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The tensile strength of roots and the friction characteristics of the root–soil interface of tree species are the indicators that play a crucial role in understanding the mechanism of soil reinforcement by roots. To calculate the effectiveness of the reinforcement of soil by tree roots based on essential influencing parameters, typical trees in the coastal region of southeastern China selected for this study were subjected to tests of the tensile mechanical properties of their roots, as well as studies on the friction characteristics of the root–soil interface and the microscopic interfaces. The results indicated that in the 1–7 diameter classes, the root tensile strength of both Pinus massoniana and Cunninghamia lanceolata was negatively correlated with the root diameter in accordance with the power function. The root tensile strength of these two trees, however, was positively correlated with the lignin content but negatively correlated with cellulose and hemicellulose contents. The shear strength at the root–soil interface and the vertical load exhibited a constitutive relationship, which followed the Mohr–Coulomb criterion. As the root diameter increased, both the cohesion and the friction coefficients at the root–soil interface gradually increased, but the growth rate stood at around 15%. The cohesion value of the root–soil interface of the two trees decreased linearly with the increase in soil moisture content within the range of 25 to 45%. At the microinterface, the root surface of C. lanceolata exhibited concave grooves and convex ridges that extended along the axial direction of roots, with their height differences increasing with the enlargement of the root diameter. The rough surface of P. massoniana roots had areas composed of polygonal meshes, with an increase observed in the mesh density with increasing root diameter. [ABSTRACT FROM AUTHOR]

Studying the pullout characteristics of roots can help to gain a deeper understanding of the interaction mechanism between plants and soil, and provide theoretical support and practical guidance for engineering applications. This paper aims to investigate the pulling characteristics of the root–soil interface in expansive soil reinforced by vetiver roots. For this purpose, the study focuses on exploring the correlation between the pullout force (PF) required to extract the roots and the pullout displacement caused by the extraction. The research also examines how the geometry of the roots affects the pulling characteristics of the root–soil using a pullout test. It is found that with increasing root diameter, root length segment (RLS), root volume, and root surface area (RSA), the maximum PF increases. The highest correlation coefficient was found between RSA and maximum PF, which explained 81.4% of the variation in maximum PF. Then, the functional relationship between RLS and PF is established. Using RSA to predict the maximum PF of vetiver root is relatively reasonable. The assessment of PF must consider RSA indicators. The increase in RSA due to a high number of roots results in the improvement of the PF of vetiver root. These findings could assist in enhancing the strength of expansive soils. [ABSTRACT FROM AUTHOR]

The tensile strength of roots and the friction characteristics of the root–soil interface of tree species are the indicators that play a crucial role in understanding the mechanism of soil reinforcement by roots. To calculate the effectiveness of the reinforcement of soil by tree roots based on essential influencing parameters, typical trees in the coastal region of southeastern China selected for this study were subjected to tests of the tensile mechanical properties of their roots, as well as studies on the friction characteristics of the root–soil interface and the microscopic interfaces. The results indicated that in the 1–7 diameter classes, the root tensile strength of both Pinus massoniana and Cunninghamia lanceolata was negatively correlated with the root diameter in accordance with the power function. The root tensile strength of these two trees, however, was positively correlated with the lignin content but negatively correlated with cellulose and hemicellulose contents. The shear strength at the root–soil interface and the vertical load exhibited a constitutive relationship, which followed the Mohr–Coulomb criterion. As the root diameter increased, both the cohesion and the friction coefficients at the root–soil interface gradually increased, but the growth rate stood at around 15%. The cohesion value of the root–soil interface of the two trees decreased linearly with the increase in soil moisture content within the range of 25 to 45%. At the microinterface, the root surface of C. lanceolata exhibited concave grooves and convex ridges that extended along the axial direction of roots, with their height differences increasing with the enlargement of the root diameter. The rough surface of P. massoniana roots had areas composed of polygonal meshes, with an increase observed in the mesh density with increasing root diameter.

In dealing with issues such as soil erosion and slope instability, plant roots enhance the shear strength of the soil mass through their anchoring effect. However, in nature, cyclic loads such as flash floods and blizzards indirectly impose fatigue effects on plant root systems. To explore the impact of cyclic loads on the anchoring capacity of plant roots, this paper selects the roots of Betula platyphylla as the research object and uses a monotonic load and cyclic load as two loading modes. Under different loading amplitudes (25%, 50%, and 75%), root diameters and burial depths (50 mm, 100 mm, and 150 mm), and soil moisture contents (11.85%, 13.85%, and 15.85%), the effects of each factor on the anchoring capacity of the roots under cyclic loading are analyzed. The results showed that the root–soil interface exhibited two failure modes under different cyclic load amplitudes, and the cyclic load significantly reduced the maximum friction of the root–soil interface. As the cyclic load amplitude increased (from 25% to 75%), the hysteretic curve envelope area increased, and the growth rate of cumulative residual slip changed from decreasing to decreasing and then increasing. A good correlation was found between cumulative residual slip and the number of loading cycles, and the three characteristic slips were correlated with loading amplitude but not significantly with diameter. The increase in soil moisture content, root embedment depth, and diameter led to an increase in the ratio of the two maximum friction forces. It was shown that a certain degree of plasticity exists at the root–soil interface to resist environmental stresses in nature. At high fatigue stress levels, the root–soil interface is more nonlinear, and as the load amplitude increases, more energy is dissipated, and bond damage between the root–soil interface becomes more pronounced. The root–soil interface gradually degraded under long-term cyclic loading, whereas the increase in root depth and soil water content could resist the negative effect of cyclic loading on anchorage capacity, and the resistance effect became more and more obvious with the increase in diameter. [ABSTRACT FROM AUTHOR]

Dynamics of soil enzyme activity during plant restoration processes have been studied extensively; however, a general understanding of the rhizosphere-specific enzyme activity during plant secondary succession is limited. Rhizosphere samples were collected from the dominant and main companion species in farmlands abandoned for up to 32 years in the Loess Plateau. We determined soil chemical properties, phospholipid fatty acid (PLFA) profiles and activities of four enzymes (β-1,4-glucosidase, BG; β-1,4-N-acetylglucosaminidase, NAG; L-leucine aminopeptidase, LAP and alkaline phosphatase, AP), specific enzyme activity per soil organic carbon (SOC) and per PLFA unit. Specific BG, NAG, LAP and AP activities per SOC unit significantly decreased 77.1%, 53.2%, 83.0% and 49.2% with plant succession. Specific BG, NAG and LAP activities per PLFA unit significantly decreased 59.6%, 18.9% and 70.9% with plant succession. Furthermore, specific BG and LAP activities in the dominant species were lower than those in the companion species. Redundancy analysis revealed that variations in specific enzyme activities per SOC and per PLFA unit were explained 76.6% and 68.6% by soil chemical properties. SOC, total nitrogen (TN) and available phosphorus (aP) significantly influenced the rhizosphere-specific enzyme activities. This study improved the understanding of the element cycle at the root–soil interface under secondary succession ecosystems. [ABSTRACT FROM AUTHOR]

[Objective] The effects of different factors on the shear strength index and shear strength of the root-soil interface were analyzed, in order to provide an understanding of the mechanism of shrub root soil fixation and slope protection in the loess region of the Northeastern Tibetan Plateau. [Methods] The dominant shrub (Caragana korshinskii) for this region was selected as the study object, and the effects of the dry density, moisture content, and salt content of soil, and of root diameter on the shear characteristics of the root-soil interface and their mechanisms were analyzed and discussed using the direct shear test (i. e., one factor was varied at a time) for root diameters of (2.20±1.00—32.00±1.80) mm and moisture contents, dry densities, and salt contents of the soil of 6.00%—22.00%, 1.20—1.60 g/cm3, and 0.59%—2.50% respectively. [Results] Root diameter had no significant effect on the shear strength indexes and shear strength of the root-soil interface (p>0.05) when other influencing factors were held constant. As soil moisture content increased from 6.00% to 22.00%, the root-soil interface cohesion initially increased and then decreased, reaching a maximum value of 6.74 kPa at a soil moisture content of 14.00%. The root-soil interface friction angle decreased linearly from 21.40° to 15.75°. The shear strength of the root-soil interface decreased linearly. As the soil dry density increased from 1.20 g/cm3 to 1.60 g/cm3, the cohesion of the root-soil interface increased exponentially from 5.70 kPa to 6.85 kPa, and the friction angle increased linearly from 20.67° to 21.67°. The shear strength of the root-soil interface increased linearly. As the soil salt content increased from 0.59% to 2.50%, the root-soil interface cohesion increased linearly from 6.71 kPa to 7.31 kPa, and the shear strength increased linearly. However, there was no significant change in the root-soil interface friction angle (p>0.05). The gray correlation analysis of these results showed that dry density had the greatest influence on the cohesion, the friction angle, and the shear strength of the root-soil interface. [Conclusion] The dry density, moisture content, and salt content of soil can all affect the shear characteristics of the root-soil interface, however, the degree of influence is different. The influence of rainfall infiltration on the shear characteristics of the root-soil interface should be fully considered when evaluating the protective effect of plant roots on the shallow soil of loess slopes.

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Copyright of Acta Prataculturae Sinica is the property of Acta Prataculturae Sinica Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Understanding mechanical interactions at the root-soil interface is essential to predict the erosion of vegetated slopes. Recently, the shear strength of vegetatedil under changing hydraulic conditions has been measured and modeled; however, root-soil interfaces have not been investigated under changing hydraulic conditions. This paper proposes (1) a novel pullout apparatus to measure the shear strength at the root-soil interface under changing suction, (2) a Mohr-Coulomb-Vilar (MCV) shear strength model of root-soil interfaces, and (3) a numerical simulation using Node-To-Segment (NTS) approach along with Finite Element Method (FEM). The pullout tests were verified using the numerical simulation, and the results showed that the combination of the MCV model and NTS/FEM approach can accurately predict the shear behavior of root-soil interfaces under changing suction. In addition, we experimentally evaluated the pullout problem of roots and showed that the present method provides reasonably predicts root-pullout problems even when the suction is changed during the pullout process. The current method, therefore, can be used for predicting root-soil interface dynamics under varying suction and soil pressure by only adding two additional parameters of the Vilar model.

Iron toxicity is a major constraint to rice production, particularly in highly weathered soils of inland valleys in sub‐Saharan Africa where the rice growing area is rapidly expanding. There is a wide variation in tolerance of iron toxicity in the rice germplasm. However, the introgression of tolerance traits into high‐yielding germplasm has been slow owing to the complexity of the tolerance mechanisms and large genotype‐by‐environment effects. We review current understanding of tolerance mechanisms, particularly those involving below‐ground plant–soil interactions. Until now these have been less studied than above‐ground mechanisms. We cover processes in the rhizosphere linked to exclusion of toxic ferrous iron by oxidation, and resulting effects on the mobility of nutrient ions. We also cover the molecular physiology of below‐ground processes controlling iron retention in roots and root‐shoot transport, and also plant iron sensing. We conclude that future breeding programmes should be based on well‐characterized molecular markers for iron toxicity tolerance traits. To successfully identify such markers, the complex tolerance response should be broken down into its components based on understanding of tolerance mechanisms, and tailored screening methods should be developed for individual mechanisms. Progress in identifying markers for Fe toxicity tolerance in rice has been slow because of the complexity of tolerance mechanisms and the importance of below‐ground plant‐soil interactions. We review current understanding of these below‐ground processes, including recent advances in the molecular physiology of tolerance. [ABSTRACT FROM AUTHOR]

In this study, an innovative method was used to assess the spatial and temporal patterns of the hydrological connectivity in soil profiles in the Yellow River Delta wetland. In this method, field dye-tracing experiments conducted in the study area (i.e., large [LWa] and small communities [LWb] of Phragmites australis and Suaeda glauca [JP]) were considered, and the root-soil interface in the region and the status of the wetland degradation were analyzed. The results revealed that the index of hydrological connectivity (IHC) significantly changed both spatially and temporally. The high IHC values reaching a 0.392 ± 0.209 gradient were concentrated in the middle and western parts of the study areas. The spatial distribution of the high hydrological connectivity has been extremely fragmented since 2010. The LWb (severely degraded wetland) and JP (extremely degraded wetland) were found to be more seriously degraded than the LWa (moderately degraded wetland) in the study area. The changes in the IHC were positively correlated with the principal component (PC) values of the wetland degradation. The IHC is a novel indicator of the status of wetland degradation. Furthermore, the soil holding capacity, soil non-capacity porosity, and soil ventilation were relatively important for the changes in the IHC. Compared with the soil properties, the hydrological responses of the roots systems can be neglected at the root-soil interface. Based on our results, we propose an alternative wetland restoration solution for the Yellow River Delta wetland: 1) a seepage layer in the surface soils (0–10 cm), shallow ploughing treatment, and litter or straw return to the soil surface should be conducted to increase the hydrological connectivity of the soil surface; 2) reeds should not be reaped every year to remove the nutrients from this area; and 3) appropriate freshwater inputs should be strengthened.

Endophytic fungus Serendipita indica can bolster plant growth and confer protection against various biotic and abiotic stresses. However, S. indica reshaped rhizosphere microecology interactions and root-soil interface processes in situ at the submicrometer scale remain poorly understood. We combined amplicon sequencing and high-resolution nano X-ray fluorescence (nano-XRF) imaging of the root-soil interface to reveal cadmium (Cd) rhizosphere processes. - reshaped rhizosphere microecology interactions and root-soil interface processes in situ at the submicrometer scale remain poorly understood. We combined amplicon sequencing and high-resolution nano X-ray fluorescence (nano-XRF) imaging of the root-soil interface to reveal cadmium (Cd) rhizosphere processes. S. indica can successfully colonize the roots of Sedum alfredii Hance, which induces a remarkable increase in shoot biomass by 211.32% and Cd accumulation by 235.72%. Nano-XRF images showed that S. indica colonization altered the Cd distribution in the rhizosphere and facilitated the proximity of more Cd and sulfur (S) to enter the roots and transport to the shoot. Furthermore, the rhizosphere-enriched microbiota demonstrated a more stable network structure after the S. indica inoculation. Keystone species were strongly associated with growth promotion and Cd absorption. For example, Comamonadaceae are closely related to the organic acid cycle and S bioavailability, which could facilitate Cd and S accumulation in plants. Meanwhile, Sphingomonadaceae could release auxin and boost plant biomass. In summary, we construct a mutualism system for beneficial fungi and hyperaccumulation plants, which facilitates high-efficient remediation of Cd-contaminated soils by restructuring the rhizosphere microbiota.

Understanding mechanical interactions at the root-soil interface is essential to predict the erosion of vegetated slopes. Recently, the shear strength of vegetatedil under changing hydraulic conditions has been measured and modeled; however, root-soil interfaces have not been investigated under changing hydraulic conditions. This paper proposes (1) a novel pullout apparatus to measure the shear strength at the root-soil interface under changing suction, (2) a Mohr-Coulomb-Vilar (MCV) shear strength model of root-soil interfaces, and (3) a numerical simulation using Node-To-Segment (NTS) approach along with Finite Element Method (FEM). The pullout tests were verified using the numerical simulation, and the results showed that the combination of the MCV model and NTS/FEM approach can accurately predict the shear behavior of root-soil interfaces under changing suction. In addition, we experimentally evaluated the pullout problem of roots and showed that the present method provides reasonably predicts root-pullout problems even when the suction is changed during the pullout process. The current method, therefore, can be used for predicting root-soil interface dynamics under varying suction and soil pressure by only adding two additional parameters of the Vilar model. [ABSTRACT FROM AUTHOR]

Vegetation can enhance the stability of slopes by increasing the shear resistance of the soil. Shear stress applied to the soil matrix is resisted by the pullout strength of the roots via the friction at contact points between the soil and the roots. The effectiveness of root reinforcement depends on interface friction between soil and roots. In this study, tests were carried out on Indigofera amblyantha Craib roots, by measuring resistance as they are pulled out of the soil where the soil has varying dry densities. The results reveal three phases in the relationship between the pullout force and the slippage of the roots, i.e. (1) steep rise, (2) steep fall, and (3) gradual decline. In the first phase, the pullout force is increasing sharply and linearly up to a maximum when the slippage is about 10mm. With continued slippage, the required pullout force decreases significantly and nonlinearly in up and down fluctuations. Eventually, the pullout force reaches zero. For soil with a given dry density, the maximum pullout force increases linearly with increasing root diameter, and the correlation coefficient is greater than 0.9. Further, for a root with a given diameter, the maximum pullout force increases with increasing soil dry density. When the root breaks on pulling, it is called tensile failure; when the root is fully pulled out, it is called friction failure. The mode of failure for all roots is friction failure, for soil with dry densities of 1.35 g/cm³, 1.45 g/ cm³, and 1.55 g/cm³ . For soil with a dry density of 1.65 g/cm³, and root diameter under 0.716 mm, the observed failure mode is generally tensile; for diameters over 0.716 mm, the failure mode changes to friction; that is, thin roots break, thick roots get pulled out. [ABSTRACT FROM AUTHOR]

To a first-order approximation, the yield of a water-limited grain crop depends (i) on how much water is available to the crop and (ii) on how crop water use is partitioned during the growing season. The latter determines the harvest index of the crop, that is, the proportion of the crop's above ground biomass that is converted into grain, which is typically optimal if about 30% of the seasonal available water supply is used during flowering and grain filling. Here, we review the role of roots in extracting water from the soil in both the amount and the timing that may lead to maximal grain yield, and the various mechanisms underlying this activity. These include architectural and anatomical traits; the biophysics of water movement from soil through roots to the leaves including especially the properties of and processes within the interface between roots and soil and the role of mucilage therein; and the physiological role of the roots in influencing the growth and transpiration of the crop canopy, which can optimize the seasonal pattern of water use. These various properties and mechanisms are discussed in the context of improving grain yield in strongly water-limited, especially semiarid, environments. [ABSTRACT FROM AUTHOR]

• Background and Aims Although it is commonly accepted that root exudation enhances plant--microbial interactions in the rhizosphere, experimental data on the spatial distribution of exudates are scarce. Our hypothesis was that root hairs exude organic substances to enlarge the rhizosphere farther from the root surface. • Methods Barley (Hordeum vulgare 'Pallas' -- wild type) and its root-hairless mutant (brb) were grown in rhizoboxes and labelled with 14CO2. A filter paper was placed on the soil surface to capture, image and quantify root exudates. • Key Results Plants with root hairs allocated more carbon (C) to roots (wild type: 13 %; brb: 8 % of assimilated 14C) and to rhizosheaths (wild type: 1.2 %; brb: 0.2 %), while hairless plants allocated more C to shoots (wild type: 65 %; brb: 75 %). Root hairs increased the radial rhizosphere extension three-fold, from 0.5 to 1.5 mm. Total exudation on filter paper was three times greater for wild type plants compared to the hairless mutant. • Conclusion Root hairs increase exudation and spatial rhizosphere extension, which probably enhance rhizosphere interactions and nutrient cycling in larger soil volumes. Root hairs may therefore be beneficial to plants under nutrient-limiting conditions. The greater C allocation below ground in the presence of root hairs may additionally foster C sequestration. [ABSTRACT FROM AUTHOR]

• Surficial cracking behavior and tensile strength of rooted lateritic soil. • Grass roots do not always enhance tensile strength and mitigate desiccation cracks. • Microscopic mechanism behind soil reinforcement by grass roots. • Formulas of the interfacial shear strength and tensile strength of rooted soil. Desiccation cracking is a major cause of shallow failure of lateritic soil slopes. Knowledge of soil tensile strength helps better understand the cracking behavior of vegetated lateritic soil. This study aims to examine the surficial cracking behavior and tensile strength of remolded lateritic soil containing horizontally arranged vetiver roots. Desiccation cracking tests, root pullout tests and direct tensile tests were performed on lateritic soil considering various porosities, degrees of saturation, and root contents. The mechanism underlying the root reinforcement was analyzed by scanning electron microscopy. The results demonstrate that adding 0.4% grass roots can reduce the crack width and crack intensity factor, and thus mitigate crack developments in lateritic soil. The interfacial shear strength exhibits a decreasing trend with increasing root diameter. As the root content increases from 0% to 1.0%, the tensile strength of lateritic soil increases to reach the peak at a root content of 0.4%-0.5% and then drops greatly. This is why the optimal root content in resisting crack development is about 0.4%. Microscopic tests show that the interfacial shear strength originates from the friction, interlocking force, cementation and capillary force between the rough root surface and surrounding soil particles. A power function-based empirical equation expressed by the root diameter and porosity is proposed to estimate the interfacial shear strength of rooted lateritic soil. Additionally, a semi-theoretical equation of the tensile strength of rooted lateritic soil is deduced based on the tensile strength of soil matrix and the interfacial shear strength between the roots and soil matrix. This equation considers the root content and root diameter distribution and is applicable to the soil of different state parameters (e.g., porosity, degree of saturation and water content). The results could provide insights into the mitigation of desiccation cracking in lateritic soil slopes with root-reinforcement technology. [ABSTRACT FROM AUTHOR]

We compared modifications of soil zymography, a new in situ technique to visualize enzyme activities, based on contact of fluorgenic substrate-saturated membranes with soil either through the gel layer ( gel zymography) or without gel application ( direct zymography). We coupled zymography with quantitative measurements of enzyme kinetics to characterize catalytic mechanisms of β-glucosidase activity at the plant-soil interface including root surface (rhizoplane), rhizosphere, and bulk soil. Direct zymography refined and focused image resolution. The area of hotspots (i.e., spots with most intensive enzyme activity) as well as color intensity ratios estimated using direct zymography exceeded by a factor of 2 the corresponding values obtained with gel zymography. As determined by direct zymography, the percentage of hotspots associated to root surfaces was 58-68 % of total hotspot area. Hotspot area comprised only 6.8 ± 0.1 % of the total area of an image and 9.0 ± 3 % of the root surface area. The intensity of β-glucosidase activity, however, was up to 20 times higher in the hotspots versus bulk soil. The contribution of rhizosphere to β-glucosidase activity of the whole image (77-82 %) was four times higher than the contribution of the root surface. Enzyme kinetic parameters indicated different enzyme systems in bulk and rhizosphere soil. Higher substrate affinity and catalytic efficiency in bulk than in rhizosphere soil suggested relative domination of microorganisms with more efficient enzyme systems in the former. Coupling direct zymography and kinetic assays enabled mapping the two-dimensional (2D) distribution of enzyme activity at the root-soil interface and estimating the catalytic properties of root-associated and soil-associated enzymes. [ABSTRACT FROM AUTHOR]

Aims: The putative role of the rhizosphere in controlling root water uptake is receiving increasing attention. Recent experiments showed that the rhizosphere turned temporarily hydrophobic after drying and subsequent rewetting. Our objective was to investigate whether the rhizosphere hydrophobicity influences the hydraulic conductivity of the rhizosphere-root continuum. Methods: Lupins were grown in aluminium containers filled with a sandy soil. When the plants were 30 days-old, the soil was let dry to a water content of 2-4 % and then it was irrigated. The soil water content during irrigation was imaged using a time-series neutron radiography. By image processing, we quantified the increase in the volume of water in the roots upon irrigation. Using this information and additional measurements of the root pressure after irrigation, we calculated the water flow into the roots and the total hydraulic conductance of the rhizosphere-root continuum, K. Results: The radiographs showed that: 1) the rhizosphere stayed temporarily dry after irrigation; 2) as the rhizosphere slowly rewetted, the roots rehydrated of 63 %. During 2 to 3 h subsequent to irrigation, K increased from 1.36 ± 1.09 × 10 m s MPa to 5.02 ± 2.13 × 10 m s MPa, approaching the conductance of lupin roots in wet soils measured in previous experiments. Based on our calculations, these values of K correspond to a rhizosphere conductivity of 3.87 ± 1.91 × 10 m s (shortly after irrigation) and 1.09 ± 1.64 × 10 m s (2-3 h after irrigation). Conclusions: We conclude that during a drying/wetting cycle, the conductivity of the root-soil interface is a temporary limit to root water uptake. We postulate that the temporary reduced hydraulic conductivity is primarily caused by the rhizosphere water repellency. [ABSTRACT FROM AUTHOR]

To investigate whether root colonisation of Scots pine (Pinus sylvestris L.) seedlings with symbiotic mycorrhizal fungi (Hebeloma sp. and Suillus luteus) could be detected in situ, classification analysis of the electric impedance spectra (IS) of the root system was carried out. The seedlings were inoculated either with Hebeloma or Suillus with some left as controls. The seedlings were firstly cultivated in long-day and high temperature (LDHT) conditions. Half of the seedlings remained in LDHT and half were moved to short-day and low temperature conditions (SDLT) to acclimatise to the cold. The electrical impedance spectra of the root systems were measured at a frequency range of 5 Hz–100 kHz. The results of principal component analysis (PCA) showed that current delivery through root system, sensed by real and imaginary parts of IS, depended upon the cold acclimation and mycorrhizal treatment. Comparison of SDLT to LDHT via correlation analysis indicated a 13% and a 27% change in PCA responses for the real and imaginary parts of the impedance, respectively. When the mycorrhizal treatments were compared with a non-mycorrhizal treatment, the respective changes in the correlation coefficients were 30% for Hebeloma sp. and 39% for S. luteus in the real part and 28% and 38% in the imaginary part, respectively. These changes in the correlation coefficients appear to indicate physicochemical changes (e.g. ionic behaviour) in the roots as a result of fungal colonisation. [Copyright &y& Elsevier]

Abstract: Applicability of root electrical impedance (EI) and electrical capacitance (EC) measurements for the detection of arbuscular mycorrhizal fungal (AMF) colonization was studied. AMF-inoculated and non-inoculated maize plants were compared whilst growing them under well-watered and drought-stressed condition with regular measurement of root EI and EC. AMF-colonized maize plants showed lower EI and higher EC than control counterparts independently of moisture condition, indicating an enhanced root–soil interface. Root scanning revealed lower root surface area in mycorrhizal plants than in non-mycorrhizal ones growing under well-watered condition, while in plants exposed to drought no significant difference between root surface areas was detected. Since fungal colonization didn''t provoke an increase in root surface area, the higher root–soil interface showed by EI and EC values was apparently due to the increased absorption surface area caused by the growth of AMF hyphae. Consequently, the simple, non-destructive EI and EC method is considered an appropriate technique for in situ investigation of AMF-colonization and function, which may partially substitute the intrusive techniques commonly used in mycorrhizal research. [Copyright &y& Elsevier]

Abstract: A novel type of planar optodes for simultaneous optical analysis of pH and oxygen dynamics in the rhizosphere is introduced. The combination of the optical, non-invasive measurement of these parameters with sterile sampling of rhizosphere solution across and along growing roots by use of a novel type of rhizobox provides a methodical step forward in the investigation of the physicochemical dynamics of the rhizosphere and its underlying matter fluxes between roots and soil. In this study, this rhizobox was used to investigate the effect of oxygen releasing roots of three Juncus species on the amount and distribution of organic acids in reductive, oxygen-deficient soils of different pH (pH 3.9–pH 5.9). Pronounced diurnal variations of oxygen concentration and pH along the roots, particularly along the elongation zone were observed. Long-term records over more than eight weeks revealed considerable spatial and temporal patterns of oxygen over a range of almost 200 μmol O2 L−1 and pH dynamics of ±1.4 pH units in the rhizosphere. A strong effect of oxidative acidification due to oxygen release by the plant roots was clearly visible for Juncus effusus, whereas the roots of Juncus articulatus alkalinized the rhizosphere. In contrast, roots of Juncus inflexus induced no effects on rhizospheric pH. Only four different organic acids (oxalate, acetate, formate and lactate) were detectable in all soil solutions. Maximal concentration of all organic acids occurred at pH 3.9, whereas the lowest concentration of each organic acid was found at pH 5.9. Hence, considering the pH-dependence of the redox potential, the acid soil provided increased reductive conditions leading to slower anaerobic degradation of organic acids to CO2 or methane (CH4). The concentration of organic acids decreased by up to 58% within a distance of only 4 mm from the bulk soil to the root surface, i.e. reciprocal to the pronounced O2-gradient. The decreasing presence of organic acids toward the oxygen releasing roots is possibly due to a change in the composition of the microbial community from anaerobic to aerobic conditions. The present study highlights the dynamic interplay between O2 concentration, pH and organic acids as key parameters of the physicochemical environment of the rhizosphere, particularly for wetland plants growing in oxygen-deficient waterlogged soils. [Copyright &y& Elsevier]

The ability of the free ion activity model (FIAM), the terrestrial biotic ligand model (TBLM), the diffusive gradients in thin films (DGT) technique and a plant-based biotest, the RHIZOtest, to predict root copper (Cu) concentration in field-grown durum wheat (Triticum turgidum durum L.) was assessed on 44 soils varying in pH (3.9–7.8) and total Cu (32–184 mg kg−1). None of the methods adequately predicted root Cu concentration, which was mainly correlated with total soil Cu. Results from DGT measurements and even more so FIAM prediction were negatively correlated with soil pH and over-estimated root Cu concentration in acidic soils. TBLM implementation improved numerically FIAM prediction but still failed to predict adequately root Cu concentration as the TBLM formalism did not considered the rhizosphere alkalisation as observed in situ. In contrast, RHIZOtest measurements accounted for rhizosphere alkalisation and were mainly correlated with total soil Cu. [Copyright &y& Elsevier]

• The index of hydrological connectivity (IHC) changed both spatially and temporally. • The spatial distribution of high IHC has been extremely fragmented since 2010. • IHC could be treated as a novel indicator of the status of wetland degradation. • Soil property had larger influences on IHC than roots systems at root-soil interface. In this study, an innovative method was used to assess the spatial and temporal patterns of the hydrological connectivity in soil profiles in the Yellow River Delta wetland. In this method, field dye-tracing experiments conducted in the study area (i.e., large [LWa] and small communities [LWb] of Phragmites australis and Suaeda glauca [JP]) were considered, and the root-soil interface in the region and the status of the wetland degradation were analyzed. The results revealed that the index of hydrological connectivity (IHC) significantly changed both spatially and temporally. The high IHC values reaching a 0.392 ± 0.209 gradient were concentrated in the middle and western parts of the study areas. The spatial distribution of the high hydrological connectivity has been extremely fragmented since 2010. The LWb (severely degraded wetland) and JP (extremely degraded wetland) were found to be more seriously degraded than the LWa (moderately degraded wetland) in the study area. The changes in the IHC were positively correlated with the principal component (PC) values of the wetland degradation. The IHC is a novel indicator of the status of wetland degradation. Furthermore, the soil holding capacity, soil non-capacity porosity, and soil ventilation were relatively important for the changes in the IHC. Compared with the soil properties, the hydrological responses of the roots systems can be neglected at the root-soil interface. Based on our results, we propose an alternative wetland restoration solution for the Yellow River Delta wetland: 1) a seepage layer in the surface soils (0–10 cm), shallow ploughing treatment, and litter or straw return to the soil surface should be conducted to increase the hydrological connectivity of the soil surface; 2) reeds should not be reaped every year to remove the nutrients from this area; and 3) appropriate freshwater inputs should be strengthened. [ABSTRACT FROM AUTHOR]

Contents Summary 11 I. Introduction 11 II. Phosphate uptake and its regulation 14 III. Phosphate uptake assisted by the arbuscular–mycorrhizal 18 symbiosis IV. Agricultural potential 21 V. Conclusions and perspectives 21 Acknowledgements 22 References 22 Phosphorus (P) is an essential plant nutrient and one of the most limiting in natural habitats as well as in agricultural production world-wide. The control of P acquisition efficiency and its subsequent uptake and translocation in vascular plants is complex. The physiological role of key cellular structures in plant P uptake and underlying molecular mechanisms are discussed in this review, with emphasis on phosphate transport across the cellular membrane at the root and arbuscular–mycorrhizal (AM) interfaces. The tools of molecular genetics have facilitated novel approaches and provided one of the major driving forces in the investigation of the basic transport mechanisms underlying plant P nutrition. Genetic engineering holds the potential to modify the system in a targeted way at the root–soil or AM symbiotic interface. Such approaches should assist in the breeding of crop plants that exhibit improved P acquisition efficiency and thus require lower inputs of P fertilizer for optimal growth. Whether engineering of P transport systems can contribute to enhanced P uptake will be discussed. [ABSTRACT FROM AUTHOR]

Summary For biochemical modification of the root–soil interface, the engineered secretion of stable enzymes from trichoblasts (= root hair bearing rhizodermal cells) is proposed. As a reporter activity, we chose to express a synthetic gene encoding a secretory phytase (PHY) directed by a trichoblast-specific promoter in root hair cells of the crop plant potato. Transgenic plants produced and secreted phytase in sufficient amounts to release phosphate from phytate in liquid medium. When grown in an unsterile substrate containing phytate, transgenic plants accumulated 40% more P in leaves than wild-type plants. The improved P nutrition driven by trichoblast-targeted expression and subsequent secretion of PHY illustrates the potential of using trichoblast-targeted expression of suitable enzymes for future applications in plant nutrition, phytoremediation and molecular farming. [ABSTRACT FROM AUTHOR]

Root biomass is quantitatively and qualitatively important in most ecosystems, but its contribution to the pool of organic matter in the soil is not clear. This work was designed to specify root ageing on an ultrastructural scale by transmission electron microscopy combined with microanalysis by electron energy loss spectroscopy. This approach is very suitable for studying the soil/plant interface, and for semi-quantitative analysis of the evolution of polyphenolic substances during root evolution. Three root segments were studied according to a gradient of root senescence: the apical and basal segments of the mycorrhiza and the mycorrhiza-carrier root. Each segment contained a certain proportion of senescent cells, some of which were of fungal origin, and this proportion increased as the root aged. In the three segments, the soil/plant interfaces were differentiated, and the micro-organisms observed in situ were described. Senescent root cells contained many polyphenolic substances and our results showed that these substances were, according to the root segment, differently associated with Ca, N and Si. When all these ultrastructural data are correlated with more global data, they can be usefully applied to root cell physiology, microbiology and pedology. This approach makes it possible to specify the evolution of organic matter in situ in soils whatever its origin. [Copyright &y& Elsevier]

To investigate the extent and size of root-soil air gaps that develop during soil drying, resign casts of roots of the desert succulent Agave deserti Engelm, were made in situ for container-grown plants and in the field. Plants that were droughted in containers for 7 and 14 had 24 and 34% root shrinkage, respectively, leading to root-soil air gaps that would reduce the hydraulic conductivity at the root-soil interface by a factor of about 5. When containers were vibrated during drought, root-soil air gaps were greatly diminished, and the predicted conductivity at the interface was similar to that of the control (moist soil). For plants in the field (4 and 6 wk after the last rainfall), root shrinkage was grater than for container-grown plants, but root-soil contact on the root periphery was greater, which led to a higher predicted hydraulic conductivity at the root-soil contact on the root periphery was greater, which led to a higher predicted hydraulic conductivity at the root-soil interface. To test the hypothesis that root-soil air gaps would help to limit water efflux from roots in drying soil, the water potentials of the soil, root, and shoot of plants from vibrated containers (with gaps eliminated or reduced) and non-vibrated containers were compared. The soil water potential was lower for vibrated containers after 14 d of drought, suggesting more rapid depletion of soil water due to better root-soil contact, and the root water potential was lower as well, suggesting greater water loss by roots in the absence of root-soil air gaps. Thus air gap could benefit A. deserti by helping to maintain a higher root water potential in the early stages of drought and later by limiting root water loss at the root-soil interface when the water potential exceeds that of the soil. [ABSTRACT FROM AUTHOR]

A common assumption in models of water flow from soil to root is that the soil can be described in terms of its representative or effective behavior. Microscale heterogeneity and structure are thereby replaced by effective descriptions, and their role in flow processes at the root‐soil interface is neglected. Here the aim was to explore whether a detailed characterization of the microscale heterogeneity at the scale of a single root impacts the relation between flow rate and pressure gradient. Numerical simulations of water flow toward a root surface were carried out in a two‐dimensional domain with a randomized configuration of spatially variable unsaturated hydraulic conductivities and varying boundary conditions, that is, increasing and decreasing root water uptake rates. By employing Matheron's method, the soil hydraulic properties were varied, while the effective hydraulic conductivity (corresponding to the geometric mean) remained unchanged. Results show that domains with a uniform conductivity could not capture important features of water flow and pressure distribution in spatially variable domains. Specifically, increasing heterogeneity at the root‐soil interface allowed to sustain higher root water uptake rates but caused a slower recovery in xylem suction after transpiration ceased. The significance of this is that, under critical conditions, when pressure gradients and flow rates are high, microscale heterogeneity may become an important determinant and should not be neglected in adequate descriptions of water flow from soil to root in dry soil. Key Points: At the scale of a single root soil heterogeneity impacts the relation between flow rate and pressure headHere the emergent relation between water flow and pressure head gradient cannot be described by a single effective conductivityExplicitly accounting for soil heterogeneity might give new insights on water flow in soil‐plant‐atmosphere systems [ABSTRACT FROM AUTHOR]