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The geometry of rooting systems is important for modeling water flows in the soil-plant-atmosphere continuum. Measured information about root density can be summarized in adjustable equations applied in hydrological models. We present such descriptive functions used to model root density distribution over depth and evaluate their quality of fit to measured crop root density profiles retrieved from the literature. An equation is presented to calculate the mean root half-distance as a function of depth from root length density profiles as used in single root models for water uptake. To assess the importance of the shape of the root length density profile in hydrological modeling, the sensitivity of actual transpiration predictions of a hydrological model to the shape of root length density profiles is analyzed using 38 years of meteorological data from Southeast Brazil. The cumulative root density distributions covering the most important agricultural crops (in terms of area) were found to be well described by the logistic function or the Gompertz function. Root length density distribution has a consistent effect on relative transpiration, hence on relative yield, but the common approach to predict transpiration reduction and irrigation requirement from soil water storage or average water content is shown to be only partially supported by simulation results.

Abstract: Efficient extraction of soil water is essential for the productivity of plant communities. However, research on the complementary use of resources in mixed plant communities, and especially the impact of plant species richness on root water uptake, is limited. So far, these investigations have been hindered by a lack of methods allowing for the estimation of root water uptake profiles. The overarching aim of our study was to determine whether diverse grassland plant communities in general exploit soil water more deeply and whether this shift occurs all the time or only during times of enhanced water demand. Root water uptake was derived by analysing the diurnal decrease in soil water content separately at each measurement depth, thus yielding root water uptake profiles for 12 experimental grasslands communities with two different levels of species richness (4 and 16 sown species). Additional measurements of leaf water potential, stomatal conductance, and root traits were used to identify differences in water relations between plant functional groups. Although the vertical root distribution did not differ between diversity levels, root water uptake shifted towards deeper layers (30 and 60 cm) in more diverse plots during periods of high vapour pressure deficit. Our results indicate that the more diverse communities were able to adjust their root water uptake, resulting in increased water uptake per root area compared to less diverse communities (52% at 20 cm, 118% at 30 cm, and 570% at 60 cm depth) and a more even distribution of water uptake over depth. Tall herbs, which had lower leaf water potential and higher stomatal conductance in more diverse mixtures, contributed disproportionately to dynamic niche partitioning in root water uptake. This study underpins the role of diversity in stabilizing ecosystem function and mitigating drought stress effects during future climate change scenarios. Furthermore, the results provide evidence that root water uptake is not solely controlled by root length density distribution in communities with high plant diversity but also by spatial shifts in water acquisition. A plain language summary is available for this article. [ABSTRACT FROM AUTHOR]

In this manuscript, we propose a new method to calculate water flow and xylem water potential distribution in hydraulic architectures (such as root systems) of any complexity. It is based on the extension of the water flow equation analytical resolution of Landsberg and Fowkes for single roots. It consists in splitting the root systems in zones of homogeneous or homogeneously changing properties and deriving the xylem potential and water flow under any given boundary conditions (plant transpiration or collar potential, and potential at soil-root interfaces) without assuming a uniform xylem potential within each zone. The method combines analytical solutions of water flow within the segmented zones with the numerical solution of flow connectivity for the whole root system. We demonstrate that the proposed solution is the asymptote of the exclusively numerical solution for infinitesimal root segment lengths (and infinite segment number). As water uptake locations and magnitudes predicted by the latter solution for finite segmentation lengths deviate from the exact solution, and are computationally more intensive, we conclude that the new methodology should always be privileged for future applications. The proposed solution can be easily coupled to soil modules (as already done with existing solutions) and further implemented in functional-structural plant models to predict water flow in the soil-plant atmosphere continuum with a better accuracy than current models. Finally the new solution may be used to calculate more accurately plant scale macroscopic parameters for crop models. [ABSTRACT FROM AUTHOR]

Root system architecture (RSA) can significantly affect plant access to water, total transpiration, as well as its partitioning by soil depth, with implications for surface heat, water, and carbon budgets. Despite recent advances in land surface model (LSM) descriptions of plant hydraulics, descriptions of RSA have not been included because of their three-dimensional complexity, which makes them generally too computationally costly. Here we demonstrate a new, process-based 1D layered model that captures the dynamic shifts in water potential gradients of 3D RSA under different soil moisture conditions: the RSA stencil. Using root systems calibrated to the rooting profiles of four plant functional types (PFT) of the Community Land Model, we show that the RSA stencil predicts plant water potentials within 2% to the outputs of a full 3D model, under the same assumptions on soil moisture heterogeneity, despite its trivial computational cost, resulting in improved predictions of water uptake and soil moisture compared to a model without RSA in a transient simulation. Our results suggest that LSM predictions of soil moisture dynamics and dependent variables can be improved by the implementation of this model, calibrated for individual PFTs using field observations. [ABSTRACT FROM AUTHOR]

Do root hairs help roots take up water from the soil? Despite the well-documented role of root hairs in phosphate uptake, their role in water extraction is controversial., We grew barley ( Hordeum vulgare cv Pallas) and its root-hairless mutant brb in a root pressure chamber, whereby the transpiration rate could be varied whilst monitoring the suction in the xylem. The method provides accurate measurements of the dynamic relationship between the transpiration rate and xylem suction., The relationship between the transpiration rate and xylem suction was linear in wet soils and did not differ between genotypes. When the soil dried, the xylem suction increased rapidly and non-linearly at high transpiration rates. This response was much greater with the brb mutant, implying a reduced capacity to take up water., We conclude that root hairs facilitate the uptake of water by substantially reducing the drop in matric potential at the interface between root and soil in rapidly transpiring plants. The experiments also reinforce earlier observations that there is a marked hysteresis in the suction in the xylem when the transpiration rate is rising compared with when it is falling, and possible reasons for this behavior are discussed. [ABSTRACT FROM AUTHOR]

Root water uptake is a component of water balance that has not been clearly understood. This study was carried out in a completely randomized design with three replications under the greenhouse condition at Shahrekord University, Shahrekord, Iran. In this study, the root water uptake (RWU) by pepper plant under various irrigation water levels was investigated. Irrigation treatments included control (full irrigation level, FI) and three deficit irrigation levels, 80%, 60% and 40% of the plant's water requirement called DI80, DI60and DI40, respectively. A no-plant cover treatment with three replications was also used to measure evaporation from the soil surface. Daily measurements of volumetric soil moisture (VSM) were made at 10 cm intervals of the soil column. The differences between the measured VSM and the VSM in the next day, and evaporation rate at the soil surface at the same layer of the no-plant cover treatment were calculated and, eventually, the RWU in each layer per day was estimated. The results showed that the maximum and minimum RWUs were found in the FI and DI40treatments, respectively. The averages of root water uptakes in the DI80, DI60, and DI40treatments were reduced by 17.08%, 48.72% and 68.25%, respectively. Furthermore, in the DI80treatment, the reduced rate of water uptake was less than the reduced rate of water applied to the plants. [ABSTRACT FROM AUTHOR]

Predicting root water uptake and plant transpiration is crucial for managing plant irrigation and developing drought-tolerant root system ideotypes (i.e. ideal root systems). Today, three-dimensional structural functional models exist, which allows solving the water flow equation in the soil and in the root systems under transient conditions and in heterogeneous soils. Yet, these models rely on the full representation of the three-dimensional distribution of the root hydraulic properties, which is not always easy to access. Recently, new models able to represent this complex system without the full knowledge of the plant 3D hydraulic architecture and with a limited number of parameters have been developed. However, the estimation of the macroscopic parameters a priori still requires a numerical model and the knowledge of the full three-dimensional hydraulic architecture. The objective of this study is to provide analytical mathematical models to estimate the values of these parameters as a function of local plant general features, like the distance between laterals, the number of primaries or the ratio of radial to axial root conductances. Such functions would allow one to characterize the behaviour of a root system (as characterized by its macroscopic parameters) directly from averaged plant root traits, thereby opening new possibilities for developing quantitative ideotypes, by linking plant scale parameters to mean functional or structural properties. With its simple form, the proposed model offers the chance to perform sensitivity and optimization analyses as presented in this study. [ABSTRACT FROM AUTHOR]

Few reports focus on the source of water used in crop uptake from the soil profile following superabsorbent polymer ( SAP) application, particularly the quantification of crop water uptake from SAP-treated soil and non- SAP-treated soil. Using column experiments, we investigated the effect of SAP on root water uptake of maize over two years and researched in depth the utilization of water from different soil layers under SAP application by employing stable isotope D/18O. The results suggest that SAP can increase root water uptake by 16.3-27.8% in SAP-treated soil layers. The water used by the crop mainly originated in the 0- to 20-cm soil layer at the jointing stage, 20- to 40-cm at heading stage and 0- to 20- cm during grain filling. [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]

Soil moisture affects and is affected by root water uptake and at the same time drives soil CO2 dynamics. Selecting root water uptake formulations in models is important since this affects the estimation of actual transpiration and soil CO2 efflux. This study aims to compare different models combining the Richards equation for soil water flow to equations describing heat transfer and air-phase CO2 production and flow. A root water uptake model (RWC), accounting only for root water compensation by rescaling water uptake rates across the vertical profile, was compared to a model (XWP) estimating water uptake as a function of the difference between soil and root xylem water potential; the latter model can account for both compensation (XWPRWC) and hydraulic redistribution (XWPHR). Models were compared in a scenario with a shallow water table, where the formulation of root water uptake plays an important role in modeling daily patterns and magnitudes of transpiration rates and CO2 efflux. Model simulations for this scenario indicated up to 20% difference in the estimated water that transpired over 50 days and up to 14% difference in carbon emitted from the soil. The models showed reduction of transpiration rates associated with water stress affecting soil CO2 efflux, with magnitudes of soil CO2 efflux being larger for the XWPHR model in wet conditions and for the RWC model as the soil dried down. The study shows the importance of choosing root water uptake models not only for estimating transpiration but also for other processes controlled by soil water content. [ABSTRACT FROM AUTHOR]

The root water uptake profile (RWUP) reflects a plant's survival strategy and controls evapotranspiration and carbon fluxes. Despite its importance, there is still no reliable method for reconstructing this profile. In this study, we applied and compared two possible approaches to a case study in a conifer plantation: an isotope-calibrated mechanistic model and a mixing model with a bell-shaped approximation. Our results show that, after calibrating the hydrologically-active root density profile, the mechanistic model gave a good estimation of the xylem water isotope delta (δx); even though the measured root density was greater in shallower soils, water uptake occurred throughout the entire soil profile, with more uptake in deeper soils. The RWUPs estimated by the mixing model were different from those estimated by the mechanistic model and were unrealistic. However, when we constrained the minimum thickness of the water uptake zone, there was good agreement between the RWUPs from the two approaches. We can therefore conclude that the mechanistic model calibrated with isotopes gave better results, and that sole use of the mixing model is not recommended unless appropriate constraints are applied. [ABSTRACT FROM AUTHOR]

The amount of moisture transpired by vegetation is critically tied to the moisture supply accessible to the root zone. In a Mediterranean climate, integrated evapotranspiration (ET) is typically greater in the dry summer when there is an uninterrupted period of high insolation. We present a 1-D model to explore the subsurface factors that may sustain ET through the dry season. The model includes a stochastic parameterization of hydraulic conductivity, root water uptake efficiency, and hydraulic redistribution by plant roots. Model experiments vary the precipitation, the magnitude and seasonality of ET demand, as well as rooting profiles and rooting depths of the vegetation. The results show that the amount of subsurface moisture remaining at the end of the wet winter is determined by the competition among abundant precipitation input, fast infiltration, and winter ET demand. The weathered bedrock retains ~30% of the winter rain and provides a substantial moisture reservoir that may sustain ET of deep-rooted (>8 m) trees through the dry season. A small negative feedback exists in the root zone, where the depletion of moisture by ET decreases hydraulic conductivity and enhances the retention of moisture. Hence, hydraulic redistribution by plant roots is impactful in a dry season, or with a less conductive subsurface. Suggestions for implementing the model in the CESM are discussed. [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]

Modeling water flow in the soil–plant–atmosphere continuum with the Richards equation requires a model for the sink term describing water uptake by plant roots. Despite recent progress in developing process-based models of water uptake by plant roots and water flow in above-ground parts of vegetation, effective models of root water uptake are widely applied and necessary for large-scale applications. Modeling root water uptake consists of three steps, (i) specification of the spatial distribution of potential uptake, (ii) reduction of uptake due to various stress sources, and (iii) enhancement of uptake in part of the simulation domain to describe compensation. We discuss the conceptual shortcomings of the frequently used root water uptake model of Feddes and suggest a simple but effective improvement of the model. The improved model parametrizes water stress in wet soil by a reduction scheme which is formulated as function of air content whereas water stress due to low soil water potential is described by the original approach of Feddes. The improved model is physically more consistent than Feddes’ model because water uptake in wet soil is limited by aeration which is a function of water content. The suggested modification is particularly relevant for simulations in heterogeneous soils, because stress parameters are uniquely defined for the entire simulation domain, irrespective of soil texture. Numerical simulations of water flow and root water uptake in homogeneous and stochastic heterogeneous soils illustrate the effect of the new model on root water uptake and actual transpiration. For homogeneous fine-textured soils, predicted root water uptake never achieves its potential rate. In stochastic heterogeneous soil, predicted water uptake is more pronounced at the interfaces between fine and coarse regions which has potential implications for plant growth, nutrient uptake and depletion. [ABSTRACT FROM AUTHOR]

As root water uptake (RWU) is an important link in the water and heat exchange between plants and ambient air, improving its parameterization is key to enhancing the performance of land surface model simulations. Although different types of RWU functions have been adopted in land surface models, there is no evidence as to which scheme most applicable to maize farmland ecosystems. Based on the 2007-09 data collected at the farmland ecosystem field station in Jinzhou, the RWU function in the Common Land Model (CoLM) was optimized with scheme options in light of factors determining whether roots absorb water from a certain soil layer ( W ) and whether the baseline cumulative root efficiency required for maximum plant transpiration ( W ) is reached. The sensibility of the parameters of the optimization scheme was investigated, and then the effects of the optimized RWU function on water and heat flux simulation were evaluated. The results indicate that the model simulation was not sensitive to W but was significantly impacted by W . With the original model, soil humidity was somewhat underestimated for precipitation-free days; soil temperature was simulated with obvious interannual and seasonal differences and remarkable underestimations for the maize late-growth stage; and sensible and latent heat fluxes were overestimated and underestimated, respectively, for years with relatively less precipitation, and both were simulated with high accuracy for years with relatively more precipitation. The optimized RWU process resulted in a significant improvement of CoLM's performance in simulating soil humidity, temperature, sensible heat, and latent heat, for dry years. In conclusion, the optimized RWU scheme available for the CoLM model is applicable to the simulation of water and heat flux for maize farmland ecosystems in arid areas. [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]

Tuolumne Meadows is a groundwater dependent ecosystem in the Sierra Nevada of California, USA, that is threatened by hydrologic impacts that may lead to a substantial loss of organic matter in the soil. In order to provide a scientific basis for management of this type of ecosystem, this paper quantifies the effect of soil organic content on soil water retention and water use by plants. First, we show a substantial dependence of soil water retention on soil organic content by correlating Van Genuchten soil water retention parameters with soil organic content, independent of soil texture. Then, we demonstrate the impact of organic content on plants by simulating the degree to which root water uptake is affected by soil water retention with the use of a physically based numerical model of variably saturated groundwater flow. Our results indicate that the increased water retention by soil organic matter contributes as much as 8.8 cm to transpiration, or 35 additional water-stress free days, during the dry summer when plants experience increased water stress. [ABSTRACT FROM AUTHOR]

• In addition to buffering plants from water stress during severe droughts, plant water storage (PWS) alters many features of the spatio-temporal dynamics of water movement in the soil-plant system. How PWS impacts water dynamics and drought resilience is explored using a multi-layer porous media model. • The model numerically resolves soil-plant hydrodynamics by coupling them to leaf-level gas exchange and soil-root interfacial layers. Novel features of the model are the considerations of a coordinated relationship between stomatal aperture variation and whole-system hydraulics and of the effects of PWS and nocturnal transpiration ( Fe;night) on hydraulic redistribution (HR) in the soil. • The model results suggest that daytime PWS usage and Fe;night generate a residual water potential gradient (∆Ψ p;night) along the plant vascular system overnight. This ∆Ψ p;night represents a non-negligible competing sink strength that diminishes the significance of HR. • Considering the co-occurrence of PWS usage and HR during a single extended dry-down, a wide range of plant attributes and environmental/soil conditions selected to enhance or suppress plant drought resilience is discussed. When compared with HR, model calculations suggest that increased root water influx into plant conducting-tissues overnight maintains a more favorable water status at the leaf, thereby delaying the onset of drought stress. [ABSTRACT FROM AUTHOR]

Vegetation water use is closely related to its biophysical functioning and is often under stress from various environmental factors. However, commonly used root water uptake models only consider the stress from root zone moisture availability. There is a need to incorporate the stress from both the above-ground factors and root zone water condition. In this study, a newly developed coupled supply- and demand-induced (S&D) root water uptake model is examined with measurements on two tree species, Guihua in the subtropical monsoon climate and Drooping Sheoak in the Mediterranean climate. The results show that the S&D model outperforms a supply-constraint water stress function (the S-shape model) for both studied species. The S&D model predicts 67% and 84% temporal variability in the measured water stress for Guihua and Drooping Sheoak, respectively. The improvement of the S&D model over the S-shape model is more significant for Guihua than for Drooping Sheoak, which might be associated with the specific climate conditions. A two-step parameterization approach is adopted in this study for the S&D model, and is recommended for future applications. These results further support the validity of the S&D model, and should be considered for the root water uptake modeling. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) have been presumed to ameliorate crop tolerance to drought. Here, we review the role of AMF in maintaining water supply to plants from drying soils and the underlying biophysical mechanisms. We used a soil-plant hydraulic model to illustrate the impact of several AMF mechanisms on plant responses to edaphic drought. The AMF enhance the soil's capability to transport water and extend the effective root length, thereby attenuating the drop in matric potential at the root surface during soil drying. The synthesized evidence and the corresponding simulations demonstrate that symbiosis with AMF postpones the stress onset limit, which is defined as the disproportionality between transpiration rates and leaf water potentials, during soil drying. The symbiosis can thus help crops survive extended intervals of limited water availability. We also provide our perspective on future research needs and call for reconciling the dynamic changes in soil and root hydraulics in order to better understand the role of AMF in plant water relations in the face of climate changes., (© The Author(s) 2023. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

• Two process-based models are used to simulate root water uptake of oil crops. • The inverse model is improved by coupling the singular value decomposition parameter optimization algorithm. • The IM-SVD model shows better modeling accuracy of soil water content. • Soybean has a higher daily and seasonal root water uptake than two canola cultivars. • Root water uptake shows a time-delayed correlation with soil water content. Oil crops are often used as cover crops by smallholders managing orchards on drylands, because they have an economic return and help regulate ecosystem services. Improving the ability of models in root water uptake (RWU) simulation is important to find the right crop species and predict the dynamics of these oil crops in water-limited regions. In this study, two different process-based models were used to simulate the RWU of three oil crops, i.e. soybean (Glycine max L., SYB) and two canola cultivars (turnip type rape (Brassica campestris L.), TTR, and cabbage type rape (Brassica napus L.), CTR), grown on the semiarid Loess Plateau of China, based on two-year in-situ measurements. The first model, IM-SVD, was developed by inverse modeling (IM) of the Richards equation and incorporating singular value decomposition (SVD) parameter optimization algorithms, which simulated the RWU pattern (including soil evaporation and plant transpiration). The second model was the Feddes model implemented in one-dimensional HYDRUS (HYDRUS 1-D). Both models reliably predicted the soil water content (SWC) in different layers. However, the IM-SVD model showed better modeling accuracy of SWC, and produced higher simulated RWU values than the HYDRUS 1-D model irrespective of crop. The results from both models implied that SYB had a higher daily and seasonal RWU than TTR and CTR, and its water source was derived mainly from the top 30 cm soil layer. Additionally, the simulated RWU was positively and significantly (p <0.05) correlated with fine root length density (FRLD). The RWU showed lagged correlations with SWC, which varied between crop species and growth stages, but overall were more positively correlated with the SWC on preceding days and more negatively correlated with the SWC on subsequent days. Our study provides insights into the improving RWU simulation for oil crops and similar crops in drylands. [ABSTRACT FROM AUTHOR]

• Root water uptake of winter wheat across distinct rainfall years was quantified by using stable isotope. • Precipitation years led to significant differences in the isotopic signatures. • The appropriate irrigation wetting depth should be set as<60 cm for all kinds of rainfall years. Quantifying water availability in different soil layers to crop root water uptake (RWU) in different rainfall years helps deliver strategies to combat water scarcity and improve water productivity. To investigate the responses of RWU to soil water availability across distinct rainfall years, a field experiment of winter wheat was conducted in 2017–2018 (wet year), 2018–2019 (dry year), and 2019–2020 (ordinary year) in the North China Plain. Stable isotopes (δ18O and δD) in precipitation, irrigation water, xylem water, and soil water, as well as soil water content (SWC), were investigated during the three wheat growth seasons and were used to determine the soil water evaporation line (SWEL: the linear relationship between δD and δ18O in soil water), the line-conditioned excess (lc-excess: the deviation between δD and the local meteoric water line in the dual-isotope space) and the proportion of RWU from different soil layers with the MixSIAR model. The results indicated that there were significant differences in δ18O and δD in soil water and xylem water across different rainfall years. The slopes of SWEL and lc-excess of the 0–100 cm soil layer in the dry year were significantly smaller than those in the ordinary and wet years, which suggests that more substantial soil evaporation occurred in the dry year. The MixSIAR results revealed that the three-year wheat RWU exhibited different patterns, which were related to soil water availability. Wheat mainly utilized soil water from the 0–20 cm and 20–60 cm layers during the greening to filling stage under the dry year, while it mainly utilized the 0–20 cm soil layer under the wet and ordinary years. However, at the harvest stage, wheat under all precipitation years could transfer its main RWU zone to the 20–60 cm soil layer. There were significantly different proportions of RWU from 0 to 60 cm under different rainfall years, shown as 75.3 %, 67.5 %, and 69.5 % in the wet year, dry year, and ordinary year, respectively. These findings indicated that the appropriate irrigation wetting depth should be set as<60 cm for all kinds of rainfall years. [ABSTRACT FROM AUTHOR]

Moderate soil drying can cause a strong decrease in the soil-root system conductance. The resulting impact on root water uptake depends on the spatial distribution of the altered conductance relatively to remaining soil water resources, which is largely unknown. Here, we analyzed the vertical distribution of conductance across root systems using a novel, noninvasive sensor technology on pot-grown faba bean and maize plants. Withholding water for 4 days strongly enhanced the vertical gradient in soil water potential. Therefore, roots in upper and deeper soil layers were affected differently: In drier, upper layers, root conductance decreased by 66%-72%, causing an amplification of the drop in leaf water potential. In wetter, deeper layers, root conductance increased in maize but not in faba bean. The consequently facilitated deep-water uptake in maize contributed up to 21% of total water uptake at the end of the measurement. Analysis of root length distributions with MRI indicated that the locally increased conductance was mainly caused by an increased intrinsic conductivity and not by additional root growth. Our findings show that plants can partly compensate for a reduced root conductance in upper, drier soil layers by locally increasing root conductivity in wetter layers, thereby improving deep-water uptake., (© 2023 The Authors. Plant, Cell & Environment published by John Wiley & Sons Ltd.)

Split root experiments have the potential to disentangle water transport in roots and soil, enabling the investigation of the water uptake pattern of a root system. Interpretation of the experimental data assumes that water flow between the split soil compartments does not occur. Another approach to investigate root water uptake is by numerical simulations combining soil and root water flow depending on the parameterization and description of the root system. Our aim is to demonstrate the synergisms that emerge from combining split root experiments with simulations. We show how growing root architectures derived from temporally repeated X-ray CT scanning can be implemented in numerical soil-plant models. Faba beans were grown with and without split layers and exposed to a single drought period during which plant and soil water status were measured. Root architectures were reconstructed from CT scans and used in the model R-SWMS (root-soil water movement and solute transport) to simulate water potentials in soil and roots in 3D as well as water uptake by growing roots in different depths. CT scans revealed that root development was considerably lower with split layers compared to without. This coincided with a reduction of transpiration, stomatal conductance and shoot growth. Simulated predawn water potentials were lower in the presence of split layers. Simulations showed that this was caused by an increased resistance to vertical water flow in the soil by the split layers. Comparison between measured and simulated soil water potentials proved that the split layers were not perfectly isolating and that redistribution of water from the lower, wetter compartments to the drier upper compartments took place, thus water losses were not equal to the root water uptake from those compartments. Still, the layers increased the resistance to vertical flow which resulted in lower simulated collar water potentials that led to reduced stomatal conductance and growth.