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IV. Forces Exerted by Roots and Shoots

IV. Forces Exerted by Roots and Shoots

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19



MECHANICAL RESISTANCE OF SOIL



Others again enable the meristem to evade stress, as does the nutant habit

of the seedling shoot in many dicotyledons.

It is not proposed to go into further detail here, as clear accounts of

mechanical adaptation are to be found in the literature. The most comprehensive source of information is still Haberlandt’s classical text

“Physiologische Pflanzenanatomie.” Numerous further examples of the

mechanical adaptations to be found in underground shoots are given by

Leonhardt ( 1915).



B. MAGNITUDE

In Pfeffer’s experiments part of a root or shoot was secured within a

gypsum block; a second but movable block was then cast around the

exposed tip or around one side of the organ, Any force exerted on the

second block by the growing organ could then be measured by balancing

against a known resistance. In practice Pfeffer was concerned solely

with static equilibria, measuring the resistance that had to be applied to

the second block to prevent it from being moved.

Pfeffer found that when an organ was so confined it soon exerted a

force. The force increased rapidly at first and then more slowly, approaching a maximum in 2 to 3 days. The maximum force corresponded to a

pressure of from 5 to 10 bar distributed over the largest cross section

within the growing region. Although he did not make many measurements, Pfeffer’s results suggest that the pressure exerted by root tips is

greater in the direction of the longitudinal axis than in the radial direction (see Table 11).On the other hand, Pfeffer found that the axial and

TABLE I1

Pfeffer’s Data on the Maximum Pressure Developed by Confined Root Tips0

Axial pressure (bar)

Species



No. of

roots



Mean max.



Range



Vicia faba L.

Zea mays L.



6

3



9.0

10.2



7.0-10.7

9.5-11.2



~



a



T: 17-20°C; length of tip



Transverse pressure (bar)

No. of

roots



Range



5.3

6.6



4.3-6.1

-



4

1



~



< 1 cm.; pressure



Mean max.



~



=



force



~~~



+ max.



cross section.



radial pressures developed by shoots were similar. Pfeffer was not altogether surprised by the magnitude of the pressures that he recorded

for roots and shoots, as Muller (1872) had found previously that a

mechanical pressure of 14 bar was needed to prevent the elongation of

pith isolated from the stem of the sunflower, Helianthus annuus L., and



20



K. P. BARLEY AND E. L. GREACEN



Krabbe (1884) had reported that a radial pressure of 10 bar was needed

to prevent an increase in the girth of trees.

After the publication of Pfeffer’s paper in 1893 the subject appears

to have been neglected until Williams (1956) measured the force exerted

by the arching hypocotyl of small-seeded legumes. Although papers of

Gill and Miller (19%) and Barley (1962) helped renew interest in the

topic, these authors were mainly concerned with the efEects of stress on

growth (see Section V, A, 3 ) . Recently, Barley and Stolzy ( 1966) have

described a method of measuring the force exerted by root tips penetrating a soil. The soil is supported by a force transducer that measures the

reaction to the root tip. Providing measurements are restricted to the

time during which the hairless part of the tip is penetrating the soil, only

a small correction is needed for skin friction.

From Pfeffer’s work it is clear that, for a given species and organ, the

pressure developed is largely independent of the diameter attained, so

that the force exerted increases with the size of the growing organ. Even

though roots apply a smaller pressure in the radial than in the axial

direction, the force exerted in the radial direction is by far the greater, as

the pressure acts over a larger area. For example, roots of the broad bean,

Viciu faba L., can exert maximum radial and axial pressures of 5 and 9

bar, respectively, but the radial and axial forces that can be exerted by

a 4 cm. length of root are 5 kg.wt. and 0.3 kg.wt. The upward acting

forces exerted by seedling shoots range from 15 g.wt. for the thin hypocotyls of alfalfa, Medicago satiua, L., (Williams, 1956) to 401) g.wt. for

the thick hypocotyls of the broad bean (Pfeffer, 18913). Evidently, any

environmental factor that changes the dimensions of a growing organ

influences the total force that can be exerted on the surroundings.

The ability of roots or shoots to exert force on the soil depends not

only on their physiological properties and shape, but also on the anchorage provided by the proximal parts of the plant; that is, the force exerted

cannot exceed the ability of the proximal parts to withstand the reaction.

Anchorage is provided by skin friction together with the resistance that

has to be overcome to dislodge the seed, root hairs and root laterals.

Pfeffer found that forces of the order of 40 g.wt. per centimeter length

were required to pull the hair-covered radicles of broad bean from soils,

and that several centimeters of branched root could stand a pull equal

to the maximum axial force exerted by the growing tip of the root.



C. PHYSIOLOGICAL

ORIGIN

1. Osmotically Induced Turgor



The exertion of force by plant organs is most readily explained in

terms of their osmotic behavior. When pressures are measured with



MECHANICAL RESISTANCE OF SOIL



21



respect to the ambient solution as datum, for a semipermeable tissue at

osmotic equilibrium

a-T=O



(9)



where x = osmotic pressure of the cell contents, and T = hydrostatic

pressure within the cell. Strictly, an equilibrium expression for an imperfectly permeable osmometer should be given here, but the nature of cell

permeability does not affect the present argument. We disregard variaand T within the turgid cell, arising from the presence of

tion in

differentially permeable cytoplasmic membranes. Treating forces directed

toward the center of the cell as positive, at the cell wall,



-T



=



W+B- P



(10)



where W = pressure exerted by the wall (“wall” pressure); B = pressure

exerted by other cells (“tissue” pressure); and P = pressure applied

externally by the plant.

Thermodynamically, osmotic and swelling pressures are identical

(Hermans, 1949); so, if we assume that meristematic cells offer little

internal resistance to water transfer, then the vacuolar liquid and protoplasm should be in or near osmotic equilibrium. Further, providing

supply of water is not limiting, osmotic equilibrium with the ambient

solution is thought to be attained, or nearly so, throughout the zone of

cell enlargement ( Ordin et al., 19.56).

If plant forces are osmotic in origin, they may be mobilized either

by an increase in x or by relieving W and B . The pressure exerted by

,

W = B = 0.

the plant attains a theoretical maximum, P,,,,, = x ~ when

Pfeffer believed that both processes were operative. Measuring ro with

the plasmolytic method of de Vries (1884) and with the “minimum

length” method often ascribed to Ursprung (1923) in modern texts,

Pfeffer (1893) concluded that in broad bean T o rose gradually after the

root tip or seedling shoot had been confined, Secondly, Pfeffer showed

that elastic strain disappeared from the cells of confined root tips. He

found that root tips confined for 48 hours or more failed to shrink when

plasmolyzed. This was not due merely to maturation of the apical

tissue, as the tips at once began to elongate when transferred to iced

water.

Unfortunately, as Pfeffer used potassium nitrate as the osmoticum,

his r0 values are excessively high ( > 15 bar). It is now known that this

salt penetrates excessively into root cells. Using sucrose at 2”C., Barley

(1962) did not find any increase in x in compressed growing radicles of

the tick bean (Viciu faba L., var. MINOR). Neither Pfeffer nor Barley

detected any increase in T in compressed radicles of corn. Whether or

not there is a buildup in x in some species, the relief of wall and tissue



22



K. P. BARLEY AND E. L. GREACEN



pressure appears to offer a ready means of mobilizing osmotic turgor to

perform external work. Although the plant material is not directly comparable, it is interesting to note that the value of P,,, found by Pfeffer for

the root tips of corn agrees with the r,,value obtained by Barley: P,.,,

= To = 11 bar.

2. Nonosmotic Contributions to Turgor



Even if we can account for the magnitude of the pressure measured

by Pfeffer without the need to invoke other than osmotic processes, this

in itself does not show that osmosis is the only process involved. However, no other process has conclusively been shown to raise the hydrostatic pressure within plant cells. Bennet-Clark ( 1959), having reviewed

the evidence in favor of “active” uptake of water by plant cells, suggested

that the strongest evidence was provided by data showing the osmotic

pressure of expressed sap to be generally less than the plasmolytically

determined value. A more straightforward explanation of this discrepancy, however, is provided by the tendency for osmoregulation to

occur during exposure to an osmoticum, either by solute transfer or by

hydrolysis of cell polymers.

In commenting on the water relations of Nitella, Dainty (1963) notes

that although small differences in electrical potential across charged

pores might theoretically lead to substantial turgor differences across

the membranes concerned, such differences could not in fact be realized

in Nitella as outward flow can occur through numerous uncharged pores.

Similar reasons may rule out electroosmotic or other “active” contributions to turgor in higher plants, but at present too little is known about

the properties of cell membranes for us to decide.

3. Other Forces of Metabolic Origin



So far we have considered only those forces that depend on cell

turgor. We also need to ask whether forces might not arise from the

propensity of growing tissues to accumulate, synthesize, or transform

materials other than water. A sol + gel transformation, for example, is

associated with cell division; before furrowing begins protoplasmic sols

are converted to gels. Furrowing and cleavage are then brought about

by the contraction of the gels, and energy used in building up the

structure of the gel can be expended as work as the gel contracts and

reverts to a sol (Landau et al., 1955). Forces that might be associated

with the surface extension of the cell wall or cell membranes also need

to be considered, whether or not they are adsorptive in origin as Bell

(1961) suggests.

Although such phenomena provide interesting examples of ways in



MECHANICAL RESISTANCE OF SOIL



23



which metabolic energy may be expended as work, it has to be remembered that the rigidity of meristematic tissue is almost wholly dependent

on cell turgor. When the tissue is turgid, the cell walls cannot themselves

be load bearing, as they are stretched, not compressed, and wall pressure

is directed centripetally. Only when turgor is fully mobilized against an

external resistance, and when wall tension is removed, can the tendency

for surface extension of the wall lead to the exertion of a force. By

measuring the force exerted by root tips of broad bean growing at incipient plasmolysis, Pfeffer (1893) concluded that wall growth gave rise to

forces about one-tenth as large as those produced by turgor. His experiment has not yet been repeated. One might expect that compression of

thin, flexible cell walls would lead to buckling and bending of the wall,

and changes of this kind have been described by Hottes (1929).

Where cell walls have been strengthened, continued growth of the

wall may well give rise to forces independent of those produced by turgor. Even so, the ability of thin-walled cells within an organ to withstand

compression may continue to set a limit to the pressures developed during

growth. In this connection it is worth noting that the pressures exerted

by enlarging trunks of trees, in which many of the cells have strong walls,

are comparable with those produced by delicate root tips (see Section

IV, A ) .



4. Energy Expended on External Work

We have considered contributions to plant forces that may be made

by osmotic and “active” uptake of water, by cell division, and by wall

growth. The forces observed arise most obviously from osmotically

induced turgor. Whatever contributions may or may not be made by

other processes, it is important to consider also the energy required for

external work in relation to the total energy available to the plant.

To give an example, a root of 1 mm. diameter, elongating at 1 mm.

hr.-l against a resistance of 10 bar, performs external work at the rate

of 0.2 erg sec.-l; whereas energy is released during respiration by the

root tip at rates of the order of lo2 erg sec.-l. Work may also be performed in stretching the cell wall, but again this is small ( Frey-Wyssling,

1952). It is clear that the energy expended on mechanical work during

growth is trivial compared with the output of respiratory energy, Because

of this, it is sometimes inferred that mechanical resistance is not likely

to be important. However, little is known about the efficiency with which

the plant “engine” performs mechanical work. Moreover, even if sufficient energy is available, growth may be altogether prevented by a

sufficient resistance, as there is a definite upper limit to the force that a

plant organ can exert on its surroundings.



24



K. P. BARLEY AND E. L. GREACEN



V.



Effects of Mechanical Stress on the Growth of Roots and Shoots



In Section I11 we saw that large pressures are often required to

create channels in soils. For example, in loams of modest strength the

pressure needed to lengthen a channel is of the order of 10 bar. Clearly,

root tips or emerging shoots experience large stresses as they penetrate

finely structured layers or peds of soil.

Although the study of stress-stain relations in a particular organ may

help us to interpret a growth response, we are much less concerned here

with the strains produced in a given organ when a stress is first applied,

than we are with the way in which growth proceeds after a stress has

been applied.

It may sometimes be overlooked that in studying underground shoots

we are dealing with dark-grown or etiolated organs, and that conclusions

reached with shoots growing in the light may not apply. Particular care

needs to be taken in extrapolating from experiments with specialized

shoots such as tendrils, that show marked growth responses both to

contact stimuli and to tension ( Brush, 1912).

as the external normal stress acting in

In what follows we define

the direction of the longitudinal axis of a plant organ, and a,, ay as the

external normal stresses acting in the direction of the remaining Cartesian

= uy we replace them by ur, the radial stress. Although we

axes. When

deal only with applied stresses we note that these are superposed on

whatever stresses arise within the plant organ.

The effects of mechanical stress on the processes of cell division, cell

enlargement, and differentiation have rarely been separated in experiments, so that it is more expedient to classify the available data according

to the nature of the applied stress. We begin by considering the influence on growth of a simple axial tension or pressure.

A. STEADYSTRESS

1 . Uniaxial Stress ( # 0,

= 0)

When devising methods to push or pull a radially unconfined plant

organ, it is simpler to use shoots than root tips; a shoot offers more points

of attachment for an object transmitting a force; furthermore, many

young shoots contain collenchyma and are less readily buckled or bent

than are root tips.

The influence of tension on stem growth has been studied intensively

by physiologists for two distinct reasons. First, following claims by

Pfeffer’s school at Leipzig, considerable interest was taken at the turn

of the nineteenth century in the question of whether applied tension led

( T ~



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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