Mechanical impedance to root growth: a review of experimental techniques and root growth responses pot

Journalof Soil Science, 1990,41,341-358 Mechanical impedance to root growth: a review of experimental techniques and root growth responses A. G. BENGOUGH & C. E. MULLINS* Cellular and Environmental Physiology Department, Scottish Crop Research Institute, Dundee OD2 5DA and *Department of Plant and Soil Science, University of Aberdeen. Aberdeen AB9 ?UE, UK SUMMARY Mechanicalimpedancetorootgrowthisoneofthemostimportant factorsdeterminingroot elongation and proliferation within a soil profile. Penetrometers overestimate resistance to root growth in soil by a factor of between two and eight and, although they remain the most convenient method for predicting root resistance, careful interpretation of results and choice of penetrometer design are essential if improved estimates of soil resistance to root elongation are to be obtained. Resistance to root growth through pressurized cells contain- ing ballotini considerably exceeds the confining pressure applied externally to these cells. Results from this work are reappraised. Existing models of soil penetration by roots and penetrometers are reviewed together with the factors influencing penetration resistance. The interpretation of results from mechanical impedance experiments is examined in some detail and root responses, including possible mechanisms of response, are discussed. INTRODUCTION The type of soil strength characteristic (i.e. the variation of soil strength with soil water content) favourable to crop growth depends on both the amount and the distribution of the annual rainfall, and on the nature of the crop. The soil must have sufficient mechanical strength to provide adequate anchorage for the plant throughout its development, and to prevent the collapse of soil water and air pathways by soil overburden pressure and the weight of vehicle and animal traffic. Dense regions of high strength may limit root growth and crop yield (Jamieson et al., 1988; Oussible, 1988) by creating a large mechanical resistance to root growth and/or restricting the rate of oxygen supply to roots. These dense regions occur in naturally compact soil horizons and also arise from compaction by heavy farm machinery and by the formation of plough pans. Mechanical impedance is experienced to varying degrees by virtually all roots growing through soil. If continuous pores of sufficiently large diameter do not already exist, a root tip must exert a force to deform the soil. This process may considerably decrease root elongation rates, increase the root diameter and change the pattern of lateral root initiation (Russell, 1977). In this paper, the effects of mechanical impedance on root morphology are reviewed and some direct comparisons between soil resistance to root growth and resistance to a penetrometer are discussed. The physical process of root growth through soil and artificial media is considered, with emphasis on the interpretation of results from different experimental techniques. Changes which occur in root elongation rate under both constant, and temporally and spatially varying levels of mechanical impedance are considered together with the complicating effects of soil aeration and water status. Finally, possible physiological mechanisms for the root responses are discussed. Terminology Penetrometers provide the best estimates of resistance to root growth in soil, short of direct measure- ment of root force. Most penetrometers consist of a metal probe with a conical tip fixed onto a 34 1 342 A. G. Bengough & C. E. Mullins cylindrical shaft (Fig. 1) that is generally of smaller diameter than the cone (normally 80% of the cone diameter; Gill, 1968; Barley & Greacen, 1967; Bengough, 1990). Penetrometer resistance, Q,, is defined in Equation (l), 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: Qp.r = Fp,rlAp,r (1) The pressure that is exerted on the soil by a growing root cannot at present be measured at every point on the root surface. In this review, root penetration resistance, Q,, is defined similarly to penetrometer resistance, but where F, is the component of force directed along the root-axis that the section of root that moves through the soil must exert on the soil in order to extend, and A, is the root cross-sectional area, measured behind the elongation region. In common with the literature, the terms mechanical impedance and root penetration resistance have been used interchangeably. - Shaft 7 Root hairs 41 ifl Elongating region Meristemat i( region Phloem Endodermis Cortex Epidermis Xylem ‘‘j ,,’/ f Muclgel sheoth ‘l; I I Fig. 1. (a) A penetrometer, where F,, A,, oN. and a are as defined in Equations (1) and (2). and (b) a root tip. EFFECTS OF MECHANICAL IMPEDANCE ON ROOT MORPHOLOGY When a root tip encounters an obstacle that resists penetration, the root cap becomes less pointed and the surface cells may slough off (Souty, 1987). Mechanical impedance decreases the rate of root elongation because of both a decrease in the rate of cell division in the meristem, and a decrease in cell length (rather than volume). Eavis (1967) found a decrease of 40% in the cell division rate at a root penetration resistance (0.34 MPa) sufficient to decrease the root elongation rate by 70%. Cell length is decreased and the volume of the inner cortical cells may decrease, but the diameter and volume of the outer cortical and epidermal cells can be considerably greater (Barley, 1965; Wilson et al., 1977). The increase in root diameter in mechanically impeded roots results mainly from an increased thickness of the cortex; this is a consequence of both the increase in the diameter of the outer cells, and an increase in the number of cells per unit length of root. Mechanical impedance to root growth 343 The apical meristem and zone of cell extension of impeded roots is shorter (Barley, 1962; Souty, 1987), and root hairs develop closer to the tip of impeded roots (Goss & Russell, 1980). Lateral initiation occurs nearer the tip and laterals occur together along the impeded axis (Goss & Russell, 1980; Barley, 1962). Where mechanical deflection causes roots to curve around an obstacle, the initiation of laterals generally occurs on the convex side of the root (Goss & Russell, 1980). Root hair development is greater on the opposite (concave) side and, in highly impeding media, the growing zone of the root is much distorted. The growth of impeded lateral roots is affected by impedance similarly to the main axis (Goss, 1977). However, if the pore size in the growing medium is such that only the main root axes are impeded, the freely penetrating laterals attain much greater length than in completely unimpeded root systems. COMPARISON OF ROOT RESISTANCE WITH PENETROMETER RESISTANCE There have been relatively few studies involving the measurement of root force (F, in Equation (1)) because of the experimental difficulties. Root force must be measured after the root has penetrated the surface of the soil to a depth of several times its diameter (since root penetration resistance is initially lower because the surface of the surrounding soil is displaced upwards; Gill, 1968), but before root hairs anchor the tip (Stolzy & Barley, 1968; Ennos, 1989). To calculate the root penetration resistance requires measurement of the root cross-sectional area. Root tip diameter increases in impeded roots and, since simultaneous measurements of root diameter and force can not normally be obtained, it is not obvious whether the initial or the final root diameter should be measured. Ideally, root diameter should be recorded just behind the elongating zone and level with the soil surface at the time of force measurement. The results of experiments involving direct comparisons of root and penetrometer resistance indicate that penetrometers experience two to eight times greater resistance than plant roots penetrating soil (Table 1). Dexter (1987) suggests that this ratio of penetrometer resistance to root resistance is positively correlated with soil strength, being greater in ‘stronger’ soils. At present there is neither theoretical basis for this suggestion nor sufficient published data to justify such a conclusion, although the need for accurate prediction of this ratio is clear. Indirect evidence for the difference between root and penetrometer resistance arises from com- paring the maximum pressures exerted by roots with penetrometer resistance in soil of sufficient strength to virtually halt root elongation. The maximum axial pressure that a root can exert is between about 0.9 MPa and 1.3 MPa (Misra et al., 1986b), whereas root elongation stops in soil with a penetrometer resistance of 0.8 to 5.0 MPa (Greacen et al., 1969). The results are variable because of differences between plant species and soil types, and possibly the temperatures at which the exper- iments were performed (Greacen, 1986). Thus, roots cease elongating in soil with a penetrometer resistance up to six or more times greater than the maximum axial pressure that they can exert. The reason for this difference must be physical differences in the way in which plant roots and metal probes penetrate soil. MODELLING MECHANICAL IMPEDANCE TO PLANT ROOTS AND TO PENETROMETERS IN SOIL Barley & Greacen (1967) comprehensively reviewed the mechanics of soil deformation and failure which occur around penetrometer probes, roots and underground shoots. There have since been several attempts to predict penetration resistances in soil and in ballotini beads from bulk mechan- ical properties. All but one of these models estimate root resistance by predicting the theoretical pressure required to expand a cavity in the soil or ballotini. Penetrometer resistance, Qp, is then given by Qp=a,(l +pcota) (Greacen et al., 1968) where uN is the pressure required to expand a cavity in the soil (and is equal to the normal stress on the surface of the penetrometer cone), p is the coefficient of soil-metal friction, 344 A. G. Bengough & C. E. Mullins penetrometer probes Table 1. Studies involving direct measurement of penetration resistance both to plant roots and to Eavis Stolzy & Whiteley Misra Bengough & (1967) Barley(1968) etal. (1981) etal. (1986a) Mullins(1988) Soil Probe diameter (mm) Probe semiangle Penetration rate (mm min- I) mm behind tip where root diameter measured ratio (orobe resistance) remoulded remoulded remoulded sandy loam sandy loam cores and undisturbed clods of sandy loam 1 3 1 to2 parabolic 30" 30" 1 0.17 3 5 3t05 4 4t08 4.5 to 6 2.6 to 5.3 clay loam undisturbed aggregates cores of sandy loam 1 I 30" 30" 3 4 I* 2 to 5 1.8 to 3.8 4.5 to 9 (root resistance) Number of replicates 12 2 120 324 14 *Root diameter was also measured in the air gap above the aggregate; it is not clear which figure was used. and a is the cone semi-angle. On the assumption that plant roots experience very little frictional resistance, Greacen et al. (1968) have shown that this equation can account for much of the large difference between the resistance experienced by plant roots and by metal probes: Equation (2) predicts that sharp penetrometers (i.e. small a) will experience a much higher component of frictional resistance than blunter penetrometers. However, with a semi-angle of more than 30", soil bodies (that move with the probe) have been observed to form around the probe tip so that soil- metal friction is no longer involved and Equation (2) ceases to be applicable (Mulqeen et al., 1977; Bengough, 1988). Farrell & Greacen (1966) and Greacen et al. (1968) calculated the pressure required to expand cavities in the soil by spherical and cylindrical deformation respectively. The advancing probe or root was accommodated by compression of the surrounding soil. This was assumed to occur in two distinct regions: an inner zone of compression with plastic failure immediately surrounding the probe, and a zone of elastic compression outside this. Sharp penetrometers (5" semi-angle) and plant roots were assumed to deform the soil cylindrically, whereas blunt penetrometers caused spherical deformation. In calculations for three sandy loam soils, the cavity pressure for cylindrical deformation was only 25 to 40% of that required for spherical deformation. The major disadvantage of the Greacen et al. (1968) model is that it requires many laborious measurements of soil mechanical properties. A simpler approach was adopted by Romkens & Miller (1971), who equated the pressure required for void-ratio changes occurring in a cylinder of soil around a root with the pressure required for one-dimensional soil consolidation. The resulting equation was used to predict the rooting densities at which further root radial expansion would be inhibited by the expansion of neighbouring roots. Unfortunately, the Romkens & Miller (1971) model is valid only for saturated cohesionless media and, therefore, is of very limited applicability to many agricultural soils. Further confirmation that less stress is required for radial (cylindrical) soil deformation than for axial (spherical) deformation was provided by Abdalla et al. (1969) and Hettiaratchi & Ferguson (1973). For any given (elastic) strain in a cylinder of soil ahead of the root tip, it was theoretically predicted that less stress is required to deform the soil radially than axially (Abdalla et al., 1969). Mechanical impedance to root growth 345 This theory was complemented by experiments using a large modified penetrometer to demonstrate that radial expansion behind a penetrometer (or root) tip can reduce axial resistance to soil pen- etration. Hettiaratchi & Ferguson (1973) predicted theoretically that the pressure required for cylindrical soil deformation in a frictionless cohesive medium was always less than for spherical deformation, the difference increasing with cohesion. Collis-George & Yoganathan (1985) used the spherical cavity expansion model of VesiC (1972) to define limiting mechanical conditions for seed germination and root growth. Although this model may be suitable to describe germination conditions, use of spherical expansion theory will have resulted in overestimates of the resistance to root growth. The VesiC model requires fewer inputs than the Greacen model, and includes a volumetric strain term. Collis-George & Yoganathan assumed the volumetric strain to be zero, so that fewer soil mechanical measurements were needed to perform their calculation. However, their zero-strain assumption is questionable because even a tiny volumetric strain may, under certain conditions, alter the cavity pressure considerably (VesiC, 1972). PHYSICAL DIFFERENCES BETWEEN SOIL PENETRATION BY PLANT ROOTS AND PENETROMETER PROBES Roots are flexible organs that follow tortuous paths through the soil, apparently seeking out the path of least resistance. They extract water from the soil, excrete mucilage from around their tips, and swell when physically impeded. In contrast, penetrometers are rigid metal probes constrained to a linear path through the soil. Penetrometers vary from about 0.1 mm in diameter for a small (needle) penetrometer (e.g. Groenevelt et al., 1984) to over lOmm for a large (field) penetrometer (the standard ASAE penetrometer cone has a diameter of 20.27mm; ASAE, 1969), and often penetrate the soil at rates up to two or more orders of magnitude greater than roots (Whiteley et al., 1981). The differences between penetrometers and roots have resulted in the expression of much doubt as to the usefulness of penetrometers (e.g. Russell, 1977, p. 188), but despite their limitations they remain the best available method of estimating resistance to root growth in soil. It is important, therefore, to determine what are the most important physical differences between the action of roots and penetrometers. Rootflexibility and spatial variation of soil strength Because roots often grow through cracks and holes in the soil, or follow planes of weakness between soil peds (Russell, 1977), penetrometers are of limited use in some structured soils. Detailed work has been done on the behaviour of roots growing along cracks and through pores (Whiteley & Dexter, 1983; Dexter, 1986; Scholefield & Hall, 1985), but is beyond the scope of this review. However, in coarsely structured soil, individual soil peds may be considered as continuous even though the soil is structured on a larger scale (Greacen et al., 1969) and root penetration into these peds may be important for nutrient uptake and plant growth. The forces required to buckle root tips growing across air gaps were measured by Whiteley & Dexter (198 1 ). The buckling stress decreased as the size of the air gap increased, but attempts to predict the buckling stress from the elastic modulus of the root tip were only partly successful. Dexter (1978) has modelled root growth through a bed of aggregates by relating root growth rates to penetrometer resistance within individual aggregates, and combining this with information on the probability of roots penetrating the aggre- gates. To date, this model has not been tested against independent experimental data over a range of realistic conditions. Penetrometers average soil resistance in a zone surrounding the probe tip; thus, they cannot detect changes in soil strength that are on a scale much smaller than the tip dimensions. Groenevelt et al. (1984) investigated small-scale variations in strength by using a 0.15mm diameter pen- etrometer to determine the proportion of linear depth in a soil core with penetrometer resistance less than 1 MPa, and inferred that this fraction of the soil has a relatively low resistance to root growth. This ‘percentage linear penetrability’ of the soil decreased at higher soil bulk density, and good correlations between penetrability and rooting density have been obtained using a larger (1 3 mm 346 A. G. Bengough & C. E. Mullins diameter) penetrometer (Jamieson et al., 1988). Spectral analysis of penetrometer data, in which the pattern of variation of penetrometer resistance with depth was examined using Fourier analysis, was used by Grant et al. (1989, but has not yet been related to root growth. Diameter and rate ofpenetration Existing experimental evidence on the effects of probe or root diameter on penetration resistance is based almost entirely on penetrometer measurements, and is often contradictory (Table 2). Richards & Greacen (1986), in their theoretical model of cavity expansion in granular media, imply that thin roots may deform the soil elastically, thereby encountering less resistance than thicker roots which cause plastic deformation. However, the limited studies of several different plant species available to date do not indicate that roots of smaller diameter are relatively less mechanically impeded by soil or by ballotini (Gooderham, 1973; Goss, 1977). In contrast to roots, which can grow around objects that offer high resistance to displacement, a small probe may have to displace soil particles of a diameter comparable to the probe. The result is that, particularly where there is an abundance of coarse sand or larger material, the effective diameter of the probe is greater than its actual diameter so that smaller probes (e.g. of 1 mm rather than 2 mm diameter) can experience a significantly greater resistance (Whiteley & Dexter, 198 1). Table 2. Studies in which resistance to probes of different diameter was measured Reference Probe Probe type Greatest Soil diameter (mm) (semiangle) resistance to Dexter & Tanner (1973) Barley ef al. (1965) Gooderham (1973, cited by *below) Bradford (1980) Whiteley ef al. (1981)* Whiteley & Dexter (1981) Bengough (1988) field soil (various textures) remoulded sandy loam undisturbed undisturbed clods and remoulded cores of sandy loam remoulded (various textures) undisturbed cores of sandy loam 10,20,30,40 3.8,5.1 1.00,1.25,1.50 1.75.2.00 1.00,1.25,1.50 1.75,2.00 0.5,l .O sphere smallest probe conical no difference - smallest probe (30") conical no difference conical no difference (307 (30") conical smallest probe conical smallest probe (307 (307 It is important to distinguish between the vertical component of frictional resistance on the tip of a penetrometer, and the friction on the shaft, which can account for a greater proportion of the total resistance to small penetrometers (Groenevelt et al., 1984; Barley et al., 1965; Greacen, 1986). Probes with a (relieved) shaft of smaller diameter than the tip are used to decrease the component of shaft friction. The success of this feature may be restricted if the trajectory of the probe tip causes the shaft to bow and come into contact with the soil, or if soil falls or deforms inwards around the shaft and rubs against it. Penetrometer resistance in soil cores that are laterally confined inside rigid cylinders may also be greater if the ratio of core diameter to probe diameter is less than about 20 (Greacen et al., 1969). This effect of confinement is smaller in more compressible soils, where the probe volume can be accommodated by the compression of a smaller cylinder of soil. Studies on penetration rate by Eavis (1967), Voorhees et al. (1975), Gerard et al. (1972) and Gooderham (1973) revealed decreases of less than 20% in penetrometer resistance for decreases in penetration rate of between one and three orders of magnitude down to penetration rates of Mechanical impedance to root growth 347 1 mm h-’ or slower. Although Waldron & Constantin (1970) found a large effect of penetration rate, an intermittently rotated penetrometer was used, which would have resulted in larger decreases in soil frictional resistance at slower rates of penetration. In very wet soil, penetrometer resistance is more clearly linked to penetration rate because of its interaction with pore water pressure (Cockroft et al., 1969). This effect will be greater in less permeable soils (especially remoulded soil) containing a higher propertion of silt and clay, than in sands. Penetrometer resistance doubled in a sandy loam soil remoulded at approximately field capacity for a 100-fold increase in penetration rate, whereas a 250-fold increase in penetration rate resulted in only a 25% increase in penetrometer resistance in air-dry sand (Bengough, 1988). Similarly, Cockroft et al. (1969) found a doubling of penetration resistance for a 350-fold increase in penetration rate in saturated remoulded clay. Thus, excluding very wet and remoulded soils, penetrometer resistance is only weakly dependent on penetration rate for speeds between those normally used in needle penetrometer measurements and typical rates of root elongation. Shape and friction Root or probe shape determines both the mode of soil deformation and the amount of frictional resistance on the tip. Observations of soil movement and density patterns surrounding probes and roots suggest that both narrowly tapered probes and plant roots deform the soil cylindrically compared with the spherical deformation caused by blunt probes (Cockroft et al., 1969; Greacen et al., 1968). By using both lubricated and rotated penetrometer probes it has been demonstrated that a large component of penetrometer resistance is frictional (Tollner & Verma, 1984; Bengough & Mullins, 1988). Greacen et al. (1968) suggested that root tips experience virtually no frictional resistance because of the lubricating action of mucilage secretion and the sloughing off of root cap cells. If this is so, the best estimate of root resistance may be obtained by measuring the resistance to a narrowly-tapered probe, and then subtracting the component of frictional resistance using Equation (2) (Greacen &Oh, 1972; Voorhees et al., 1975). Interactions between roots, water extraction by roots, root swelIing and root nutation Because roots seldom grow through soil in complete isolation, it is important to consider inter- actions between neighbouring roots. Greacen et al. (1969) measured resistance to penetration of a narrowly-tapered probe, surrounded by six identical probes. Penetration resistance for the central probe of a group was considerably lower than when the probe was used on its own. Tensile cracking occurred between the probes, and similar cracks were also observed in a separate experiment between neighbouring pea radicles growing into a loam. The drying action of roots is very important in the formation of such cracks, which must facilitate the subsequent growth of lateral roots in a soil of high resistance (Gerard et al., 1972). Although this cracking is generally likely to be advan- tageous, there are soils of high tensile strength (e.g. Mullins et al., 1987, 1990) which may not crack readily under the drying action of roots. In such soils, the increase in penetration resistance caused by the soil drying may further impede root growth. It has been suggested by Abdalla et al. (1969) that roots penetrate soil by an alternating series of radial and axial enlargements. Graf & Cooke (1980) used a finite-element model and, assuming a low coefficient of root-soil friction and that the soil behaved as a homogenous linear elastic medium, predicted that the radial expansion of impeded root tips could reduce the axial stress on the root cap. A zone of stress relief caused by radial enlargement was also predicted by Richards & Greacen (1986), and by Hettiaratchi & O’Callaghan (1974, 1978) who also analysed theoretically the mech- anics of the changes in cell size and shape that can occur in mechanically impeded root tips. It is clear that rigid penetrometers cannot mimic this radial expansion of roots, but the potential importance of this as a mechanism for soil penetration by roots has not yet been fully investigated. Finally, oae factor rarely commented on with respect to soil penetration is the tendency for roots to nutate (Greacen et al., 1969; Ney & Pilet, 1981). The magnitude of this motion in highly resistant soil is probably very small, but it could be a process by which roots locate low-resistance pathways through heterogeneous soils, and may also reduce soil frictional resistance to root tip penetration. Nutation has also been suggested as a mechanism that aids soil penetration by rhizomes (Fisher, 1964). 348 A. G. Bengough & C. E. Mullins EFFECTS OF MECHANICAL IMPEDANCE ON ROOT GROWTH Experimental techniques Existing experimental techniques can be divided into several different catagories. Soil. Experiments in soil are more realistic, but it is difficult to ensure that mechanical impedance is the only soil factor limiting root growth. If a soil is compacted to increase resistance to root growth, the resulting decrease in porosity may result in poor aeration. Similarly, increasing soil- water tension to increase soil strength may result in water stress. However, the greatest difficulty is in determining the penetration resistance experienced by roots in soil. This is ideally determined by direct measurement (e.g. Stolzy & Barley, 1968; Eavis & Payne, 1969; Bengough & Mullins, 1988), but practical difficulties have led most researchers to use penetrometer resistance measurements to estimate root resistance (e.g. Greacen &Oh, 1972). Pressurizedcelis. Root growth has been studied in artificial systems where it was intended that a constant, uniform and known value of mechanical impedance could be imposed, while simul- taneously maintaining a carefully controlled supply of aerated nutrient solution to the roots. Goss (1977), Abdalla et al. (1969) and Barley (1963) grew roots in ballotini contained in flexible-sided cells which could be subjected to an external confining pressure (Fig. 2). (a) Pressure inlet Root inlet (b) Root inlet Gas inlet Gas under pressure, ud Polythene diaphragm Nylon cloth u Lu 024 01 2 cm cm Fig. 2. (a) A triaxial cell (after Barley, 1963), and (b) a pressurized diaphragm apparatus (after Barley, 1962). Fig. 2(a) is reproduced from K.P. Barley, Influence of soil strength on growth of roots, Soil Science 1963,96(3), 175-180 (0 Williams & Wilkins, 1963). Resistance to root growth in ballotini has been taken as either equal to (Russell & Goss, 1974; Goss, 1977; Goss & Russell, 1980; Veen, 1982), or an order of magnitude greater than (Richards & Greacen, 1986; Bengough & Mullins, 1990) the pressure applied externally to the boundary of the growth medium (u3 in Fig. 2). Goss (1977), Veen (1982), Richards & Greacen (1986) and Bengough & Mullins (1990) all estimated resistance to root growth by measuring the pressure required to inflate a small rubber tube within the pressurized ballotini cell. The tube inflation pressure depends entirely on the detailed procedure followed (Bengough & Mullins, 1990). Goss (1977), Veen (1982) and Bengough & Mullins (1990) inserted a tube inflated under atmospheric pressure only and unsealed at one end into an unpressurized cell of ballotini. An external confining pressure was then applied to the cell, causing the tube partly to deflate. The Mechanical impedance to root growth 349 pressure required to reflate the tube to its original volume was then found to be equal to the pressure applied externally to the ballotini cell. In contrast, when the tube is not allowed to deflate during pressurizing of the cell, the pressure required to expand the tube beyond its initial volume is between 5 and 10 times higher than the pressure applied externally to the ballotini cell (Richards & Greacen, 1986; Bengough & Mullins, 1990). This is attributable to the frictional resistance to deformation of the ballotini. Since roots penetrating the ballotini must exert pressure to expand a new cavity in previously undisturbed ballotini, the latter experiment gives more accurate representation of the resistance experienced by growing roots. Although the pressure required to inflate a tube in the ballotini will depend on both the tube diameter and on the frictional properties of the tube walls, it seems reasonable to conclude that the resistance to root growth in the cells considerably exceeded the external confining pressure. Richards & Greacen (1986) also used a finite-element model to predict the effect of tube inflation pressure on tube diameter in ballotini and in sand. Tube diameter increased slowly until the internal pressure reached a certain critical value, when the diameter increased much more rapidly (Fig. 3). Predicted inflation pressures were much higher than the external confining pressure, and the difference was greater for sand than for ballotini. E E v I I I I I I I I i I I I i I I 3.0} I / I 36~ I I I I I I I / / / I 1 I 1 I I 0 0.05 0.10 0.15 0.20 0.25 Inflation pressure (MPa) Fig. 3. Outside diameter of tube vs inflation pressure in ballotini at several constant external cell pressures (indicated at the top of each curve), after Richards & Greacen (1986). Experimental (-); theoretical ( ) Pressurized diaphragms. Barley (1962) and Gill & Miller (1956) adopted slightly different tech- niques, using an externally-pressurized rubber diaphragm to exert pressure on roots growing down a porous plate, separated from the diaphragm by nylon cloth or ballotini respectively (Fig. 2). In these experiments, asymmetric stress was applied to the whole root length. These authors suggested that resistance to root growth might be considerably greater than the diaphragm pressure (o,, in Fig. 2). Barley (1962) estimated resistance to root growth by measuring the pressure required to inflate a small tube placed under the diaphragm. He assumed a coefficient of soil-root friction of 0.25 and estimated that resistance to root elongation varied between 2 and 7 times the diaphragm pressure. Gill & Miller (1956) observed that if roots were well-covered with ballotini, elongation ceased at lower diaphragm pressures. The authors suggested that ‘arching’ of the ballotini caused small displacements of single glass beads to require larger displacements of the diaphragm. Thus, resistance to root elongation was considerably greater than the diaphragm pressure. Pressurized airlwater. Chaudhary & Aggarwal(l984) proposed growing seedlings in moist sand inside a pressure vessel as an experimental technique to measure the effect of mechanical resistance on root elongation. However, although application of air pressure would cause a corresponding 350 A. G. Bengough & C. E. Mullins increase in the absolute value of root cell turgor pressure, the decrease in osmotic potential or cell wall tension required for root cell extension would be independent of the externally applied pressure (assuming the cell permeability to water remained unchanged). Thus, the experiment does not truly represent the situation of roots elongating against an external mechanical resistance. The reason for the large decrease in root growth rates observed by the authors could be the 10-fold increase in dissolved gas concentration in the water inside the pressure chamber, and ultimately in the plant, resulting from the 10-fold increase in air pressure that they applied (Henry's law). Effect of a constant mechanical impedance on root elongation rate Plots of relative root elongation rate against pressure are shown in Figs 4 and 5, and the techniques used are summarized in Table 3. Root elongation rate varies approximately inversely with increas- ing soil penetrometer resistance (Fig. 4). Because penetrometers experience a resistance 2 to 8 times greater than that experienced by roots, the maximum penetrometer resistance measured in a medium when roots can only just elongate far exceeds the maximum stress which roots can exert (about 0.9 to 1.3 MPa). In contrast, results from studies in artificial systems (Fig. 5) show root elongation virtually halted by pressures applied to the outside of ballotini-filled cells of less than 0.1 MPa (Goss, 1977; Abdalla et al., 1969; Barley, 1963). This is only one-tenth of the maximum stress that roots can exert and, as already explained, is due to the resistance to root growth being considerably greater than the externally applied pressure. m m ? 5; 60 t m W 3 0 - c 0 5 20 c 0 W - Penetrometer resistance (MPa) Fig. 4. Root elongation vs penetrometer resistance obtained by Voorhees et al., 1975 (A = sandy loam, B =clay) and by Taylor & Ratliff, 1969 (C = peanuts, D =cotton). Details are summarized in Table 3. There have been only three studies in which both root elongation rate and root resistance (as directly determined from the root force) have been measured (Eavis, 1967; Stolzy & Barley, 1968; Bengough & Mullins, 1988). The results of Eavis (1967) have been replotted in Fig. 5 to show the relationship between root elongation rate and root penetration resistance. This shows that root elongation rate was reduced by about 50% by a resistance to root growth of approximately 0.3 MPa. Bengough & Mullins (1988) found that maize root elongation rates in cores of two sandy loam soils were reduced to between 50% and 90% of that of control plants grown in loose sieved soils, by root penetration resistances of between 0.39 and 0.48 MPa (based on the initial root tip cross- sectional area). Stolzy & Barley (1968) found that the elongation rates of two pea radicles were reduced to 44% of their unimpeded rate by a root penetration resistance of 0.46 MPa. [...]... corn root elongation Canadian Journal of Soil Science 53,383-388 MISRA, R.K., DEXTER, A. R &ALSTON, A. M l98 6a Penetration ofsoil aggregates of finite size 11 Plant roots Plant and Soil 94,59-85 MISRA, R.K., DEXTER, A. R & ALSTON, A. M 1986b Maximum axial and radial growth pressures of plant roots, Plant andSoil95,315-326 M.B 1988 Moss, G.I., HALL,K.C and JACKSON, Ethylene and the responses of roots of maize... 1985), and are potentially of considerable use for investigating the largely unresearched ability of different species and varieties to grow against mechanical impedance (Goss, 1974) The changes in root growth rate which occur as a root enters or leaves a zone of high mechanical impedance are of considerable relevance to real soils, but have received surprisingly little attention The mechanism of root. .. penetration of a loam by plant roots Australian Journal of Soil Research 3,69-79 BARLOW, P.W 1989 Anatomical controls of root growth Aspects of Applied Biology 22,57-66 BENGOUGH, A. G 1988 The use of penetrometers in estimating mechanical impedance to root growth Ph.D thesis, University of Aberdeen BENGOUGH, A. G 1990 The penetrometer in relation to mechanical resistance to root growth In Soil Analysis:... interact with mechanical impedance, because soil strength and root penetration resistance increase as the matric potential decreases Taylor & Ratliff (1969) measured the root elongation rates of cotton and peanuts in remoulded soil at several different bulk densities and matric potentials in the range -17 to -700 kPa for cotton, and - 19 to - 1250 kPa for peanuts Root elongation rate clearly depended on... deficiency and mechanical impedance is still uncertain, but may be due to mechanical impedance changing the morphology of the cortical root cells and the spaces between these cells This might result in a shorter and a more tortuous path for oxygen transport along the root to the tip, and for ethylene transport away from the tip It is more difficult to ascertain whether low matric potentials interact with mechanical. .. observed 'root elongation rate, and similarly anchorage of the tip by root hairs can result in the resistance being underestimated All the experimental relationships in Figs 4 and 5 show similar patterns in which elongation rate decreases with increasing mechanical resistance If the results of direct experimental measurements of root resistance and elongation rate (Stolzy & Barley, 1968; Eavis, 1967) are... penetrometer resistance of the soil and not on the matric potential per se Similar results were obtained by Greacen & Oh (1972) for peas at matric potentials below -0.5 MPa, and by Taylor & Gardner (1963) for cotton root penetration at matric potentials between - 20 kPa and - 67 kPa These results imply that there is no interaction between matric potential and mechanical impedance In contrast to these findings,... elongation and branching of seminal roots Journal of Experimental Botany 28,96-11 I Goss, M.J & RUSSELL, 1980 Effects of mechanR.S ical impedance on root growth in barley (Hordeum vulgare L.) 111 Observations on the mechanism Mechanical impedance to root growth of response Journal of Experimental Botany 31, 577-588 Goss, M.J., BARRACLOUGH, & POWELL, P.B B .A 1989 The extent to which physical factors... from the total resistance to a narrowly tapered probe The effects on penetration resistance of penetrometer or root diameter still require a fuller investigation Root elongation rate is progressively decreased by increasing mechanical resistance to growth, and ceases at root penetration resistances of about 1 MPa Resistance to root elongation within a ballotini cell confined by an externally applied... DEXTER, A. R 1987 Mechanics of root growth Hunt andSoil98,303-312 DEXTER, & TANNER, A. R D.W 1973 The force on spheres penetrating soil Journal of Terramechanics 9,31-39 EAVIS,B.W 1967 Mechanical impedance to root growth Agricultural Engineering Symposium, Silsoe Paper 4/F/39, pp 1-1 1 EAVIS, B.W 1972 Soil physical conditions affecting seedling root growth I Mechanical impedance aeration and moisture availability . mechanical impedance probably undergo physiological changes to adapt to the stress. Interaction of mechanical impedance with matric potential and aeration. Journalof Soil Science, 1990,41,341-358 Mechanical impedance to root growth: a review of experimental techniques and root growth responses A. G.