Soil structure governs ecosystem functioning across scales, but its complexity requires integrative approaches that capture geometric and functional properties. This study proposes a methodological framework that integrates field-based visual evaluation of soil structure (VESS), X-ray computed tomography (CT) and soil hydraulic property (SHP) assessment to quantify structural attributes for contrasting land-use types (arable land, grassland, forest). This approach assesses soil structure from three perspectives: aggregate architecture, macropore connectivity and hydraulic function. As a conceptual framework to isolate structural and texture effects and quantify differences related to land use, we chose three sites in Switzerland with similar topsoil texture and close proximity (similar to 1 km). Undisturbed topsoil samples were collected for CT and SHP measurements (250 mL, 5-10 cm depth) while VESS was performed in situ (5-10 cm and 0-30 cm depth). Assessment of SHP included measuring the soil water retention curve and the saturated and unsaturated hydraulic conductivity. CT imaging (91 mu m pixel size) quantified macropore volume and connectivity metrics (Euler-Poincar & eacute; characteristic EPC and gamma indicator). Saturated hydraulic conductivity data aligned closely with CT metrics, especially macroporosity and the EPC, highlighting their utility in bridging structural observations with functional implications. Despite smaller total porosity, soils at the arable site showed a better VESS score and greater macroporosity and saturated hydraulic conductivity than soils at the grassland site, underscoring the importance of combining different metrics in structural interpretation. The combined methods capture complementary aspects of soil structure, ranging from aggregate-scale features to pore connectivity and hydraulic function, and improve structural interpretation for soil health assessment. Following upon this methodological framework with a small sample size (11 samples) and results related to site specific conditions, future research should validate whether relationships between field-based VESS scores and laboratory metrics hold across broader pedological conditions, to potentially make VESS a quantitative predictor of soil structural functionality for large-scale monitoring.

Root hairs are considered to be crucial for facilitating root water uptake, but the specific conditions influencing their impact remain unclear. We present a model that explores the efficacy of root hairs as a function of soil properties, root type, and tissue maturation. In particular, we investigate how the location of root hairs along the root axis, relative to root development, influences root water uptake. The analysis focuses on spatial distribution instead of individual traits such as hair density and length. Using a hydraulic model based on the finite-difference approach, water flow in single maize roots (lateral, seminal, crown, and brace) in two contrasting soil textures (sand and loam) was simulated. Soil water potential, root hair length, and the longitudinal extent of the root hair zone were varied to assess root hair impact on water uptake. (i) Root hairs become increasingly relevant in facilitating root water uptake as soil water potentials decrease. (ii) Root hair effectiveness is greater in sand than loam, due to the lower hydraulic conductivity of sand in the dry range. (iii) In contrast to loam, all root types increase total root water uptake with increasing rhizosphere conductance in sand drier than-2300 hPa. The positive role of hairs in water uptake depends on the profile of root conductivities along the root axis as long as the soil is more conductive than the root. Additionally, root hair functionality depends on soil hydraulic properties and hence is soil texture specific.

Gas exchange in the soil is determined by the size and connectivity of air-filled pores. Root mucilage can partially reduce air-filled pore connectivity and thus reduce gas diffusivity. However, it remains unclear to what extent mucilage affects soil pore connectivity and tortuosity. The aim of this study was to gain a better understanding of gas diffusion processes in the rhizosphere by explaining the geometric alterations of the soil pore space induced by mucilage. We quantified the effect of a root mucilage analogue collected from chia seeds without intrinsic respiratory activity on oxygen diffusion at different water contents during wetting-drying cycles in a diffusion chamber experiment. In addition, we used X-ray computed tomography (CT) imaging to visualize the distribution of air and water in the pore space, and quantified the connectivity of the gas phase. Furthermore, we used environmental scanning electron microscopy (ESEM) to visualize mucilage bridges in the dry soil samples. Quantification of oxygen diffusion showed that mucilage decreased the gas diffusion coefficient in dry soil without affecting air-filled porosity. Without mucilage, a hysteresis in gas diffusion coefficient during a drying-rewetting cycle could be observed for fine sandy soil as well as silt and clay soils. The effect diminished with increasing mucilage content. CT imaging indicated a hysteresis in the connectivity of the gas phase during a drying-rewetting cycle for samples without mucilage. This effect was attenuated with increasing mucilage content. Electron microscopy showed that mucilage forms membrane-like liquid bridges during drying. With increasing mucilage content cylindrical structured are created and at high content interconnected structures are observed throughout the pore space, thereby progressively reducing the connectivity of the gas phase. Our results suggest that the release of mucilage into the soil may be a plant adaptation strategy to balance soil oxygen availability and water content.

Drought impacts trees in varied temporal and spatial patterns, suggesting that heterogeneity of below-ground water stores influences the fate of trees under water stress. Karst ecosystems rely on shallow soil overlying bedrock that can store available water in primary pores. A contribution of rock moisture to tree water status has been previously demonstrated, but actual mechanisms and rates of rock-to-root water delivery remain unknown. We report accurate measurements of hydraulic properties of two rock types (Breccia and Dolostone), of typical Karst red soil, and of roots of a common Karst tree species grown under different rock-soil combinations. Experimental data were used to build a water exchange model that supported the hypothesis that roots can extract water from porous and highly conductive rocks (Breccia), but not from more compact ones (Dolostone), especially when plants grow in rocky substrate or experience water stress, and thus have low root hydraulic conductivity and low rates of water extraction from rocks. Our data support the hypothesis that rocks represent important water stores for plants growing in rock-dominated habitats. Heterogeneous rock properties translate into different rates of water delivery to root systems, underlying complex patterns of tree mortality under severe drought stress.

To address the challenge of soil degradation among different land uses, development of precise indicators that accurately reflect the current state of soil health is crucial. Soil structural attributes, such as the volume of percolating pores and the connectivity of the pore network are inextricably linked to processes such as nutrient dynamics, carbon cycling, root penetration, biological activity, and rainfall partitioning. Hence, they play a significant role in determining the soil susceptibility to erosion and offer great potential as soil health indicators. These attributes are directly reflected in the hydraulic properties of the soil, particularly in its capacity for water infiltration and retention. Notably, high rates of infiltration and drainage are associated with the presence of well-connected macropores. However, these structural attributes typically have to be quantified using costly and time-consuming imaging methods, while obtaining accurate estimates in lab and field experiments has proven challenging. Our multilevel approach is designed to link directly measured structural attributes (macropore volume and connectivity) to standard field or lab measurements.More specifically, macropore volume and connectivity were quantified using X-ray imaging across diverse land use types, including arable land, grassland, and forest. Structural characteristics were then correlated with key hydraulic properties, such as water retention and both saturated and unsaturated hydraulic conductivity, measured using the Hyprop system. We further compared the imaged and measured hydraulic properties with predictions from the European soil texture-based pedotransfer function EUPTF, to contrast texture- and structure-related soil hydraulic properties. As an additional exploratory angle, we related mid-infrared (MIR) spectral reflectance to our previously obtained hydraulic property data, to evaluate if MIR could serve as a less laborious alternative to traditional lab-based analyses. Finally, to develop applicable user-friendly and sensitive indicators, we correlated our findings with the classifications from in-situ Visual Evaluation of Soil Structure (VESS) and infiltration experiments.Preliminary results of X-ray CT data and Hyprop measurements revealed significant differences in the volumetric fraction and drainage capacity of macropores as well as in the saturated hydraulic conductivity between arable land, grassland, and forest. Forest soil showed the largest drainage capacity of macropores, but also the largest variability between samples. Despite exhibiting similar pore size distributions, arable land samples showed, as a result of tillage, larger pore connectivity than grassland. Larger connectivity did, interestingly, not result in larger hydraulic conductivity of macropores. Our novel multilevel approach reveals clear distinction of land use regarding the complex interplay between soil structural continuity, soil texture, and hydraulic behavior. Such knowledge is crucial in formulating sensitive, quantifiable, and scalable indicators for soil health evaluation and management. These indicators are instrumental for creating more accurate models, for designing sensitive monitoring networks and ultimately advancing sustainable practices in agriculture, forestry, and environmental conservation.

In the rhizosphere, all transport processes considered fundamental in regulating resource availability and accessibility for plants and microorganisms are controlled by water retention and its temporal dynamics in the soil pore space, the rhizosphere liquid architecture (RLA). As the soil dries, root water and nutrient uptake becomes increasingly limited as the cross-sectional area and connectivity of the pore water declines. At the same time, diffusive transport ceases, negatively affecting root exudate transport and limiting microbial activity as enzyme diffusion and activity drop. The extent to which soil structural and biological processes influence local water retention and, in turn, related transport processes in the rhizosphere remains a challenging task. This study aimed to elucidate the effect of root growth and extracellular polymeric substances (EPS) on soil water retention in the rhizosphere of maize. High-resolution X-ray tomography was used to capture gradients in water distribution as a function of rhizosphere age and distance from the root surface. This combination of techniques allows distinguishing between soil structure versus primarily biologically induced modification. This study is a step toward a better understanding of the feedbacks between plants, microorganisms, and soil in controlling rhizosphere transport properties in this complex process aimed at optimizing resource availability and acquisition.

By substantially increasing the surface area of roots available for soil resource capture, root hairs have been hypothesised to facilitate root water uptake, particularly in dry soil conditions. However, existing experimental and computational studies have shown that the effect of root hairs on water uptake cannot be generalised across soils and plant species. The objective of our study is to investigate to what extent and under which soil conditions root hairs facilitate root water uptake. Ultimately we aim to gain a mechanistic understanding of the effect of root hairs on root water uptake across soil textures. We scanned maize (Zea Mays L.) roots grown in two soil types (loamy and sandy soil) using synchrotron-based X-ray CT. We utilized an image‐based modelling approach to simulate water flow through the soil-root continuum by solving the flow equations numerically. This approach allowed us to incorporate rhizosphere properties (e.g. root-soil contact) and root hair shrinkage into the image-based model. Experimental and numerical results show that under dry soil conditions (-1 to -0.1 MPa) root hairs attenuate the gradient in soil matric potential across the rhizosphere. This results in a more effective water extraction compared to a hairless root. Our model revealed that the effect of hairs is determined by soil properties (e.g. soil porosity), root hair traits (e.g. length and density) and the capacity of hairs to remain turgid under drought stress. Compared to densely packed fine textured soils, the effect of hairs is more pronounced in coarse textured soils and loosely packed fine textured soil. This is explained by the steeper hydraulic conductivity curves of these soils. In conclusion, our results show that the effect of root hairs is determined by root-soil contact, which depends on soil properties, and root hair shrinkage.

Despite its limited vertical extent, the thin soil layer provides essential functions for climate and ecosystems globally. For accurate large scale process description, land use models compute the water distribution in soils based on spatial domains with a width-to-thickness ratio of about 1000:1: a geometry as thin as a sheet of paper. Most models simulate the water flow in these ‘soil sheets’ by solving the Richardson-Richards equation in 1D, neglecting smaller scale structures and lateral flow, and implicitly making strong assumptions on the relations between water content, matric potential, hydraulic conductivity, and water flux. To quantify the accuracy of this conceptualization, we compare drainage simulations of wet soils for the 1D column simplification with the full 2D-and 3D geometry using the correct sheet-like size ratio. The role of different climates, soil types, and heterogeneities at smaller scale is analyzed. These simulations based on the full geometry provide guidelines for (i) the applicability of Richardson-Richards equation in land surface models and (ii) the development of appropriate averaging schemes of soil hydraulic properties in the 1D scenario.

The hydraulic properties of the rhizosphere are essential for understanding root-soil interactions. Diverging from common assumptions in soil modelling, which often equate rhizosphere properties with those of bulk soil, research shows that the rhizosphere is distinct in its physical, biological, and chemical attributes. There is a broad agreement on the role of mucilage and extracellular polymeric substances (EPS) in modifying soil water dynamics in the rhizosphere. However, the mechanisms of how these substances interact with the soil matrix and impact its hydraulic properties remain unclear. In this study, we assessed the forces exerted by Xanthan gum, used as a stand-in for EPS, maize root mucilage, and water - formed liquid bridges on particle pairs. Forces were quantified for 1 microL liquid bridge between a pair of glass beads — one standing on a precision balance and the other fixed to a static stand. While the water bridges broke upon drying due to capillary forces, mucilage and Xanthan gum formed resilient filaments that maintain connectivity and tensile forces between the glass beads. The continuous recordings of weight changes by the balance provided crucial data for quantifying the force exerted on the beads during drying. Our results show that both Xanthan gum and maize mucilage liquid bridges exert tensile strengths that are substantially greater than those of water bridges. The polymer solutions initially behave similarly to water, but the forces exerted on particle pairs deviate as the solutions dry, becoming progressively stronger. The tensile strength of water reaches around 10-1 mN, while maize and Xanthan gum are respectively 1 and 2 order of magnitudes bigger. This increase is caused by the stretching of the polymer network and the development of elastic forces. The significant aggregating force observed in our study suggests that EPS and mucilage play a crucial role on the mechanics of the root-soil interface. They contribute to soil structure formation in the rhizoshere and to maintain root and soil contact as roots shrink in drying soils.

Root hair emergence is affected by heterogeneities in water availability in the growth medium. Root hairs preferentially emerge into air, whereas their emergence into water is inhibited. Yet, these results were based either on destructive methods or on roots grown on an agar-air interface. Additionally, there is a lack of knowledge about the spatial distribution of root hairs as hairs elongate radially across the rhizosphere. Therefore, root hair growth in soils remains largely unexplored. Maize (Zea Mays L.) plants were grown in microcosms which were scanned with a synchrotron-based X-ray μ CT. The distribution of root hairs along the root epidermis and radially across the rhizosphere (i.e. as function of distance from the root epidermis) was analysed using spatial point pattern analysis. While hairs emerged randomly in air-filled pores, their emergence was inhibited where the root was in contact with the soil matrix. As hairs elongated radially into the soil, they were preferentially located in the close proximity of soil particles. In maize, we rarely observed root hairs penetrating into soil aggregates. We conclude that in maize, root hairs grow in air-filled pores at the root-soil interface, where the flow of nutrients and water is impeded. Across the rhizosphere, hairs establish contact to the soil by growing in the proximity to soil particles. The effect of hairs on uptake processes, plant anchorage and rhizosheath formation might be limited (in maize) as they hardly penetrate into soil aggregates.

Gas exchange in the soil is determined by the size and connectivity of air-filled pores. Root mucilage reduces air-filled pore connectivity and thus gas diffusivity. It is unclear to what extent mucilage affects soil pore connectivity and tortuosity. The aim of this study was to gain a better understanding of gas diffusion processes in the rhizosphere by explaining the geometric alterations of the soil pore space induced by mucilage. We quantified the effect of a root mucilage analogue collected from chia seeds without intrinsic respiratory activity on oxygen diffusion at different water contents during drying-rewetting cycles in a diffusion chamber experiment. Quantification of oxygen diffusion showed that mucilage decreased the gas diffusion coefficient in dry soil without affecting air-filled porosity. Without mucilage, a hysteresis in gas diffusion coefficient during a drying-rewetting cycle was observed. The effect depended on particle size and diminished with increasing mucilage content. X-ray computed tomography imaging indicated a hysteresis in the connectivity of the gas phase during a drying-rewetting cycle for samples without mucilage. This effect was attenuated with increasing mucilage content. Furthermore, electron microscopy showed that mucilage structures formed in drying soil increase with mucilage content, thereby progressively reducing the connectivity of the gas phase. In conclusion, the effect of mucilage on soil gas diffusion highly depends on soil texture and mucilage content. The diminishing hysteresis with the addition of mucilage suggests that plant roots secrete mucilage to balance oxygen availability and water content, even under fluctuating moisture conditions.

Root hairs and soil water content are crucial in controlling the release and diffusion of root exudates and shaping profiles of biochemical properties in the rhizosphere. But whether root hairs can offset the negative impacts of drought on microbial activity remains unknown. Soil zymography, 14 C imaging and neutron radiography were combined to identify how root hairs and soil moisture affect rhizosphere biochemical properties. To achieve this, we cultivated two maize genotypes (wild-type and root-hair-defective rth3 mutant) under ambient and drought conditions. Root hairs and optimal soil moisture increased hotspot area, rhizosphere extent and kinetic parameters (Vmax and Km ) of β-glucosidase activities. Drought enlarged the rhizosphere extent of root exudates and water content. Colocalization analysis showed that enzymatic hotspots were more colocalized with root exudate hotspots under optimal moisture, whereas they showed higher dependency on water hotspots when soil water and carbon were scarce. We conclude that root hairs are essential in adapting rhizosphere properties under drought to maintain plant nutrition when a continuous mass flow of water transporting nutrients to the root is interrupted. In the rhizosphere, soil water was more important than root exudates for hydrolytic enzyme activities under water and carbon colimitation.

Modeling plant responses to drought over short-to long-term is crucial under rising global warming threats. Roots form the critical gateway between a plant and its water sources in the soil, yet their connection with the soil is still poorly understood. As the soil dries, roots shrink gradually disconnecting from the surrounding soil. This progressive reduction of root-soil contact interrupts the liquid-phase continuity and limits the water movement. The importance of the loss of contact between soil and roots depends on the water potential at which this occurs. If roots lose contact at potential close or beyond the wilting point, when the low soil hydraulic conductivity is already limiting, the loss of contact might not be as important. But if this occurs in still relatively wet conditions, it might trigger an earlier limitation of root water uptake.Currently, it is known at what water potential roots lose contact with the soil. Furthermore, we expect that this critical water potential is not unique, but it depends on soil properties, soil particle size and porosity, and root properties, such as root hair density and mucilage production.Here we present an analysis to identify and quantify the forces that bind the soil to the root for different soil textures. We estimate the adhesive forces that hold roots in contact with the soil and that counteract root shrinkage caused by decreasing water potential and cells losing turgor. The ingredients of our analysis are: root hairs, capillary forces and mucilage elastic properties. Thresholds of gap formation at the root-soil interface are identified for varying soil particle size and porosity and for varying root hair density and mucilage elastic properties.This analysis shows that root-soil contact dynamics do not depend only on the root cell turgor loss point, but also on soil properties, and helps to identify the mechanisms impacting the hydraulic continuity across the root-soil interface.

<p>Root hairs, tubular protrusions of epidermal root cells, are considered a key rhizosphere feature: by substantially increasing the contact area between roots and soil, they enhance the ability of plants to capture soil resources. Hence, they are considered a breeding target for improving drought tolerance and yield stability of crops. While their pivotal role in the uptake of immobile nutrients such as phosphorus is well accepted, their effect on root water uptake remains controversial as it varies across plant species. <br />By means of image-based modelling, our objective was to identify environmental conditions (e.g. soil water content) and hair traits (e.g. root hair length and density) that determine the effectiveness of root hairs in root water uptake. Furthermore, we investigated the effect of drought stress-induced root hair shrinkage on root water uptake.</p> <p>We scanned root compartments of 8 days old maize seedlings (<em>Zea Mays</em> L.) grown in a loamy soil using synchrotron radiation X-ray CT. Based on the collected image-data, we implemented a 3D root water uptake model. By solving Richards equation numerically, we computed the propagation of water potential gradients across the root-soil continuum which allowed to quantify root water uptake. The high spatial resolution of the acquired images enabled us to explicitly take rhizosphere features, such as root hairs and root-soil matrix contact into account. We determined the key parameters governing the effectiveness of root hairs in water uptake by comparing a set of six maize root compartments before and after digitally removing their hairs. The quantification of root hair turgor-loss in response to progressive soil drying allowed us to implement hair shrinkage within our model.</p> <p>We found that the effect of root hairs in root water uptake is governed by 1) the root hair induced increase in root soil contact and 2) root hair length. Furthermore, our results suggest that root hairs potentially facilitate root water uptake under dry soil conditions (< -0.1MPa). However, in the dry range, root hair shrinkage severely reduces the effect of hairs. Depending on their turgor-loss curve, root hairs may still provide a positive effect on root water uptake in a narrow range of soil matric potential. </p> <p>In summary, the effect of root hairs on root water uptake depends on soil water content, root-soil contact, root hair length and the turgor-loss point of hairs.</p>

The effect of root hairs on water uptake remains controversial. In particular, the key root hair and soil parameters that determine their importance have been elusive. We grew maize plants (Zea mays) in microcosms and scanned them using synchrotron-based X-ray computed microtomography. By means of image-based modelling, we investigated the parameters determining the effectiveness of root hairs in root water uptake. We explicitly accounted for rhizosphere features (e.g. root-soil contact and pore structure) and took root hair shrinkage of dehydrated root hairs into consideration. Our model suggests that > 85% of the variance in root water uptake is explained by the hair-induced increase in root-soil contact. In dry soil conditions, root hair shrinkage reduces the impact of hairs substantially. We conclude that the effectiveness of root hairs on root water uptake is determined by the hair-induced increase in root-soil contact and root hair shrinkage. Although the latter clearly reduces the effect of hairs on water uptake, our model still indicated facilitation of water uptake by root hairs at soil matric potentials from -1 to -0.1 MPa. Our findings provide new avenues towards a mechanistic understanding of the role of root hairs on water uptake.

Aims Root hairs are important for uptake, especially for nutrients with low mobility in soils with high sorption capacity. Mutants with defective root hairs are expected to have lower nutrient uptake, unless they compensate with more root growth. Since root hairs can also contribute to the plant's water uptake their importance could change over the course of a growing season. It was our objective to investigate the role of root hairs under field conditions. Methods The root hair mutant rth3 of Zea mays and the corresponding wild-type were grown for two years under field conditions on sand and loam. Results Shoot growth and P and K uptake of the plants were promoted by the presence of hairs at all growth stages. Differences between genotypes were greater on loam than on sand until tassel emergence, presumably as additional exploitation by hairs is more relevant in loam. Compensation for the absence of root hairs by increased root growth was not observed in absolute terms. The root to shoot ratio was higher for rth3 than for wild-type. Root traits showed high plasticity in response to texture, the most salient being a greater mean root diameter in sand, irrespective of genotype. The mechanism causing the increase in mean root diameter is still unknown. Root length density was higher in sand, which can be explained by a greater need for exploration than exploitation in this substrate. Conclusion The role of hairs for nutrient uptake could be confirmed under field conditions. The large impact of texture on root growth and consequences for carbon balance require further investigations.

Purpose Simultaneously interacting rhizosphere processes determine emergent plant behaviour, including growth, transpiration, nutrient uptake, soil carbon storage and transformation by microorganisms. However, these processes occur on multiple scales, challenging modelling of rhizosphere and plant behaviour. Current advances in modelling and experimental methods open the path to unravel the importance and interconnectedness of those processes across scales. Methods We present a series of case studies of state-of-the art simulations addressing this multi-scale, multi-process problem from a modelling point of view, as well as from the point of view of integrating newly available rhizosphere data and images. Results Each case study includes a model that links scales and experimental data to explain and predict spatial and temporal distribution of rhizosphere components. We exemplify the state-of-the-art modelling tools in this field: image-based modelling, pore-scale modelling, continuum scale modelling, and functional-structural plant modelling. We show how to link the pore scale to the continuum scale by homogenisation or by deriving effective physical parameters like viscosity from nano-scale chemical properties. Furthermore, we demonstrate ways of modelling the links between rhizodeposition and plant nutrient uptake or soil microbial activity. Conclusion Modelling allows to integrate new experimental data across different rhizosphere processes and scales and to explore more variables than is possible with experiments. Described models are tools to test hypotheses and consequently improve our mechanistic understanding of how rhizosphere processes impact plant-scale behaviour. Linking multiple scales and processes including the dynamics of root growth is the logical next step for future research.

Background and aims Impact of drought on crop growth depends on soil and root hydraulic properties that determine the access of plant roots to soil water. Root hairs may increase the accessible water pool but their effect depends on soil hydraulic properties and adaptions of root systems to drought. These adaptions are difficult to investigate in pot experiments that focus on juvenile plants. Methods A wild-type and its root hairless mutant maize ( Zea mays ) were grown in the field in loam and sand substrates during two growing seasons with a large precipitation deficit. A comprehensive dataset of soil and plant properties and monitored variables were collected and interpreted using simulations with a mechanistic root water uptake model. Results Total crop water use was similar in both soils and for both genotypes whereas shoot biomass was larger for the wild type than for the hairless mutant and did not differ between soils. Total final root length was larger in sand than in loam but did not differ between genotypes. Simulations showed that root systems of both genotypes and in both soils extracted all plant available soil water, which was similar for sand and loam, at a potential rate. Leaf water potentials were overestimated by the model, especially for the hairless mutant in sand substrate because the water potential drop in the rhizosphere was not considered. Conclusions A direct effect of root hairs on water uptake was not observed but root hairs might influence leaf water potential dependent growth.

This is the dataset of the enzyme kinetics, and other biochemical properties obtained from zymography, 14C images and water images of the work entitled "The spatial distribution of rhizosphere microbial activities under drought: water availability is more important than root-hair controlled exudation".

Although root hairs significantly increased root–soil contact, in maize, their shrinkage during soil drying is initiated at relatively high soil matric potentials (between −10 and −310 kPa).