Root exudates alter the rhizosphere's physical properties, but the impact that these changes have on solute transport is unknown. In this study, we tested the effects of chia mucilage and wheat root exudates (WREs) on the transport of iodide and potassium in saturated or unsaturated soil. Saturated solute breakthrough experiments, conducted in loamy sand soil or coarser textured quartz sand, revealed that increasing the exudate concentration in soil resulted in non-equilibrium solute transport. This behavior was demonstrated by an initial solute breakthrough after fewer pore volumes (PVs) and the arrival of the peak solute concentration after greater PVs in soil mixed with exudates compared to soil without exudates. These patterns were more pronounced for the coarser textured quartz sand than for the loamy sand soil, and in soil mixed with mucilage than in soil mixed WREs. Parameter fits to these breakthrough curves with a mobile-immobile transport model indicated the fraction of immobile water increased as the concentration of exudates increased. For example, in quartz sand the estimated immobile fraction increased from 0 without exudates to 0.75 at a mucilage concentration of 0.2%. The solutes' breakthrough under unsaturated conditions was also altered by the exudates, demonstrated by a smaller volume of water extracted from soil mixed with exudates, compared to soil without exudates, before the arrival of the peak solute concentration. The results indicate that exudates alter the rhizosphere's transport properties; we hypothesize that this is due to exudates creating low-conducting flow paths that result in a physical non-equilibrium solute transport.

Induced polarization (IP) is increasingly applied for hydrological, environmental and agricultural purposes. Interpretation of IP data is based on understanding the relationship between the IP signature and the porous media property of interest. Mechanistic models on the IP phenomenon rely on the Poisson-Nernst-Plank equations, where diffusion and electromigration fluxes are the driving forces of charge transport and are directly related to IP. However, to our knowledge, the impact of advection flux on IP was not investigated experimentally and was not considered in any IP model. In this work, we measured the spectral IP (SIP) signature of porous media under varying flow conditions, in addition to developing and solving a model for SIP signature of porous media, which takes flow into consideration. The experimental and the model results demonstrate that as bulk velocity increases, polarization and relaxation time decrease. Using a numerical model, we established that fluid flow near the particle deforms the electrical double layer (EDL) structure, accounting for the observed reduction in polarization. We found a qualitative agreement between the model and the measurements. Still, the model overestimates the impact of flow rate on SIP signature, which we explain in terms of the flow boundary conditions. Overall, our results demonstrate the sensitivity of the SIP signature to fluid flow, highlighting the need to consider fluid velocity in the interpretation of the SIP signature of porous media, and opening an exciting new direction for noninvasive measurements of fluid flow at the EDL scale.

Monitoring the growth, architecture, and function of plant roots is of great interest. One promising noninvasive geoelectrical monitoring approach is spectral induced polarization (SIP). However, the roots' SIP signature is underexplored, not well understood, and a mechanistic model has not been proposed. Here, we developed a mechanistic model for SIP's response of roots, which is based on the Poisson-Nernst-Planck equation. The modeling results suggest that the magnitude of root polarization is linearly related to the root's external surface area and that the polarization length scale is the root's diameter. We suggest that injecting a current to the plant's stem results in higher polarization associated with the root-cells' total surface area. In this case, the polarization length scale is the cell diameter. Overall, we quantified the link between the root's dimensions and their electrical signature, which may inspire SIP application for root phenotyping.

ABSTRACT Root axial conductance which describes the ability of water to move through the xylem, contributes to the rate of water uptake from the soil throughout the whole plant lifecycle. Under the rainfed wheat agro-system, grain-filling is typically occurring during declining water availability (i.e. terminal drought). Therefore, preserving soil water moisture during grain filling could serve as a key adaptive trait. We hypothesized that lower wheat root axial conductance can promote higher yields under terminal drought. A segregating population derived from a cross between durum wheat and its direct progenitor wild emmer wheat was used to underpin the genetic basis of seminal root architectural and functional traits. We detected 75 QTL associated with seminal roots morphological, anatomical, and physiological traits, with several hotspots harboring co-localized QTL. We further validated the axial conductance and central metaxylem QTL using wild introgression lines. Field-based characterization of genotypes with contrasting axial conductance suggested the contribution of low axial conductance as a mechanism for water conservation during grain filling and consequent increase in grain size and yield. Our findings underscore the potential of harnessing wild alleles to reshape the wheat root system architecture and associated hydraulic properties for greater adaptability under changing climate. This article is protected by copyright. All rights reserved.

The properties of clays and oxides govern many environmental processes, consequently, ongoing effort is invested in developing non-destructive, in-situ analytical tools that reflect these properties. Herein, the physicochemical properties of montmorillonite (MMT) and iron-oxide coated montmorillonite (FeOx-MMT) were characterized using common analytical techniques, and the results were compared to spectral induced polarization (SIP) measurements. FeOx-MMT particles showed a lower CEC, higher pH dependency of the surface charge, and lower suspension stability. Also, the size of the primary particles increased following iron-oxide deposition. SIP measurements over a range of salinities showed that the effective polarization length of the clays was in the order of several microns, suggesting the measurements of aggregates (not primary particles). Moreover, FeOx-MMT particles were more compact than MMT, and their size decreased with increasing salinity due to compaction of the EDL and arrangement of primary particles in the aggregate. The SIP-response to pH changes agreed with zeta potential measurements; at low pH values, MMT exhibited higher polarization due to the higher CEC. However, at a high pH, the differences diminish due to deprotonation of the Fe-OH surface groups. These findings suggest that SIP is a sensitive method that can detect changes in the surface chemistry of soil particles. (C) 2020 Elsevier Inc. All rights reserved.

Measuring root properties, the ``hidden half'' of the plant, is challenging due to their heterogeneous and dynamic nature. A promising method for noninvasive mapping of roots and their activity, spectral induced polarization (SIP), has been introduced. However, measurements of root properties together with their SIP responses are missing, limiting the interpretation of a root's SIP signature. In this study, we coupled SIP measurements of roots in hydroponic solution with measurements of root biomass, surface area, and diameter. Furthermore, we monitored the SIP response of roots poisoned by cyanide, which results in depolarization of the root's cell membrane potential. We found a linear correlation between root biomass and surface area, and the low-frequency electrical polarization. In addition, we demonstrate the relationship between root cell membrane potential and root polarization. Based on the results, we suggest that in comparison with the stem-based approach used by other researchers, the polarization in the contact-free method used in this study is related to the external surface area of the root and external architectural structures such as root diameter and root hair. Overall, a direct link between root properties and their electrical signature was established.

This paper provides an update on the fast-evolving field of the induced polarization method applied to biogeophysics. It emphasizes recent advances in the understanding of the induced polarization signals stemming from biological materials and their activity, points out new developments and applications, and identifies existing knowledge gaps. The focus of this review is on the application of induced polarization to study living organisms: soil microorganisms and plants (both roots and stems). We first discuss observed links between the induced polarization signal and microbial cell structure, activity and biofilm formation. We provide an up-to-date conceptual model of the electrical behaviour of the microbial cells and biofilms under the influence of an external electrical field. We also review the latest biogeophysical studies, including work on hydrocarbon biodegradation, contaminant sequestration, soil strengthening and peatland characterization. We then elaborate on the induced polarization signature of the plant-root zone, relying on a conceptual model for the generation of biogeophysical signals from a plant-root cell. First laboratory experiments show that single roots and root system are highly polarizable. They also present encouraging results for imaging root systems embedded in a medium, and gaining information on the mass density distribution, the structure or the physiological characteristics of root systems. In addition, we highlight the application of induced polarization to characterize wood and tree structures through tomography of the stem. Finally, we discuss up- and down-scaling between laboratory and field studies, as well as joint interpretation of induced polarization and other environmental data. We emphasize the need for intermediate-scale studies and the benefits of using induced polarization as a time-lapse monitoring method. We conclude with the promising integration of induced polarization in interdisciplinary mechanistic models to better understand and quantify subsurface biogeochemical processes. © 2019 European Association of Geoscientists & Engineers

Monitoring plant root within the subsurface is important but challenging, due to the opacity of the soil. Recently, it was demonstrated that the spectral induced polarization (SIP) method has the potential to image roots, but the mechanisms governing the SIP signal of roots remain poorly understood. Here, we present a numerical model and experimental setup that was designed to establish relationships between root properties and the SIP response and to enhance our understanding of the polarization mechanisms of roots. Our preliminary results show a positive correlation between root mass and quadrature conductivity in nutrient solution. Surprisingly, a negative relation was found in soil. Overall, the results from this study further demonstrate the potential of the SIP method to monitor roots. © 2018 SEG.

Seminal roots constitute the initial wheat root system and provide the main route for water absorption during early stages of development. Seminal root number (SRN) varies among species. However, the mechanisms through which SRN is controlled and in turn contribute to environmental adaptation are poorly understood. Here, we show that SRN increased upon wheat domestication from 3 to 5 due to the activation of 2 root primordia that are suppressed in wild wheat, a trait controlled by loci expressed in the germinating embryo. Suppression of root primordia did not limit water uptake, indicating that 3 seminal roots is adequate to maintain growth during seedling development. The persistence of roots at their primordial state promoted seedling recovery from water stress through reactivation of suppressed primordia upon rehydration. Our findings suggest that under well-watered conditions, SRN is not a limiting factor, and excessive number of roots may be costly and maladaptive. Following water stress, lack of substantial root system suppresses growth and rapid recovery of the root system is essential for seedling recovery. This study underscores SRN as key adaptive trait that was reshaped upon domestication. The maintenance of roots at their primordial state during seedling development may be regarded as seedling protective mechanism against water stress.

Abstract The flow of water between soil and plants follows the gradient in water potential and depends on the hydraulic properties of the soil and the root. In models for root water uptake (RWU), it is usually assumed that the hydraulic properties near the plant root (i.e., in the rhizosphere) and in the bulk soil are identical. Yet a growing body of evidence has shown that the hydraulic properties of the rhizosphere are affected by root exudates (specifically, mucilage) and markedly differ from those of the bulk soil. In this work, we couple a 3-D detailed description of RWU with a model that accounts for the rhizosphere-specific properties (i.e., rhizosphere hydraulic properties and a nonequilibrium relation between water content and matric head). We show that as the soil dries out (due to water uptake), the higher water holding capacity of the rhizosphere results in a delay of the stress onset. During rewetting, nonequilibrium results in a slower increase of the rhizosphere water content. Furthermore, the inverse relation between water content and relaxation time implies that the drier is the rhizosphere the longer it takes to rewet. Another outcome of nonequilibrium is the small fluctuation of the rhizosphere water content compared to the bulk soil. Overall, our numerical results are in agreement with recent experimental data and provide a tool to further examine the impact of various rhizosphere processes on RWU and water dynamics.