Paul D. Hallett

Matching roots to their environment

BACKGROUNDnPlants form the base of the terrestrial food chain and provide medicines, fuel, fibre and industrial materials to humans. Vascular land plants rely on their roots to acquire the water and mineral elements necessary for their survival in nature or their yield and nutritional quality in agriculture. Major biogeochemical fluxes of all elements occur through plant roots, and the roots of agricultural crops have a significant role to play in soil sustainability, carbon sequestration, reducing emissions of greenhouse gasses, and in preventing the eutrophication of water bodies associated with the application of mineral fertilizers.nnnSCOPEnThis article provides the context for a Special Issue of Annals of Botany on Matching Roots to Their Environment. It first examines how land plants and their roots evolved, describes how the ecology of roots and their rhizospheres contributes to the acquisition of soil resources, and discusses the influence of plant roots on biogeochemical cycles. It then describes the role of roots in overcoming the constraints to crop production imposed by hostile or infertile soils, illustrates root phenotypes that improve the acquisition of mineral elements and water, and discusses high-throughput methods to screen for these traits in the laboratory, glasshouse and field. Finally, it considers whether knowledge of adaptations improving the acquisition of resources in natural environments can be used to develop root systems for sustainable agriculture in the future.

Soil strength and macropore volume limit root elongation rates in many UK agricultural soils

BACKGROUND AND AIMSnSimple indicators of crop and cultivar performance across a range of soil types and management are needed for designing and testing sustainable cropping practices. This paper determined the extent to which soil chemical and physical properties, particularly soil strength and pore-size distribution influences root elongation in a wide range of agricultural top soils, using a seedling-based indicator.nnnMETHODSnIntact soil cores were sampled from the topsoil of 59 agricultural fields in Scotland, representing a wide geographic spread, range of textures and management practices. Water release characteristics, dry bulk density and needle penetrometer resistance were measured on three cores from each field. Soil samples from the same locations were sieved, analysed for chemical characteristics, and packed to dry bulk density of 1.0 g cm(-3) to minimize physical constraints. Root elongation rates were determined for barley seedlings planted in both intact field and packed soil cores at a water content close to field capacity (-20 kPa matric potential).nnnKEY RESULTSnRoot elongation in field soil was typically less than half of that in packed soils. Penetrometer resistance was typically between 1 and 3 MPa for field soils, indicating the soils were relatively hard, despite their moderately wet condition (compared with <0.2 MPa for packed soil). Root elongation was strongly linked to differences in physical rather than chemical properties. In field soil root elongation was related most closely to the volume of soil pores between 60 µm and 300 µm equivalent diameter, as estimated from water-release characteristics, accounting for 65.7 % of the variation in the elongation rates.nnnCONCLUSIONSnRoot elongation rate in the majority of field soils was slower than half of the unimpeded (packed) rate. Such major reductions in root elongation rates will decrease rooting volumes and limit crop growth in soils where nutrients and water are scarce.

Root hairs improve root penetration, root–soil contact, and phosphorus acquisition in soils of different strength

Root hairs are a key trait for improving the acquisition of phosphorus (P) by plants. However, it is not known whether root hairs provide significant advantage for plant growth under combined soil stresses, particularly under conditions that are known to restrict root hair initiation or elongation (e.g. compacted or high-strength soils). To investigate this, the root growth and P uptake of root hair genotypes of barley, Hordeum vulgare L. (i.e. genotypes with and without root hairs), were assessed under combinations of P deficiency and high soil strength. Genotypes with root hairs were found to have an advantage for root penetration into high-strength layers relative to root hairless genotypes. In P-deficient soils, despite a 20% reduction in root hair length under high-strength conditions, genotypes with root hairs were also found to have an advantage for P uptake. However, in fertilized soils, root hairs conferred an advantage for P uptake in low-strength soil but not in high-strength soil. Improved root-soil contact, coupled with an increased supply of P to the root, may decrease the value of root hairs for P acquisition in high-strength, high-P soils. Nevertheless, this work demonstrates that root hairs are a valuable trait for plant growth and nutrient acquisition under combined soil stresses. Selecting plants with superior root hair traits is important for improving P uptake efficiency and hence the sustainability of agricultural systems.

Root:soil adhesion in the maize rhizosphere: the rheological approach

This study was designed to investigate the strength of attachment of plant seedling roots to the soil in which they were grown. The study also assessed the effects of differing soil textures and differing soil matric potentials upon the strength of the root:soil attachment. A device for growing roots upon a soil surface was designed, and was used to produce roots which were attached to the soil. In order to quantify root:soil adhesion, roots of maize seedlings, grown on the soil surface, were subsequently peeled off using a universal test machine, in conjunction with simultaneous time-lapse video observation. To clarify the partitioning of energy in the root:soil peeling test, separate mechanical tests on roots, and on two adherent remoulded topsoil balls were also carried out. The seedling root was characterised by a low bending stiffness. The energy stored in bending was negligible, compared to the root:soil adhesion energy. The mechanical properties of two adherent remoulded topsoil balls were a decrease of the soil:soil adhesion energy as the soil:soil plastic energy increased. These two parameters were therefore interdependent. Using a video-camera system, it was possible to separate the different processes occurring during the root:soil peeling test, in particular, the seed:soil adhesion and the root:soil soil adhesion. An interpretation of the complex and variable force:displacement curves was thus possible, enabling calculation of the root:soil interfacial rupture energy. At a given suction (10 kPa), the results of the peeling test showed a clear soil texture effect on the value of the root:soil interfacial rupture energy. In contrast, for the same silty topsoil, the effect of the soil water suction on the value of the interfacial rupture energy was very moderate. The root:soil interfacial rupture energy was controlled mainly by a product of microscopic soil specific surface area and the macroscopic contact surface area between the root and the soil. Biological and physical interactions contributing to root:soil adhesion such as root:soil interlocking mechanics were also analysed and discussed.

Biomechanics of nodal, seminal and lateral roots of barley: effects of diameter, waterlogging and mechanical impedance

Background and aimsBiomechanical properties of cereal root systems largely control both resistance to root lodging and their ability to stabilise soil. Abiotic stresses can greatly modify root system growth and form. In this paper the effect of waterlogging and moderate mechanical impedance on root biomechanics is studied for both lateral roots and the main axes of barley.MethodsBarley (Hordeum vulgare) plants were subjected to transient water-logging and moderate mechanical impedance in repacked soil columns. Roots were excavated, separated into types (nodal, seminal or lateral) and tested in tension to measure strength and elastic modulus.ResultsWater-logging and mechanical impedance substantially changed root system growth whilst root biomechanical properties were affected by waterlogging. Root strength was generally greater in thin roots and depended on root type. For example, seminal roots 0.4–0.6xa0mm in diameter were approximately seven times stronger and five times stiffer than lateral roots of the same diameter when mechanically impeded. Root sample populations typically exhibited negative power-law relationships between root strength and diameter for all root types. Mechanical impedance slowed seminal root elongation by approximately 50xa0% and resulted in a 15xa0% and 11xa0% increase in the diameter of in nodal and seminal roots respectively. Power-law relationships between root diameter and root biomechanical properties corresponded to the different root types. Coefficients for between root diameter, strength and elastic modulus improved when separated by root type, with R2 values increasing in some roots from 0.05 to 0.71 for root strength and 0.08 to 0.74 for elastic modulus.ConclusionsModerate mechanical impedance did not influence the tensile strength of roots, but, waterlogging diminished the relationship between root strength and diameter. Separation of root type improved predictions of root strength and elastic modulus using power-law regressions.

Plant exudates may stabilize or weaken soil depending on species, origin and time

&NA; We hypothesized that plant exudates could either gel or disperse soil depending on their chemical characteristics. Barley (Hordeum vulgare L. cv. Optic) and maize (Zea mays L. cv. Freya) root exudates were collected using an aerated hydroponic method and compared with chia (Salvia hispanica L.) seed exudate, a commonly used root exudate analogue. Sandy loam soil was passed through a 500‐μm mesh and treated with each exudate at a concentration of 4.6 mg exudate g−1 dry soil. Two sets of soil samples were prepared. One set of treated soil samples was maintained at 4°C to suppress microbial processes. To characterize the effect of decomposition, the second set of samples was incubated at 16°C for 2 weeks at −30 kPa matric potential. Gas chromatography‐mass spectrometry (GC‐MS) analysis of the exudates showed that barley had the largest organic acid content and chia the largest content of sugars (polysaccharide‐derived or free), and maize was in between barley and chia. Yield stress of amended soil samples was measured by an oscillatory strain sweep test with a cone plate rheometer. When microbial decomposition was suppressed at 4°C, yield stress increased 20‐fold for chia seed exudate and twofold for maize root exudate compared with the control, whereas for barley root exudate decreased to half. The yield stress after 2 weeks of incubation compared with soil with suppressed microbial decomposition increased by 85% for barley root exudate, but for chia and maize it decreased by 87 and 54%, respectively. Barley root exudation might therefore disperse soil and this could facilitate nutrient release. The maize root and chia seed exudates gelled soil, which could create a more stable soil structure around roots or seeds. HighlightsRheological measurements quantified physical behaviour of plant exudates and effect on soil stabilization.Barley root exudates dispersed soil, which could release nutrients and carbon.Maize root and chia seed exudates had a stabilizing effect on soil.Physical engineering of soil in contact with plant roots depends on the nature and origin of exudates.

Effect of root age on the biomechanics of seminal and nodal roots of barley (Hordeum vulgare L.) in contrasting soil environments

Background and aimsThe biomechanics of root systems influence plant lodging resistance and soil structural stabilisation. Tissue age has the potential to influence root biomechanical properties through changes in cell wall chemistry, root anatomy and morphology. Within a root system the internal structures of roots are known to vary markedly within different root types. Nodal, seminal and lateral roots of Barley (Hordeum vulgare) have differing biomechanical behaviour in tension. This study examines the effects of root age on biomechanical properties of barley root types (Hordeum vulgare) under abiotic stress.MethodsRoot age was determined as a function of the distance from root tip with abiotic stresses consisting of waterlogging and restriction to root elongation rate through increased soil bulk density. Linear regression analyses were performed on log-transformed tensile strength and Young’s modulus data with best fits determined for single and multiple parameter models to root morphological properties.ResultsRegression co-efficients and Akaike values showed that distance from root tip (taken as a proxy of root age) was the best single variable for prediction of both root tensile strength and Young’s modulus. Incorporation of both distance from root tip and root diameter and root type increased the reliability of predictions for root biomechanical properties from 47 to 57xa0% for tensile strength and 35 to 62xa0% for Young’s modulus.ConclusionsThe age effect may partly explain some scatter in both Young’s modulus and tensile strength to diameter relationship, commonly cited in the literature.

The effect of natural seed coatings of Capsella bursa-pastoris L. Medik. (shepherd’s purse) on soil-water retention, stability and hydraulic conductivity

Backgrouand and aimMyxospermous seeds become bound by mucilage upon hydration and this trait is ecologically important. Major impacts could be enhancing seed-soil contact and improving water retention, which we quantify in this study.MethodsMyxospermous or demucilaged seeds of Capsella bursa-pastoris L. Medik. (shepherd’s purse) were added to a test sandy clay loam at seed : soil densities of 5 and 10xa0% [w/w]. The soil water retention and hydraulic conductivity were assessed. Soil rheology was also assessed using extracted mucilage only amendment at 0.5 and 1xa0% [w/w].ResultsShepherd’s purse seeds increased soil water retention and reduced soil hydraulic conductivity for myxospermous and demucilaged seeds. Soil rheological properties (complex shear modulus, viscosity and yield stress) increased in response to seed mucilage addition, and became more pronounced as soil dried. The mucilage had greatest impact on the yield stress compared to the other rheology parameters.ConclusionsThe densities of myxospermous and non-myxospermous seeds, and mucilage tested here reflect that may be found naturally in soil seedbanks. The findings provide the first evidence that the soil seedbank provided from a wild arable species may regulate the soil water retention and enhance soil stability, and that this capacity is greater for myxospermous seeds.

How do enzymes catalysing soil nitrogen transformations respond to changing temperatures

Biological processes in soils are regulated in part by soil temperature, and there is currently considerable interest in obtaining robust information on the temperature sensitivity of carbon cycling process. However, very little comparable information exists on the temperature regulation of specific nitrogen cycling processes. This paper addresses this problem by measuring the temperature sensitivity of nitrogen cycling enzymes in soil. A grassland soil was incubated over a range of temperatures (−2 to 21 °C) reflecting 99xa0% of the soil temperature range during the previous 50xa0years at the site. After 7 and 14xa0days of incubation, potential activities of protease, amidase and urease were determined. Activities of protease and urease were positively related to temperature (activation energy; Eau2009=u200949.7 and 73.4xa0kJxa0mol−1, respectively, and Q10u2009=u20092.97 and 2.78, respectively). By contrast, amidase activity was relatively insensitive to temperature, but the activity was significantly increased after the addition of glucose. This indicated that there was a stoichiometric imbalance with amidase activity only being triggered when there was a supply of exogenous carbon. Thus, carbon supply was a greater constraint to amidase activity than temperature was in this particular soil.

The rheological properties of the seed coat mucilage of Capsella bursa-pastoris L. Medik. (shepherd's purse).

The outer surface of myxospermous seed coats contains mucilage which absorbs large amounts of water relative to its dry weight. Ecologically, the seed mucilage can affect seed germination and dormancy. Upon hydration, a large proportion of the seed mucilage is lost to the soil and the physics of soil-seed mucilage interactions has not been assessed. Towards that end, the dynamic rheological properties of mucilage extracted from Capsella bursa-pastoris L. Medik. (shepherds purse) seeds were assessed as a function of mucilage concentration (1-10% [w/w]), temperature (0-80°C) and shear frequency (0.1-100 rad s-1). The seed mucilage was shear thinning and was classified as a highly viscous weak gel. The relationship between the viscoelastic parameters (viscosity, η*, storage and loss modulus, G and G″, yield and flow stresses, τy and τf) and mucilage concentration were well fitted by power law models. The values of η*, G and G″ increased as temperature increased above 40°C and were also slightly frequency dependent. The shepherds purse seed mucilage is more viscous than that from other plant parts, such as fruits and roots. These properties highlight the possibility that seed mucilage may affect soil conditions and therefore present an additional facilitative ecological role (beyond that already reported, which directly affect seed biology); and this is discussed.