W. R. Whalley

How do roots penetrate strong soil

The mechanical and physiological bases for root growth against high mechanical impedance are reviewed. The best estimates of maximum axial root growth pressure (σmax) in completely impeded pea roots appear to be from 0.5 to 0.6 MPa, which results from a turgor pressure of about 0.8 MPa. When roots are incompletely impeded, a range of responses has been reported. Roots do not change elongation rate in a simple mechanical way in response to changes in mechanical impedance. Instead, ethylene might play a key role in mediating an increase in root diameter and a decrease in elongation rate. These changes persist for some hours or days after impedance is removed. Differences between species in their ability to penetrate strong soil layers are not related to differences in σmax, but appear to be due to differences in root diameter. In rice, differences between cultivars in the ability of their roots to penetrate strong wax layers are not related to their elongation rates through uniformly strong media. Differences between species or cultivars in their ability to penetrate strong layers may be due to differences in the tendency of roots to deflect or buckle when they grow from a weak to a strong environment.

Does Soil Strength Play a Role in Wheat Yield Losses Caused by Soil Drying

Shoot growth in wheat is sensitive to high soil strength, but as high strength and drying tend to occur together it has proved difficult to separate the effects of water stress and mechanical impedance. The results of two field experiments in 2003 and 2004, where soil strength was manipulated by compaction and irrigation, demonstrated that the yield of wheat (Triticum aestivum L.) was sensitive to physical stress in the root zone. We obtained linear relationships between yield and soil strength and between yield and accumulated soil moisture data (accumulation analogous to thermal time), with similar slopes for both seasons. We were unable to detect root-sourced signals of xylem-sap ABA concentration, despite changes in stomatal conductance. When mechanical impedance and matric potential were varied independently in controlled environments, the growth of wheat was sensitive to mechanical impedance, but not to small changes in matric potential. While the response of stomatal conductance to soil drying in the field could be interpreted as evidence of hydraulic signalling, we suggest that the role of high soil strength, in limiting growth rates on moderately dry soil, requires further research.

Root penetration of strong soil in rainfed lowland rice: comparison of laboratory screens with field performance

Rice cvs with better hardpan penetration would be expected to be more drought resistant in the rainfed lowlands. Although laboratory methods to facilitate the identification or breeding of cvs with good root-penetration ability have been described, there is a need to validate such screens against field performance. Here, we compare previous field measurements with laboratory screening measurements in eight cvs (IR20, CT9993, KDML 105, IR58821, NSG 19, IR62266, Mahsuri and IR52561). These were screened (together with Moroberekan, SG329 and IR36 for comparative purposes) using a flooded wax-layer screen. Of the eight cvs, IR58821 gave the best penetration of a 60% wax layer, with a mean penetration of 5.8 root axes per plant. The worst performer was IR52561, with a mean of 0.6 axes per plant. The cvs IR20, CT9993, KDML 105, IR58821 were also screened (together with Azucena, Bala, Moroberekan, Kinandang Patong and IR36 for comparative purposes) using a (non-flooded) sand-core screen. The sand-core screen allowed mechanical impedance of the whole sand core to be varied independently of aeration and water status. High impedance treatments were obtained by placing weights on the sand cores, which greatly decreased root growth, although differences between cvs in response to impedance in the sand-core screen were small. The ability of rice roots to penetrate wax layers did not appear to be related to their elongation through strong sand, but rather to their ability to resist buckling on encountering the wax layer. Comparison with field measurements showed that cvs with good performance in the wax-layer screen did not necessarily have good hardpan penetration in the field, although IR58821 was the best performer in the field. It is concluded that further work is required to compare root penetration in the field with root penetration in laboratory screens.

How do roots elongate in a structured soil

In this review, we examine how roots penetrate a structured soil. We first examine the relationship between soil water status and its mechanical strength, as well as the ability of the soil to supply water to the root. We identify these as critical soil factors, because it is primarily in drying soil that mechanical constraints limit root elongation. Water supply to the root is important because root water status affects growth pressures and root stiffness. To simplify the bewildering complexity of soil-root interactions, the discussion is focused around the special cases of root elongation in soil with pores much smaller than the root diameter and the penetration of roots at interfaces within the soil. While it is often assumed that the former case is well understood, many unanswered questions remain. While low soil-root friction is often viewed as a trait conferring better penetration of strong soils, it may also increase the axial pressure on the root tip and in so doing reduce the rate of cell division and/or expansion. The precise trade-off between various root traits involved in root elongation in homogeneous soil remains to be determined. There is consensus that the most important factors determining root penetration at an interface are the angle at which the root attempts to penetrate the soil, root stiffness, and the strength of the soil to be penetrated. The effect of growth angle on root penetration implicates gravitropic responses in improved root penetration ability. Although there is no work that has explored the effect of the strength of the gravitropic responses on penetration of hard layers, we attempt to outline possible interactions. Impacts of soil drying and strength on phytohormone concentrations in roots, and consequent root-to-shoot signalling, are also considered.

Genetic and management approaches to boost UK wheat yields by ameliorating water deficits

Faced with the challenge of increasing global food production, there is the need to exploit all approaches to increasing crop yields. A major obstacle to boosting yields of wheat (an important staple in many parts of the world) is the availability and efficient use of water, since there is increasing stress on water resources used for agriculture globally, and also in parts of the UK. Improved soil and crop management and the development of new genotypes may increase wheat yields when water is limiting. Technical and scientific issues concerning management options such as irrigation and the use of growth-promoting rhizobacteria are explored, since these may allow the more efficient use of irrigation. Fundamental understanding of how crops sense and respond to multiple abiotic stresses can help improve the effective use of irrigation water. Experiments are needed to test the hypothesis that modifying wheat root system architecture (by increasing root proliferation deep in the soil profile) will allow greater soil water extraction thereby benefiting productivity and yield stability. Furthermore, better knowledge of plant and soil interactions and how below-ground and above-ground processes communicate within the plant can help identify traits and ultimately genes (or alleles) that will define genotypes that yield better under dry conditions. Developing new genotypes will take time and, therefore, these challenges need to be addressed now.

A bioinformatic and transcriptomic approach to identifying positional candidate genes without fine mapping: an example using rice root-growth QTLs

Fine mapping can accurately identify positional candidate genes for quantitative trait loci (QTLs) but can be time consuming, costly, and, for small-effect QTLs with low heritability, difficult in practice. We propose an alternative approach, which uses meta-analysis of original mapping data to produce a relatively small confidence interval for target QTLs, lists the underlying positional candidates, and then eliminates them using whole-genome transcriptomics. Finally, sequencing is conducted on the remaining candidate genes allowing identification of allelic variation in either expression or protein sequence. We demonstrate the approach using root-growth QTLs on chromosomes 2, 5, and 9 of the Bala x Azucena rice mapping population. Confidence intervals of 10.5, 9.6, and 5.4 cM containing 189, 322, and 81 genes, respectively, were produced. Transcriptomics eliminated 40% of candidate genes and identified nine expression polymorphisms. Sequencing of 30 genes revealed that 57% of the predicted proteins were polymorphic. The limitations of this approach are discussed.

Sensing the physical and nutritional status of the root environment in the field: a review of progress and opportunities

The challenge that faces agriculture at the start of the 21st Centuary is to provide security of food production in a sustainable way. Achieving this task is difficult enough, but against a background of climate change, it becomes a moving target. However, one certainty is that soil factors that limit crop growth must be taken into account as new strategies for crop management are developed. To achieve this, it is necessary to measure the physical and nutritional status of the root environment in the field. Before considering measurement methods, our understanding of how the plant interacts with its soil environment is reviewed, so that it is clear what needs to be measured. Soil strength due to soil drying is identified as an important stress that limits agricultural productivity. The scope to measure Soil factors that directly affect plant growth is reviewed. While in situ sensors are better developed, progress in the development of remote sensors of soil properties are also reviewed. A robust approach is needed to interpret soil measurements at the field scale and here geostatistics has much to offer. The present review takes a forward look and explores how our understanding of plant responses to soil conditions, the newly emerging sensing technologies and geostatistical tools can be drawn together to develop robust tools for Soil and crop management. This is not intended to be an exhaustive review. Instead, file authors focus on those aspects that they consider to be most important and where the greatest progress is being made.

Measurement of the matric potential of soil water in the rhizosphere

The availability of soil water, and the ability of plants to extract it, are important variables in plant research. The matric potential has been a useful way to describe water status in a soil-plant system. In soil it is the potential that is derived from the surface tension of water menisci between soil particles. The magnitude of matric potential depends on the soil water content, the size of the soil pores, the surface properties of the soil particles, and the surface tension of the soil water. Of all the measures of soil water, matric potential is perhaps the most useful for plant scientists. In this review, the relationship between matric potential and soil water content is explored. It is shown that for any given soil type, this relationship is not unique and therefore both soil water content and matric potential need to be measured for the soil water status to be fully described. However, in comparison with water content, approaches for measuring matric potential have received less attention until recently. In this review, a critique of current methods to measure matric potential is presented, together with their limitations as well as underexploited opportunities. The relative merits of both direct and indirect methods to measure matric potential are discussed. The different approaches needed in wet and dry soil are outlined. In the final part of the paper, the emerging technologies are discussed in so far as our current imagination allows. The review draws upon current developments in the field of civil engineering where the measurement of matric potential is also important. The approaches made by civil engineers have been more imaginative than those of plant and soil scientists.

Maximum axial root growth pressure in pea seedlings: effects of measurement techniques and cultivars

Values of the maximum axial growth pressure (σmax) of seedling pea (Pisum sativum L.) roots reported in the literature, obtained using different apparatuses and cultivars, range from 0.3 MPa to 1.3 MPa. To investigate possible reasons for this large range, we studied the effect of apparatus and cultivar on measurements of σmax in peas. We describe four types of apparatus which can be used to measure axial root growth force and hence σmax, and used them to measure σmax in seedling pea roots using cultivar Meteor. Two of these apparatuses were also used to compare σmax for three pea cultivars (Helka, Meteor and Greenfeast). Both cultivar and apparatus significantly affected σmax , but there were greater differences between apparatuses than between the three cultivars. Estimating root cross-sectional area from the diameter of cross-sections, rather than from in situ measurements (i.e. measurements made with the root still in place in the apparatus) may explain these differences.

The velocity of shear waves in unsaturated soil

The velocities of shear waves Vs in two soils, a loamy sand and a sandy clay loam, were measured at various matric potentials and confining pressures. We used a combination of Haines apparatus, pressure plate apparatus and a Bishop and Wesley tri-axial cell to obtain a range of saturation and consolidation states. We proposed a single effective stress variable based on a modification to Bishop’s equation which could be used in a published empirical model (Santamarina et al., 2001) to relate shear wave velocity to soil physical conditions. Net stress required a nonlinear transformation. Matric potential was converted into suction stress with the function proposed by Khallili and Khabbaz (1998), thus requiring an estimate of the air entry potential. We found it was possible to fit Vs to void ratio, net stress and matric potential with a set of four parameters which were common to all soils at various states of saturation and consolidation. In addition to the data collected for this study we also used previously published data (Whalley et al., 2011). The utility of shear wave measurements to deduce soil physical properties is discussed. 2012 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +44 01582 763133; fax: +44 01582 760 981. E-mail address: Richard.whalley@rothamsted.ac.uk (W.R. Whalley).