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Aims: Gradients in water potential of soil and plant system drives the water movement in soil-plant-atmospheric continuum. Here, we demonstrate how root water potential measured directly from the roots upon changes in soil water potential would contribute to the understanding of the drought response in Chrysopogon zizanoides. Methods: Plants of Chrysopogon zizanoides L. were sampled at different soil water status (inducing drought) and growth periods (3-, 4- and 5- months). The roots and leaves of the plants were dissected to measure the root water potential and specific leaf area, respectively. The root water potential was measured in a WP4C dew-point potentiometer. Root diameter corresponding to the roots measured for root water potential was also measured. Results: Our findings showed a logarithmic increase in gradient between soil and root water potential under the induced drought stress, similar to the existing findings of root hydraulic conductance. Specific leaf area significantly decreased with root water potential, indicating the hydraulic continuity between roots and leaves. A new power law correlation between root diameter and root water potential established a trait-based understanding of root water uptake. Conclusion: The aggregation of such root water potential measurements using potentiometer would offer strategies to explore the implications of below-ground plant behaviour in applications such as slope stability and irrigation. [ABSTRACT FROM AUTHOR]

Background and aims: The ever-increasing demand of fresh water from irrigated agriculture and the water scarcity condition that characterizes the Mediterranean region, encourages the adoption of efficient water saving strategies. In this study, the electrical resistivity imaging (ERI) technique was applied for characterizing the mass exchange mechanisms affecting the continuous soil-plant system of heterogeneous micro-irrigated orchards. Methods: Time-lapse ERI surveys were carried out in a multi-variety and mixed-age orange orchard located in Eastern Sicily (insular Italy). The time-lapse ERI monitoring was coupled with the use of independent soil water content (SWC) measurements. Results and conclusions: A clear relationship between the soil electrical resistivity (ER) and the measured SWC changes was identified in the soil under the mixed-age orchard (with an overall coefficient of determination value of 0.63). Specifically, this study highlights the dependency of SWC dynamics as function of tree age. Overall, an increase in ER was obtained in correspondence to the soil domain where the younger trees were located (< = 3 years). This behaviour was inferred to result from the higher soil evaporation process active at these locations, due to less vegetation cover of the younger trees and, consequently to their lesser root extension in comparison to the mature trees (> = 8 years). On the other hand, in mature trees, characterized by denser root biomass, there were more evident decreasing patterns of ER (i.e., greater changes in SWC), due to greater transpiration rates that caused lower initial SWC under these conditions in comparison to the younger trees. [ABSTRACT FROM AUTHOR]

Background and aims: Root water uptake (RWU) depends on root development, influenced by water and salt stress (WAS). Modeling RWU requires calibration of soil and root parameters using lysimeter data, which is challenging. The study introduces a global sensitivity index-based estimability method for parameter assessment and selection, optimizing them using observations from lysimeter experiments.A variance-based global sensitivity and estimability analyses was performed on the HYDRUS-1D model. The analyses evaluate the impact of soil hydraulic and stress parameters on pressure head, soil moisture, bottom flux, and electrical conductivity. Interactions among parameters across the root zone were analyzed for model parameter selection. The selected parameters were calibrated from soil-column lysimeter experiments on berseem (Trifolium alexandrinum) under saline and non-saline conditions.The analyses identify water stress and residual soil moisture as less estimable, while major soil and salt stress parameters as more estimable, especially from bottom flux and soil moisture data. Excluding saturated soil moisture, which strongly influenced parameter estimability, improved optimization results. For the maximum scenarios, the simulated salt-water dynamics showed fair agreement with the observed data (r2 > 0.7). For moderate salinity, reduced RWU was compensated by an increase of 0.01 d−1, while high salinity significantly reduced this compensation to 0.002 d−1 with uniform RWU of 0.004 d−1.The study demonstrates how different data sets contribute to accurate parameter estimation. Under high salinity, the compensation mechanism for reduced RWU was diminished, and the prolonged uniform RWU pattern suggested potential permanent root morphological changes.Methods: Root water uptake (RWU) depends on root development, influenced by water and salt stress (WAS). Modeling RWU requires calibration of soil and root parameters using lysimeter data, which is challenging. The study introduces a global sensitivity index-based estimability method for parameter assessment and selection, optimizing them using observations from lysimeter experiments.A variance-based global sensitivity and estimability analyses was performed on the HYDRUS-1D model. The analyses evaluate the impact of soil hydraulic and stress parameters on pressure head, soil moisture, bottom flux, and electrical conductivity. Interactions among parameters across the root zone were analyzed for model parameter selection. The selected parameters were calibrated from soil-column lysimeter experiments on berseem (Trifolium alexandrinum) under saline and non-saline conditions.The analyses identify water stress and residual soil moisture as less estimable, while major soil and salt stress parameters as more estimable, especially from bottom flux and soil moisture data. Excluding saturated soil moisture, which strongly influenced parameter estimability, improved optimization results. For the maximum scenarios, the simulated salt-water dynamics showed fair agreement with the observed data (r2 > 0.7). For moderate salinity, reduced RWU was compensated by an increase of 0.01 d−1, while high salinity significantly reduced this compensation to 0.002 d−1 with uniform RWU of 0.004 d−1.The study demonstrates how different data sets contribute to accurate parameter estimation. Under high salinity, the compensation mechanism for reduced RWU was diminished, and the prolonged uniform RWU pattern suggested potential permanent root morphological changes.Results: Root water uptake (RWU) depends on root development, influenced by water and salt stress (WAS). Modeling RWU requires calibration of soil and root parameters using lysimeter data, which is challenging. The study introduces a global sensitivity index-based estimability method for parameter assessment and selection, optimizing them using observations from lysimeter experiments.A variance-based global sensitivity and estimability analyses was performed on the HYDRUS-1D model. The analyses evaluate the impact of soil hydraulic and stress parameters on pressure head, soil moisture, bottom flux, and electrical conductivity. Interactions among parameters across the root zone were analyzed for model parameter selection. The selected parameters were calibrated from soil-column lysimeter experiments on berseem (Trifolium alexandrinum) under saline and non-saline conditions.The analyses identify water stress and residual soil moisture as less estimable, while major soil and salt stress parameters as more estimable, especially from bottom flux and soil moisture data. Excluding saturated soil moisture, which strongly influenced parameter estimability, improved optimization results. For the maximum scenarios, the simulated salt-water dynamics showed fair agreement with the observed data (r2 > 0.7). For moderate salinity, reduced RWU was compensated by an increase of 0.01 d−1, while high salinity significantly reduced this compensation to 0.002 d−1 with uniform RWU of 0.004 d−1.The study demonstrates how different data sets contribute to accurate parameter estimation. Under high salinity, the compensation mechanism for reduced RWU was diminished, and the prolonged uniform RWU pattern suggested potential permanent root morphological changes.Conclusion: Root water uptake (RWU) depends on root development, influenced by water and salt stress (WAS). Modeling RWU requires calibration of soil and root parameters using lysimeter data, which is challenging. The study introduces a global sensitivity index-based estimability method for parameter assessment and selection, optimizing them using observations from lysimeter experiments.A variance-based global sensitivity and estimability analyses was performed on the HYDRUS-1D model. The analyses evaluate the impact of soil hydraulic and stress parameters on pressure head, soil moisture, bottom flux, and electrical conductivity. Interactions among parameters across the root zone were analyzed for model parameter selection. The selected parameters were calibrated from soil-column lysimeter experiments on berseem (Trifolium alexandrinum) under saline and non-saline conditions.The analyses identify water stress and residual soil moisture as less estimable, while major soil and salt stress parameters as more estimable, especially from bottom flux and soil moisture data. Excluding saturated soil moisture, which strongly influenced parameter estimability, improved optimization results. For the maximum scenarios, the simulated salt-water dynamics showed fair agreement with the observed data (r2 > 0.7). For moderate salinity, reduced RWU was compensated by an increase of 0.01 d−1, while high salinity significantly reduced this compensation to 0.002 d−1 with uniform RWU of 0.004 d−1.The study demonstrates how different data sets contribute to accurate parameter estimation. Under high salinity, the compensation mechanism for reduced RWU was diminished, and the prolonged uniform RWU pattern suggested potential permanent root morphological changes. [ABSTRACT FROM AUTHOR]

Aims: We aimed at assessing the influence of above- and below-ground environmental conditions over the performance of Centaurea jacea L., a drought-resistant grassland forb species. Methods: Transpiration rate, CO2 assimilation rate, leaf water potential, instantaneous and intrinsic water use efficiency, temperature, relative humidity, vapor pressure deficit and soil water content in one plant and root length density in four plants, all grown in custom-made columns, were monitored daily for 87 days in the lab. The soil water isotopic composition in eleven depths was recorded daily in a non-destructive manner. The isotopic composition of plant transpiration was inferred from gas chamber measurements. Vertical isotopic gradients in the soil column were created by adding labeled water. Daily root water uptake (RWU) profiles were computed using the multi-source mixing model Stable Isotope Analysis in R (Parnell et al. PLoS ONE 5(3):1–5, 2010). Results: RWU occurred mainly in soil layer 0–15 cm, ranging from 79 to 44%, even when water was more easily available in deeper layers. In wet soil, the transpiration rate was driven mainly by vapor pressure deficit and light intensity. Once soil water content was less than 0.12 cm3 cm− 3, the computed canopy conductance declined, which restricted leaf gas exchange. Leaf water potential dropped steeply to around − 3 MPa after soil water content was below 0.10 cm3 cm− 3. Conclusion: Our comprehensive data set contributes to a better understanding of the effects of drought on a grassland species and the limits of its acclimation in dry conditions. [ABSTRACT FROM AUTHOR]

Background and aims: In humid regions, the transpiration rate is determined by transpiration demand because of the sufficiently moist soil. However, inhibition of plant water uptake capacity due to soil waterlogging can significantly constrain the transpiration rate even after drainage. This study aimed to evaluate plant hydraulic conductance during soil waterlogging and subsequent reoxygenation and its impact on whole plant transpiration.Two experiments were conducted to assess the ecophysiological responses of soybeans during waterlogging (Experiment 1) and reoxygenation (Experiment 2). Transpiration rate, stomatal conductance, leaf water potential, and leaf area were measured. In addition, plant hydraulic conductance was calculated using the root water uptake equation. A simple transpiration model incorporating the response of plant hydraulic conductance to waterlogging was used to evaluate the impact of waterlogging on transpiration estimation.Waterlogging for more than 3 days reduced plant hydraulic conductance, which persisted even during the post-waterlogging reoxygenation period. Furthermore, leaf water potential, stomatal conductance, and transpiration rate in waterlogging treatment exhibited a lower value than those in control during both waterlogging and reoxygenation. The constructed model effectively reproduced the responses of plant hydraulic conductance and transpiration rate, especially during reoxygenation.Soil waterlogging significantly reduce the hydraulic conductance of soybean plants, resulting in leaf water stress and depression of transpiration, even during reoxygenation. Our results highlight the importance of integrating plant hydraulic responses with water dynamics models in the soil-plant-atmosphere system.Methods: In humid regions, the transpiration rate is determined by transpiration demand because of the sufficiently moist soil. However, inhibition of plant water uptake capacity due to soil waterlogging can significantly constrain the transpiration rate even after drainage. This study aimed to evaluate plant hydraulic conductance during soil waterlogging and subsequent reoxygenation and its impact on whole plant transpiration.Two experiments were conducted to assess the ecophysiological responses of soybeans during waterlogging (Experiment 1) and reoxygenation (Experiment 2). Transpiration rate, stomatal conductance, leaf water potential, and leaf area were measured. In addition, plant hydraulic conductance was calculated using the root water uptake equation. A simple transpiration model incorporating the response of plant hydraulic conductance to waterlogging was used to evaluate the impact of waterlogging on transpiration estimation.Waterlogging for more than 3 days reduced plant hydraulic conductance, which persisted even during the post-waterlogging reoxygenation period. Furthermore, leaf water potential, stomatal conductance, and transpiration rate in waterlogging treatment exhibited a lower value than those in control during both waterlogging and reoxygenation. The constructed model effectively reproduced the responses of plant hydraulic conductance and transpiration rate, especially during reoxygenation.Soil waterlogging significantly reduce the hydraulic conductance of soybean plants, resulting in leaf water stress and depression of transpiration, even during reoxygenation. Our results highlight the importance of integrating plant hydraulic responses with water dynamics models in the soil-plant-atmosphere system.Results: In humid regions, the transpiration rate is determined by transpiration demand because of the sufficiently moist soil. However, inhibition of plant water uptake capacity due to soil waterlogging can significantly constrain the transpiration rate even after drainage. This study aimed to evaluate plant hydraulic conductance during soil waterlogging and subsequent reoxygenation and its impact on whole plant transpiration.Two experiments were conducted to assess the ecophysiological responses of soybeans during waterlogging (Experiment 1) and reoxygenation (Experiment 2). Transpiration rate, stomatal conductance, leaf water potential, and leaf area were measured. In addition, plant hydraulic conductance was calculated using the root water uptake equation. A simple transpiration model incorporating the response of plant hydraulic conductance to waterlogging was used to evaluate the impact of waterlogging on transpiration estimation.Waterlogging for more than 3 days reduced plant hydraulic conductance, which persisted even during the post-waterlogging reoxygenation period. Furthermore, leaf water potential, stomatal conductance, and transpiration rate in waterlogging treatment exhibited a lower value than those in control during both waterlogging and reoxygenation. The constructed model effectively reproduced the responses of plant hydraulic conductance and transpiration rate, especially during reoxygenation.Soil waterlogging significantly reduce the hydraulic conductance of soybean plants, resulting in leaf water stress and depression of transpiration, even during reoxygenation. Our results highlight the importance of integrating plant hydraulic responses with water dynamics models in the soil-plant-atmosphere system.Conclusion: In humid regions, the transpiration rate is determined by transpiration demand because of the sufficiently moist soil. However, inhibition of plant water uptake capacity due to soil waterlogging can significantly constrain the transpiration rate even after drainage. This study aimed to evaluate plant hydraulic conductance during soil waterlogging and subsequent reoxygenation and its impact on whole plant transpiration.Two experiments were conducted to assess the ecophysiological responses of soybeans during waterlogging (Experiment 1) and reoxygenation (Experiment 2). Transpiration rate, stomatal conductance, leaf water potential, and leaf area were measured. In addition, plant hydraulic conductance was calculated using the root water uptake equation. A simple transpiration model incorporating the response of plant hydraulic conductance to waterlogging was used to evaluate the impact of waterlogging on transpiration estimation.Waterlogging for more than 3 days reduced plant hydraulic conductance, which persisted even during the post-waterlogging reoxygenation period. Furthermore, leaf water potential, stomatal conductance, and transpiration rate in waterlogging treatment exhibited a lower value than those in control during both waterlogging and reoxygenation. The constructed model effectively reproduced the responses of plant hydraulic conductance and transpiration rate, especially during reoxygenation.Soil waterlogging significantly reduce the hydraulic conductance of soybean plants, resulting in leaf water stress and depression of transpiration, even during reoxygenation. Our results highlight the importance of integrating plant hydraulic responses with water dynamics models in the soil-plant-atmosphere system. [ABSTRACT FROM AUTHOR]

Aims: (1) Monitoring 'root-felt' salinity by using rhizoslides as a non-invasive method, (2) Studying how transpiration rate, salinity in irrigation water, and root water uptake affect sodium distribution around single roots, (3) Interpreting experimental results by using simulations with a 3-D root system architecture model coupled with water flow and solute transport models. Methods: Tomato plants were grown on rhizoslides under various salinity levels and two transpiration rates: high and low. Daily root images were processed with GIMP and incorporated into a 3-D numerical model. The experiments were simulated with R-SWMS, a 3-dimensional numerical model that simulates water flow and solute transport in soil, into the root and inside root systems. Results: Both experimental and simulation results displayed higher root-felt sodium concentrations compared with the bulk concentrations, and larger accumulation at higher transpiration rate. The simulations illustrated that the root-felt to bulk concentration ratio changed during the experiment depending both on the irrigation water salinity and transpiration rate. Conclusions: Changes in sodium concentrations with transpiration rates are most likely caused by root water uptake and ion exclusion. Simulation results indicate that root-scale process models are required to link root system architecture, environmental, and soil conditions with root-felt salinities. [ABSTRACT FROM AUTHOR]

Aim: Understanding the source water utilization of rice-based cropping systems helps develop improving water management strategies for paddy management. We investigated the effects of altered flooding regimes and crop diversification on plant root water uptake on a fully-replicated field trial at the International Rice Research Institute in the Philippines.Methods: All potential water pools, e.g., plant and soil extracted water, were analyzed for their water stable isotopic compositions (δ2H and δ18O). We determined the relative contributions from different water sources to root water uptake (RWU) of rice plants by applying a multi-source mixing model (Stable Isotopes Analysis in R, SIAR). The sensitivity of the model to the incorporation of prior information based on in-situ measurements of soil water content and root length density was investigated as well.Results: The modeling results showed that wet rice plants mainly extracted surface ponded water (~56-72%) during both wet and dry seasons followed by soil surface (0-0.02 m) water (~17-19%) during growth. Dry rice extracted ~40-50% of its water from shallow soil (0-0.5 m) and ~35% from 0.1 to 0.3 m depth when the plants were matured.Conclusions: The mixing model results were better constrained with the additional information on soil water content and root length density. The relative contributions of the soil water sources to RWU decreased with depth and reflected the exponential shape of the root density profile. The main water source for wet rice was surface ponded water (independent of the season), whereas shallow soil water was the main source for dry rice. [ABSTRACT FROM AUTHOR]

Background and Aims: Recent experiments showed that rhizosphere of several plant species turns temporarily hydrophobic after severe drying and subsequent rewetting. Whether or not such hydrophobicity limits root water uptake is not known.Methods: A set of experiments was performed to test whether rhizosphere water repellency negatively affects root water uptake. To this end, a commercial surfactant was used as a rewetting agent to facilitate the wettability of the rhizosphere of lupins (Lupinus albus) in a sandy soil. Lupin plants were grown in rhizoboxes and were subjected to a severe drying cycle. Then half of the plants were irrigated with water and half with the surfactant solution. Time-series neutron radiography technique was used to monitor water redistribution in the rhizosphere during irrigation. In a second experimental set-up, lupins were grown in a sandy soil partitioned in five vertical compartments separated by a 1-cm layer of coarse sand (acting as a capillary barrier). Water and surfactant were injected in different compartments and the rehydration of the root tissues beyond the irrigated compartments was monitored with neutron radiography for 2-3 h. Root rehydration rates were used to estimate the water fluxes across the root-soil interface.Results: The rhizosphere of lupin roots in sandy soil irrigated with water remained partly dry for at least 2-3 h, while it was rapidly rewetted when irrigated with surfactant. Water flow into the roots irrigated with surfactant solution was 6.5 times faster than into the roots irrigated with water.Conclusions: These results prove that water repellency of the rhizosphere of lupins in sandy soils limited the water fluxes into the roots and root rehydration during the first two to three hours after irrigation. This might not always be negative, because it can limit water losses from roots to dry soil and therefore avoid severe root dehydration. [ABSTRACT FROM AUTHOR]

Background: The role of the root-soil interface on soil-plant water relations is unclear. Despite many experimental studies proved that the soil close to the root surface, the rhizosphere, has different properties compared to the adjacent bulk soil, the mechanisms underlying such differences are poorly understood and the implications for plant-water relations remain largely speculative. Scope: The objective of this review is to identify the key elements affecting water dynamics in the rhizosphere. Special attention is dedicated to the role of mucilage exuded by roots in shaping the hydraulic properties of the rhizosphere. We identified three key properties: 1) mucilage adsorbs water decreasing its water potential; 2) mucilage decreases the surface tension of the soil solution; 3) mucilage increases the viscosity of the soil solution. These three properties determine the retention and spatial configuration of the liquid phase in porous media. The increase in viscosity and the decrease in surface tension (quantified by the Ohnesorge number) allow the persistence of long liquid filaments even at very negative water potentials. At high mucilage concentrations these filaments form a network that creates an additional matric potential and maintains the continuity of the liquid phase during drying. Conclusion: The biophysical interactions between mucilage and the pore space determine the physical properties of the rhizosphere. Mucilage forms a network that provides mechanical stability to soils upon drying and that maintains the continuity of the liquid phase across the soil-root interface. Such biophysical properties are functional to create an interconnected matrix that maintains the roots in contact with the soil, which is of particular importance when the soil is drying and the transpiration rate is high. [ABSTRACT FROM AUTHOR]

Aims: Hydro-biogeochemical processes in the rhizosphere regulate nutrient and water availability, and thus ecosystem productivity. We hypothesized that two such processes often neglected in rhizosphere models - diel plant water use and competitive cation exchange - could interact to enhance availability of K and NH , both high-demand nutrients. Methods: A rhizosphere model with competitive cation exchange was used to investigate how diel plant water use (i.e., daytime transpiration coupled with no nighttime water use, with nighttime root water release, and with nighttime transpiration) affects competitive ion interactions and availability of K and NH . Results: Competitive cation exchange enabled low-demand cations that accumulate against roots (Ca, Mg, Na) to desorb NH and K from soil, generating non-monotonic dissolved concentration profiles (i.e. 'hotspots' 0.1-1 cm from the root). Cation accumulation and competitive desorption increased with net root water uptake. Daytime transpiration rate controlled diel variation in NH and K aqueous mass, nighttime water use controlled spatial locations of 'hotspots', and day-to-night differences in water use controlled diel differences in 'hotspot' concentrations. Conclusions: Diel plant water use and competitive cation exchange enhanced NH and K availability and influenced rhizosphere concentration dynamics. Demonstrated responses have implications for understanding rhizosphere nutrient cycling and plant nutrient uptake. [ABSTRACT FROM AUTHOR]

Aims: A soil-plant-atmosphere continuum (SPAC) model for simulating tree transpiration ( Ep) with variable water stress and water distribution in the soil is presented. The model couples a sun/shade approach for the canopy with a discrete representation of the soil in different layers and compartments. Methods: To test its performance, the outputs from the simulations are compared to those from an experiment using trees of olive 'Picual' and almond 'Marinada' with the root system split into two. Trees are subjected to different irrigation phases in which one side of the root system is dried out while the other is kept wet. Results: The model is able to accurately predict Ep (R and the efficiency factor (EF) around 0.9) in the two species studied. The use of a function that modulates the uptake capacity of a root according to the soil water content was necessary to track the fluxes observed from each split part. It was also appropriate to account for root clumping to match the measured and modelled leaf water potential. Conclusions: Coupling the sun/shade approach with the soil multi-compartment solution provides a useful tool to explore tree Ep for different degrees of water availability and distribution. [ABSTRACT FROM AUTHOR]

Aims: Maize ( Zea mays L.) is one of the most important crops worldwide. Despite several studies on maize roots, there is limited information on the function of different root types in extracting water from soils. Aim of this study was to investigate the location of water uptake in maize roots. Methods: We used neutron radiography to image the spatial distribution of maize roots in soil and trace the transport of deuterated water (DO) in soil and roots. Maize plants were grown in aluminum containers filled with a sandy soil that was kept homogeneously wet throughout the experiment. When the plants were 16 days old, we injected DO into selected soil regions. The transport of DO was simulated using a diffusion-convection numerical model. By fitting the observed DO transport we quantified the diffusion coefficient and the water uptake of the different root segments. Results: The root architecture of a 16 day-old maize consisted of a primary root, 4-5 seminal roots and many lateral roots. Laterals emerged from the proximal 15 cm of the primary and seminal roots. During both day and night measurements, DO entered more quickly into lateral roots than into primary and seminal roots. The quick transport of DO into laterals was caused by the small radius of lateral roots. The diffusion coefficient of lateral roots (4.68 × 10 cm s) was similar to that of the distal unbranched segments of seminal roots (4.72 × 10 cm s) and higher than that of the proximal branched segments (1.42 × 10 cm s). Water uptake of lateral roots (1.64 × 10 cm s) was much higher than the uptake of seminal roots, which was 5.34 × 10 cm s in the proximal branched segments and only 1.18 × 10 cm s in the distal unbranched segments. Conclusions: We conclude that the function of lateral roots is to absorb water from the soil, while the function of the primary and seminal roots is to axially transport water to the shoot. [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]

Aims: Soil salinity can cause salt plant stress by reducing plant transpiration and yield due to very low osmotic potentials in the soil. For predicting this reduction, we present a simulation study to (i) identify a suitable functional form of the transpiration reduction function and (ii) to explain the different shapes of empirically observed reduction functions. Methods: We used high resolution simulations with a model that couples 3D water flow and salt transport in the soil towards individual roots with flow in the root system. Results: The simulations demonstrated that the local total water potential at the soil-root interface, i.e. the sum of the matric and osmotic potentials, is for a given root system, uniquely and piecewise linearly related to the transpiration rate. Using bulk total water potentials, i.e. spatially and temporally averaged potentials in the soil around roots, sigmoid relations were obtained. Unlike for the local potentials, the sigmoid relations were non-unique functions of the total bulk potential but depended on the contribution of the bulk osmotic potential. Conclusions: To a large extent, Transpiration reduction is controlled by water potentials at the soil-root interface. Since spatial gradients in water potentials around roots are different for osmotic and matric potentials, depending on the root density and on soil hydraulic properties, transpiration reduction functions in terms of bulk water potentials cannot be transferred to other conditions, i.e. soil type, salt content, root density, beyond the conditions for which they were derived. Such a transfer could be achieved by downscaling to the soil-root interface using simulations with a high resolution process model. [ABSTRACT FROM AUTHOR]

Aims: To determine soil water diffusivity, D(θ), on undisturbed field soil at medium to low water content (suction range from 10 to 150 m of water), for the purpose of modeling the uptake of water by plant roots. Methods: The method is based on the analysis of one-step outflow induced by a turbulent stream of dry air over the exposed end of a soil core, with the other end of the core enclosed. The outflow is measured through time as the change in the weight of the core as it sits on a recording balance. D(θ) is calculated by deconvoluting the measured outflow function. Results: Over the suction range of 10 to 150 m of water, D(θ) calculated on the undisturbed soil ranged from 20 × 10 to 10 × 10 [m s], substantially higher than other published estimates over this range in suction. Conclusions: These unusually large values cast doubt on the view that flow of water to roots limits uptake of water from the targeted subsoil. [ABSTRACT FROM AUTHOR]

Aim: To test for the presence of an impediment to water flow at the soil-root interface. Methods: Wheat plants were grown in repacked and undisturbed field soil. Their transpiration rate, E, was varied in several steps from low to high and then back to low again, while the hydrostatic pressure in the leaf xylem, ψ, was measured non-destructively and continuously. These measurements were compared to a mathematical model that calculated ψ by assuming that the hydraulic resistance across the plant was constant and that the radial flow of water to unit length of a typical plant root generated gradients in pressure in the soil water. Results: For the repacked soil, the radial flow model could not match the experiment during the falling phase of E, unless it was assumed that either an additional, constant, interfacial resistance between the soil and the roots had developed when E was large and ψ was rapidly falling, or that the resistance within the plant had changed. For the undisturbed field soil, the radial flow model did not agree with the experiment. Plausible agreement was achieved when plant water uptake was accounted for using a distributed sink model in HYDRUS-1D, with E integrated across the rootzone. This approach was based on the measured large variation in the vertical distribution of roots. Conclusions: There was no strong evidence of large drawdowns of soil water in the rhizosphere, even when ψ was falling rapidly when E was large and the soil was moderately dry. Thus, there seems to have been an additional impediment to water flow from soil to plant, either within the plant, or at the interface between the two. [ABSTRACT FROM AUTHOR]

Aims: Roots need to be in good contact with the soil to take up water and nutrients. However, when the soil dries and roots shrink, air-filled gaps form at the root-soil interface. Do gaps actually limit the root water uptake, or do they form after water flow in soil is already limiting? Methods: Four white lupins were grown in cylinders of 20 cm height and 8 cm diameter. The dynamics of root and soil structure were recorded using X-ray CT at regular intervals during one drying/wetting cycle. Tensiometers were inserted at 5 and 18 cm depth to measure soil matric potential. Transpiration rate was monitored by continuously weighing the columns and gas exchange measurements. Results: Transpiration started to decrease at soil matric potential ψ between −5 kPa and −10 kPa. Air-filled gaps appeared along tap roots between ψ = −10 kPa and ψ = −20 kPa. As ψ decreased below −40 kPa, roots further shrank and gaps expanded to 0.1 to 0.35 mm. Gaps around lateral roots were smaller, but a higher resolution is required to estimate their size. Conclusions: Gaps formed after the transpiration rate decreased. We conclude that gaps are not the cause but a consequence of reduced water availability for lupins. [ABSTRACT FROM AUTHOR]

Background and aims: Knowledge of plant water fluxes is critical for assessing mechanistic processes linked to biogeochemical cycles, yet resolving root water transport dynamics has been a particularly daunting task. Our objectives were to demonstrate the ability to non-invasively monitor individual root functionality and water fluxes within Zea mays L. (maize) and Panicum virgatum L. (switchgrass) seedlings using neutron imaging. Methods: Seedlings were propagated for 1-3 weeks in aluminum chambers containing sand. Pulses of water or deuterium oxide were then tracked through the root systems by collecting consecutive radiographs during exposure to a cold-neutron source. Water flux was manipulated by cycling on a growth lamp to alter foliar demand for water. Results: Neutron radiography readily illuminated root structure, root growth, and relative plant and soil water content. After irrigation there was rapid root water uptake from the newly wetted soil, followed by hydraulic redistribution of water through the root system to roots terminating in dry soil. Water flux within individual roots responded differentially to foliar illumination based on supply and demand of water within the root system. Conclusions: Sub-millimeter scale image resolution revealed timing and magnitudes of root water uptake, redistribution within the roots, and root-shoot hydraulic linkages-relationships not well characterized by other techniques. [ABSTRACT FROM AUTHOR]

Background and aims: Michaelis-Menten (MM) kinetics and a physical-mathematical (PM) model are the popular approaches to describe root N uptake (RNU). This study aimed to examine RNU and compare the two model approaches. Methods: A hydroponic experiment (Exp.1) investigated the effects of root length, root N mass, transpiration, plant age and solution N concentration on RNU of wheat ( Triticum aestivum L. cv. Jingdong 8). The two models were applied to simulate the RNU and soil N dynamics in a soil-wheat system (Exp.2), and the results were compared to the measured data. Results: Under the hydroponic conditions, RNU was better correlated with root N mass and transpiration than root length. The influences of solution N concentration on RNU rate per root length (MM1) and RNU rate per root N mass (MM2) were described well with MM kinetics. The kinetic parameters for MM1 changed with plant age but the parameters for MM2 were not age dependant. The description of RNU with the PM model was also independent of plant age, and was more reliable when the RNU factor decreased as a power function with the solution N concentration (PM2) than an assumed constant (PM1). In Exp.2, the root mean squared errors between the simulated and measured soil solution N concentration and the relative errors between the simulated and measured N uptake mass for MM kinetics were much larger than those for the PM model. Conclusions: Both the MM and PM models successfully described RNU under the hydroponic conditions, but the PM model (especially PM2) was more reliable than the MM model in the soil-wheat system. [ABSTRACT FROM AUTHOR]

The estimation of root water uptake and water flow in plants is crucial to quantify transpiration and hence the water exchange between land surface and atmosphere. In particular the soil water extraction by plant roots which provides the water supply of plants is a highly dynamic and non-linear process interacting with soil transport processes that are mainly determined by the natural soil variability at different scales. To better consider this root-soil interaction we extended and further developed a finite element tree hydro-dynamics model based on the one-dimensional (1D) porous media equation. This is achieved by including in addition to the explicit three-dimensional (3D) architectural representation of the tree crown a corresponding 3D characterisation of the root system. This 1D xylem water flow model was then coupled to a soil water flow model derived also from the 1D porous media equation. We apply the new model to conduct sensitivity analysis of root water uptake and transpiration dynamics and compare the results to simulation results obtained by using a 3D model of soil water flow and root water uptake. Based on data from lysimeter experiments with young European beech trees ( Fagus silvatica L.) is shown, that the model is able to correctly describe transpiration and soil water flow. In conclusion, compared to a fully 3D model the 1D porous media approach provides a computationally efficient alternative, able to reproduce the main mechanisms of plant hydro-dynamics including root water uptake from soil. [ABSTRACT FROM AUTHOR]

Water flow from soil to plants depends on the properties of the soil next to roots, the rhizosphere. Although several studies showed that the rhizosphere has different properties than the bulk soil, effects of the rhizosphere on root water uptake are commonly neglected. To investigate the rhizosphere’s properties we used neutron radiography to image water content distributions in soil samples planted with lupins during drying and subsequent rewetting. During drying, the water content in the rhizosphere was 0.05 larger than in the bulk soil. Immediately after rewetting, the picture reversed and the rhizosphere remained markedly dry. During the following days the water content of the rhizosphere increased and after 60 h it exceeded that of the bulk soil. The rhizosphere’s thickness was approximately 1.5 mm. Based on the observed dynamics, we derived the distinct, hysteretic and time-dependent water retention curve of the rhizosphere. Our hypothesis is that the rhizosphere’s water retention curve was determined by mucilage exuded by roots. The rhizosphere properties reduce water depletion around roots and weaken the drop of water potential towards roots, therefore favoring water uptake under dry conditions, as demonstrated by means of analytical calculation of water flow to a single root. [ABSTRACT FROM AUTHOR]

Correct modeling of root water uptake partitioning over depth is an important issue in hydrological and crop growth models. Recently a physically based model to describe root water uptake was developed at single root scale and upscaled to the root system scale considering a homogeneous distribution of roots per soil layer. Root water uptake partitioning is calculated over soil layers or compartments as a function of respective soil hydraulic conditions, specifically the soil matric flux potential, root characteristics and a root system efficiency factor to compensate for within-layer root system heterogeneities. The performance of this model was tested in an experiment performed in two-compartment split-pot lysimeters with sorghum plants. The compartments were submitted to different irrigation cycles resulting in contrasting water contents over time. The root system efficiency factor was determined to be about 0.05. Release of water from roots to soil was predicted and observed on several occasions during the experiment; however, model predictions suggested root water release to occur more often and at a higher rate than observed. This may be due to not considering internal root system resistances, thus overestimating the ease with which roots can act as conductors of water. Excluding these erroneous predictions from the dataset, statistical indices show model performance to be of good quality. [ABSTRACT FROM AUTHOR]

Alternate partial root zone irrigation (APRI) is a new water-saving irrigation technique. It can reduce irrigation water and transpiration without reduction in crop yield, thus increase water and nutrient use efficiency. Understanding of soil moisture distribution and dynamic under the alternate partial root zone drip irrigation (APDI) can help to develop the efficient irrigation schemes. In this paper, a two-dimensional (2D) root water uptake model was proposed based on soil water dynamic and root distribution of grape vine, and a function of soil evaporation related to soil water content was defined under the APDI. Then the soil water dynamic model of APDI (APRI-model) was developed based on the 2D root water uptake model and soil evaporation function combined with average measured soil moisture content at 0–10 cm soil layer. Soil water dynamic in APDI was respectively simulated by Hydrus-2D model and APRI-model. The simulated soil water contents by two models were compared with the measured value. The results showed that the values of root-mean-square-error (RMSE) range from 0.01 to 0.022 cm3/cm3 for APRI-model, and from 0.012 to 0.031 cm3/cm3 for Hydrus-2D model. The average relative error between the simulated and measured soil water content is about 10% for APRI-model, and from 11% to 29% for Hydrus-2D model, indicating that two models perform well in simulating soil moisture dynamic under the APDI, but the APRI-model is more suitable for modeling the soil water dynamic in the arid region with greater soil evaporation and uneven root distribution. [ABSTRACT FROM AUTHOR]

A 2-year trial was performed on autumn-sown sugar beet grown in pots in order to study the influence of irrigation frequency on the water used by plants along the soil profile. The outdoor pots, containing one plant each, were 1.3 m high and had circular openings, through which Time Domain Reflectometry (TDR) apparatus wave guides could be inserted. Three irrigation intervals were compared and plants were watered whenever the soil layer explored by roots had lost 30% (SWD1), 50% (SWD2) and 70% (SWD3) of the total available water (TAW). During the irrigation season, the water extracted by the plants from each layer along the soil profile (RWU) was determined by monitoring volumetric soil moisture content (θ), by TDR. At harvest time, root length density (RLD) along the soil profile was assessed using the Tennant method. The applied irrigation frequencies significantly affected the RWU. With the SWD3 protocol, irrigation was at longer irrigation intervals (9 days) and watering volumes were as high as 84 mm. In this treatment, the plants lost almost 60% of total water from the lower soil layer (0.6–1.0 m). In treatment SWD1, the irrigation interval was very short (3 days), and water extraction from 0.0–0.6 m soil depth was 92.0%. In the intermediate treatment, the irrigation interval was 5.5 days and a more uniform water depletion was observed along the root zone, approximately equal between the 0–0.6 and 0.6–1.0 m soil layer. Water extraction of sugar beet plants at the deeper soil layers in response to long irrigation intervals was related to an increase in water uptake efficiency of the deeper younger roots and not to an increase in root length density, which, on the contrary, decreased. This morpho-physiological acclimatization to progressive soil water deficit was coupled with an increase of the root/shoot ratio. [ABSTRACT FROM AUTHOR]

An efficient new numerical method for simultaneously simulating soil water flow and plant root growth is presented. It allows the calculation of the water uptake of an entire root system while preserving the local impact of single roots. The approach is based on the adaptive finite-element method, which enables a flexible fine resolution along individual roots, which cannot be achieved by classical non-adaptive algorithms. [ABSTRACT FROM AUTHOR]

Background: Our recent publication (Warren et al., Plant Soil 366:683-693, ) described how pulses of deuterium oxide (DO) or HO combined with neutron radiography can be used to indicate root water uptake and hydraulic redistribution in maize. This technique depends on the large inherent differences in neutron cross-section between D and H atoms resulting in strong image contrast. Scope and Conclusions: However, as illustrated by Carminati and Zarebanadkouki () there can be a change in total water content without a change in contrast simply by a change in the relative proportions of DO and HO. We agree with their premise and detailed calculations (Zarebanadkouki at al. , ), and present further evidence that mixing of DO and HO did not confound evidence of hydraulic redistribution in our study. [ABSTRACT FROM AUTHOR]

Introduction: In a recent paper, Warren et al. () illustrated the potential of neutron radiography to visualize water dynamics in soil and plants. Methods: After injection of deuterated water (DO) in soil, the authors could monitor the changes of DO concentration in roots. Results: Based on the radiographs, the authors concluded that DO was transported from roots growing in a wet soil region to roots in a dry region, proving hydraulic redistribution between roots. However, this interpretation depends on the correct estimation of DO concentration in soil. Conclusions: The experiments of Warren et al. () could also be explained by diffusion of DO from soil to roots, without hydraulic redistribution within the root system. [ABSTRACT FROM AUTHOR]