In this manner elongating root tips or

underground shoots are accommodated to the short-term perturbations

in stress that they experience when they grow through mechanically

heterogeneous soils.

In experiments stress may be perturbed suddenly by applying a force.

A single or small number of stress cycles may have little effect, but much

depends on the duration and intensity of the stress. Providing the tissues

have not been damaged by excessive stress, roots or shoots that have

been subjected to compression for several hours elongate mole rapidly

when the stress is removed. Acceleration continues until they regain the



30



K. P. BARLEY AND E. L. GREACEN



rate that prevailed before compression (Pfeffer, 1893). If a main root is

grown under compression for more than a few hours, on release of the

pressure a cluster of laterals develops from that part of the main root

which grew during the period of compression. If the stress is maintained

for several days, the main apex may fail to regain its initial, unstressed

rate of growth, possibly because of the diversion of translocated material

to the numerous laterals (Barley, 1962).

It has long been known that repeated cycles of stress produce tropic

responses in a wide variety of etiolated shoots, even when the stress is

small. For example, Stark (1917) showed that lightly rubbing one side

of the etiolated stem of seedlings of certain dicotyledons and grasses

caused the stem to bend during growth toward the rubbed side. In 1941,

Bunning claimed that etiolation could be inhibited equally well by light

or by perturbation of stress. In Bunning's experiment epicotyls of

Phaseolus multiflurous Willd., were bent at lbminute intervals in the

dark. It would be interesting to know whether a response would have

been obtained, as in Stark's work, without any change in orientation.

It is curious to find high sensitivity to small perturbations of stress in

what are normally subterranean organs. Possibly continuous local perturbation of the stress, of the kind experienced when a shoot grows through

a heterogeneons soil, causes the organ to lose its sensitivity. In this

connection it is interesting to note that repeated perturbation of stress

reduces the degree of seismonasty shown by Mimosa, and that, with

continuous shaking, for example, the leaves may regain their original

position ( Stiles, 1950).

VI.



Growth in the Soil



A. GROWTHIN MEDIAOF KNOWN MECHANICAL

PROPERTIES

A copious literature describing the adverse effects of excessive soil

compaction on plant growth has been reviewed recently by Rosenberg

( 1964). Many of the experiments described by Rosenberg merely demonstrate empirical relations between bulk density and growth, and cannot

explain why growth is affected. Here it will be more profitable to consider the relatively few experiments in which the design makes it

possible to evaluate the role of mechanical factors.

1. Homogeneous Media

a. Rigid matrix. In field soils hardpans are sometimes found that

behave essentially as materials having a rigid matrix (Lutz, 1952). The

soil in such pans has a high strength, and the matrix is subject only to

elastic strain. Because of the concentration of stress at contacts between



MECHANICAL RESISTANCE OF SOIL



31



grains ( McMurdie and Day, 1958), the elastic strain is not negligible.

Nevertheless, root growth does not seem to occur in such media, other

than in continuous pores commensurate in width with the normal, unstressed root tip. Wiersum (1957) found, for example, that the roots of

tomatoes having tips of 0.3 mm. diameter grew through sintered glass

disks in which the diameter of the pores ranged from 0.20 to 0.50 mm.,

but they were unable to penetrate disks in which the range was 0.15

to 0.20 mm.

b. Deformable matrix. Little attempt has been made to utilize the

relatively simple properties of purely cohesive media in experiments

with growing plants. Taylor and Gardner (1960) and Gardner and

Danielson (1964) measured the penetration of roots into cohesive

waxes of different hardness; these workers used the waxes simply as convenient materials with which to rank the penetrating ability of plant

roots that had been subject to various treatments or grown in various

soils ( Section VI, B, 1 ) .

Although saturated clays are often treated as purely cohesive ($I =

0 ) media in the classical theory of consolidation and bearing capacity

(Terzaghi, 1943), it is not profitable to regard them as such when dealing with deformations by plants. The reasons for this have been described

in Section 111; here we need only remind the reader that effective normal

stresses are in general not negligible around growing plant organs even

in saturated, impermeable clays.

As described in Section 11, seedlings often emerge from cohesive

soils by rupturing and lifting slabs of the overlying soil. When this kind

of deformation occurs, it is appropriate to relate emergence to the breaking forces, obtained from the dimensions of the slab and the modulus of

rupture of the soil. Relations of this kind were measured by Richards

(1953) and by Allison (1956), but their data are of little value as the

strength tests were conducted on slabs reconstituted in the laboratory

from fine earth and dried at 50°C., rather than on the crusts through

which the seedlings actually emerged. Moduli of rupture pertaining to

the moist crusts through which seedlings emerged were measured by

Hanks and Thorp (1956, 1957). But it is doubtful whether this measure

was appropriate in their experiment; emergence was not reduced by

increasing the thickness of the crust, as would have been expected if

tensile failure were the means of emergence. Rather, the shoots may have

penetrated the crust by causing continuous, local failure. On the other

hand, Parker and Taylor (1965) related the emergence of guar, Cyamopsis tetragorwloba ( L . ) Taub., to indentation test data, even though the

seedlings emerged in their experiment by rupturing a crust.

Clearly, the kind of deformation involved should be ascertained



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V. Effects of Mechanical Stress on the Growth of Roots and Shoots

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