Penetrometer resistance,root penetration resistance and root elongation rate in two sandy loam soils

Root penetration resistance and elongation of maize seedling roots were measured directly in undisturbed cores of two sandy loam soils. Root elongation rate was negatively correlated with root penetration resistance, and was reduced to about 50 to 60% of that of unimpeded controls by a resistance of between 0.26 and 0.47 MPa. Resistance to a 30° semiangle, 1 mm diameter penetrometer was between about 4.5 and 7.5 times greater than the measured root penetration resistance. However, resistance to a 5° semiangle, 1 mm diameter probe was approximately the same as the resistnace to root penetration after subtracting the frictional component of resistance. The diameter of roots grown in the undisturbed cores was greater than that of roots grown in loose soil, probably as a direct result of the larger mechanical impedance in the cores.     

Measurement of leaf biomechanical properties in studies of herbivory: Opportunities,problems and procedures

Leaf biomechanical properties have the potential to act as antiherbivore defences. However, compared with studies on chemical defences, there are few studies that have demonstrated that the physical or biomechanical structure of plants can prevent or influence herbivory. This difference in focus by ecologists may relate to the dominant paradigm of plant chemical defences in ecological research and the perceived difficulties that ecologists have with the engineering principles embedded in biomechanics. The advantage of using materials engineering concepts is that each property is precisely defined and quantifiable, although the latter may be difficult in leaves because of their composite and anisotropic nature. Most herbivory studies have used simple penetrometers to measure leaf properties, often termed ‘toughness’. As defined in materials engineering, the measured properties are ‘force to fracture’ and ‘strength’, not toughness. Measurement of strength, the resistance to crack initiation, is relevant to understanding herbivory. Measurement of ‘toughness’ as defined by materials engineering is also relevant. Toughness is the resistance to crack propagation and is a measure of the energy required to fracture the leaf. This requires more sophisticated equipment than simple penetrometers because it requires a simultaneous measure of the punch displacement. In addition, purists would argue that a punch cannot be used to measure true toughness because the crack is not controlled and plastic deformation is also involved. However, it may be the only method that allows detection of fine‐scale pattern in mechanical properties across a leaf surface at a scale that is relevant to herbivory. There is very little work on the scale at which these properties vary, particularly with regard to different sized herbivores. In addition, few studies have investigated a broad range of relevant biomechanical properties in relation to herbivory. Therefore, it is not possible yet to be definitive about the relative merits of the various types of tests. A single test might show a pattern in relation to herbivore damage at a gross level. However, to really understand the functional and ecological significance of leaf texture in relation to herbivory, a more reductionist approach is needed. Only then can we move on to the larger scales of pattern that many ecologists are seeking.     

Are penetrometer measurements useful in predicting emergence of sugar beet (Beta vulgaris L.) seedlings through a crust?

Soil crusting strongly affects seedling emergence. Laboratory experiments were carried out to analyse the emergence of sugar beet (Beta vulgarisL.) seedlings from beneath a crust, and to determine whether mechanical models using penetrometer measurements could predict the final emergence rates. Wet and dry crusts were created under a rainfall simulator. An ascending micropenetrometer was used to measure the crust strength in conditions as close as possible to those encountered by seedlings. The mode of the emergence was observed for both micropenetrometer probe and seedlings. While 94% of the emerged seedlings penetrated the wet crusts, only 6% broke it. The probe penetrated the wet crust in only 46% of cases, and broke it in the other 54%. In contrast, 39% of the seedlings emerged directly through a crack in the dry crust and 55% emerged by breaking (5% lifted a detached fragment and 1% penetrated the crust). In the same conditions, the probe emerged directly through a crack in only 4% of cases; 67% by breaking and 29% by lifting a crust fragment. The force recorded by the penetrometer was 0.05 – 0.80 N, and it varied with the mode of emergence. The seedling growth force distribution was 0 – 0.30 N (mean = 0.09 N). Mechanical models comparing the force exerted by the micropenetrometer probe to the seedling force distribution according to the mode of the emergence were used to predict final emergence rates. Penetrometer measurements appeared to overestimate the mechanical resistance encountered by the seedlings, leading to an underestimation of emergence rates under all emergence conditions. This overestimation of crust strength was attributed to the rigidity of the probe as opposed to the flexibility of hypocotyls.     

A biophysical analysis of root growth under mechanical stress

The factors controlling root growth in hard soils are reviewed alongside summarised results from our recent studies. The turgor in cells in the elongation zone of roots pushes the apex forward, resisted by the external pressure of the soil and the tension in the cell walls. The external pressure of the soil consists of the pressure required to deform the soil, plus a component of frictional resistance between the root and soil. This frictional component is probably small due to the continuous sloughing of root cap cells forming a low-friction lining surrounding the root. Mechanically impeded roots are not only thicker, but are differently shaped, continuing to increase in diameter for a greater distance behind the root tip than in unimpeded roots. The osmotic potential decreases in mechanically impeded roots, possibly due to accumulation of solutes as a result of the slower root extension rate. This more negative osmotic potential is not always translated into increased turgor pressure, and the reasons for this require further investigation. The persistent effect of mechanical impedance on root growth is associated with both a stiffening of cell walls in the axial direction, and with a slowing of the rate of cell production.     

Growth and morphology of eucalypt seedling-roots, in relation to soil strength arising from compaction

Information on the response of root growth and morphology to soil strength is useful for testing suitability of existing and new tillage methods and/or for selecting plants suitable for a specific site with or without tillage. Although there is extensive published information on the root growth-soil strength relationships for annual agricultural plants, such information is scarce for woody, perennial tree species. The purpose of this study is to examine growth and morphology of the root systems of 17-day-old eucalypt seedlings with respect to variation in soil strength. Soil strength in this study was varied by compaction of a well-aggregated clay soil to bulk densities of 0.7–1.0 Mg m^-3 whilst maintaining adequate water availability and aeration for plant growth. Lengths and tip-diameters of primary and lateral roots were measured on the excavated root systems of seedlings.With increase in bulk density and also soil strength (expressed as penetrometer resistance), total length of primary and lateral roots decreased. There were 71 and 31% reduction in the lengths of primary and lateral roots respectively with an increase in penetrometer resistance from 0.4 to 4.2 MPa. This indicated primary roots to be more sensitive to high soil strength than the lateral roots. Average length of lateral roots and diameters of both primary and lateral root tips increased with an increase in soil strength as well. There was greater abundance of lateral roots (no. of lateral roots per unit length of primary root) and root hairs with increased soil strength. The observed root behaviour to variable soil strength is discussed in the context of compensatory growth of roots and overall growth of plants.     

Tribology of the root cap in maize (Zea mays) and peas (Pisum sativum)

Frictional resistance to a penetrating body can account for more than 80% of the total resistance to penetration of soil. We measured the frictional resistance between growing root caps of maize and pea and ground and smooth glass surfaces, which was linearly correlated to load, allowing calculation of the coefficient of kinetic friction and adhesion. Coefficients of kinetic friction between the root caps and the ground and smooth glass surfaces were approximately 0.04 and 0.02, respectively, the first measurements of the frictional properties of root tips at rates approaching those of root elongation, and an order of magnitude smaller than those previously reported. Results suggest that roots are well designed for penetrating soil, and encounter only small frictional resistance on the root cap. These data provide important parameters for modelling soil stresses and deformation around growing root tips.     

The effect of soil compaction on growth and P uptake by Trifolium subterraneum: interactions with mycorrhizal colonisation

The effects of vesicular-arbuscular mycorrhizal (VAM) colonisation on phosphorus (P) uptake and growth of clover (Trifolium subterraneum L.) in response to soil compaction were studied in three pot experiments. P uptake and growth of the plants decreased as the bulk density of the soil increased from 1.0 to 1.6 Mg m^-3. The strongest effects of soil compaction on P uptake and plant growth were observed at the highest P application (60 mg kg^-1 soil). The main observation of this study was that at low P application (15 mg kg^-1 soil), P uptake and shoot dry weight of the plants colonised by Glomus intraradices were greater than those of non-mycorrhizal plants at similar levels of compaction of the soil. However, the mycorrhizal growth response decreased proportionately as soil compaction was increased. Decreased total P uptake and shoot dry weight of mycorrhizal clover in compacted soil were attributed to the reduction in the root length. Soil compaction had no significant effect on the percentage of root length colonised. However, total root length colonised was lower (6.6 m pot^-1) in highly compacted soil than in slightly compacted soil (27.8 m pot^-1). The oxygen content of the soil atmosphere measured shortly before the plants were harvested varied from 0.18 m³m^-3 in slightly compacted soil (1.0 Mg m^-3) to 0.10 m³m^-3 in highly compacted soil (1.6 Mg m^-3).     

The effect of soil strength on the growth of pigeonpea radicles and seedlings

The effect of soil strength on the growth of pigeonpea radicles and seedlings was investigated in cores of three clay soils prepared at different water contents and bulk densities in the laboratory.Radicle elongation directly into soil cores was reduced from 50–70 mm d^-1 at strengths less than 0.5 MPa to 0 mm d^-1 at 3.5–3.7 MPa. The response to soil strength was affected by the water content of the soil, presumably as a result of reduced oxygen availability in wetter soil. This effect was apparent in soils wet to air-filled porosities less than 0.15 m³ m^-3.Radicles were more sensitive to high soil strength (>1.5 MPa) than were seedling roots which encountered the same conditions at 60 mm in the profile. Radicle growth ceased at 3.5 MPa which reduced seedling root growth by only 60%.Despite a 60% reduction in root length in the high strength zone, seedling roots compensated in zones of loose soil above and below the compacted layer, and total root length and shoot growth were unaffected. There was no evidence of a root signal response which results in reduced shoot growth in some species in response to high soil strength.The proliferation of roots in surface layers and the delayed penetration of the root system to depth in compacted soil are likely to expose seedlings to a greater risk of water-deficit in the field, particularly under dryland conditions where plants rely on stored subsoil water for growth.     

Chemical and mechanical changes during leaf expansion of four woody species of dry Restinga woodland

Young leaves are preferential targets for herbivores, and plants have developed different strategies to protect them. This study aimed to evaluate different leaf attributes of presumed relevance in protection against herbivory in four woody species (Erythroxylum argentinum, Lithrea brasiliensis, Myrciaria cuspidata, and Myrsine umbellata), growing in a dry restinga woodland in southern Brazil. Evaluation of leaf parameters was made through single-point sampling of leaves (leaf mass per area and leaf contents of nitrogen, carbon, and pigments) at three developmental stages and through time-course sampling of expanding leaves (area and strength). Leaves of M. umbellata showed the highest leaf mass per area (LMA), the largest area, and the longest expansion period. On the other extreme, Myrc. cuspidata had the smallest LMA and leaf size, and the shortest expansion period. Similarly to L. brasiliensis, it displayed red young leaves. None of the species showed delayed-greening, which might be related to the high-irradiance growth conditions. Nitrogen contents reduced with leaf maturity and reached the highest values in the young leaves of E. argentinum and Myrc. cuspidata and the lowest in M. umbellata. Each species seems to present a different set of protective attributes during leaf expansion. Myrciaria cuspidata appears to rely mostly on chemical defences to protect its soft leaves, and anthocyanins might play this role at leaf youth, while M. umbellata seems to invest more on mechanical defences, even at early stages of leaf growth, as well as on a low allocation of nitrogen to the leaves. The other species display intermediate characteristics.