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Plant, Cell & Environment, 46 (3), ISSN:0140-7791, ISSN:1365-3040

Root hairs and soil water content are crucial in controlling the release and diffusion of root exudates and shaping profiles of biochemical properties in the rhizosphere. But whether root hairs can offset the negative impacts of drought on microbial activity remains unknown. Soil zymography

The spatial arrangement of the soil surrounding the root can improve plant resource acquisition under drought and is closely related to the fate of soil organic carbon (SOC). Thus, the formation of soil structure and the establishment of a stable rhizosheath can potentially improve plant drought resistance and contribute to maintained crop yields during drought events. Yet, soil structure formation is a complex process determined by the interaction between various functional plant and soil properties, such as the soil (micro)biome, root exudation, or root morphological characteristics. To date, it is not understood how water scarcity affects soil aggregation in the vicinity of roots, by which functional traits these drought effects can be modified, and how this feedbacks on the cycling of SOC. Thus, we investigated drought effects on rhizosheath properties and their link with functional plant traits. We conducted a greenhouse experiment with 38 maize varieties where half of the plants were grown under optimum moisture, while the second half of replicates were subjected to drought stress after an initial establishment phase. For each plant, the rhizosheath soil was sampled and its aggregate size distribution, carbon (C) and nitrogen (N) content, and the proportion of newly maize-derived C were analysed via natural abundance 13C. In addition, we recorded functional plant and rhizosphere traits, such as morphological and chemical root properties, microbial enzyme activities, and plant biomass.Drought-stressed plants formed lower amounts of rhizosheath, with a decreased physical aggregate stability and increased concentrations of SOC, N, and newly maize-derived C. Furthermore, under drought larger proportions of the elements were allocated into the microaggregate fractions. In particular, maize-derived C, along with N, accumulated under drought stress in the smaller aggregate size classes of the rhizosheath. Maize varieties forming larger amounts of roots under drought stress tended to maintain higher macroaggregate stability in the rhizosheath. In contrast, cultivars that invested little in root growth but promoted higher microbial enzyme activities in the rhizosheath and maintained root N contents under drought were associated with a strong accumulation of maize-C and N in the smaller aggregate size classes. Trait-based experimental approaches, such as the one presented here, are deepening our mechanistic understanding of drought effects in the crop rhizosheath and can thus help to guide future crop selection for improved drought resistance.

The partitioning between soil evaporation and transpiration from plants is an important process of water and carbon cycles and surface energy balance. Its quantification is prone to errors because of the complexity of flow geometry, which is affected by the variation of root length density over depth and time and the dynamics of soil hydraulic properties in the rhizosphere. Root water uptake and concurrent evaporation depend on the forces (capillarity, gravity, and viscous losses) controlling water flow and propagation of the drying front. We simulate the water flow from the soil to the atmosphere using invasion percolation models, draining elements as a function of the retaining forces depending on the lengths of the potential flow paths. The partitioning between evaporation and transpiration is simulated for different pore size distributions, root length densities, and vegetation covers controlling the transpiring area. Starting with a three dimensional percolation model (to reproduce the connectivity of the liquid phase) at the column scale consisting of elements in the submillimeter range, we deduce one-dimensional partitioning rules for wet and dry soils. As an outlook, we discuss how these rules can be (i) implemented in large scale models and (ii) tested by measuring vapor fluxes above and below canopy.

Sufficient water supply to the roots is needed to sustain transpiration demand. However, detailed measurements of water fluxes into the roots and the gradients in water potential driving these fluxes are rare, particularly in the field, mainly due to the difficulty in accessing roots and performing measurements on them. As a result, field monitoring of tree water use often neglects the root system, and the water fluxes from the soil to the shoot of trees remain a frontier in soil plant water relations. This measurement gap and lack of understanding of water fluxes in roots makes it difficult to properly determine what drives root water uptake and how it might vary depending on rooting depth, soil matric potential and transpiration rate in the field.During dry summer 2022, we equipped beech and spruce trees with sap flow sensors and dendrometers on the stem and on roots accessing different soil depths. This allowed us to monitor water fluxes and potentials in the different plant parts, from the integrated fluxes in the stem to the water uptake of single roots. We conducted the measurements at the “Waldlabor” ecoydrological monitoring forest in Zurich, where we comprehensively monitor the soil-plant-atmosphere continuum of beech and spruce with dendrometers, sap flow sensors and frequently measured stem & leaf water potential as well as stomatal conductance. To uncover the roots with minimal disturbance we removed the soil with a special air pressurizer and vacuum pump that allowed soil removal without damaging the roots, installed sensors at roots accessing different soil depths and afterwards covered the roots with soil again. Soil matric potential was measured at 10, 20, 40 and 80 cm depth in proximity to selected roots. At the canopy level, we measured stomatal conductance and leaf water potential throughout the summer.Here, we demonstrate how the collected data help to understand to which extent trees diversify their water uptake depending on water availability at different soil depths. Because of the scarce precipitation and the limiting soil water availability during the summer, pre-dawn leaf water potential, stomatal conductance as well as sap flow decreased, indicating a reduction in transpiration. Beech trees reduced their stomatal conductance more dramatically than spruce trees, thereby using soil water more quickly. The comparison of sap flow in the roots and the integrated signal measured along the stem reveals differences between roots, with roots accessing deeper soil upholding higher sap flow velocities than roots accessing shallow soil in both species.The results allow to assess the interplay between aboveground tree hydraulics and water status and belowground water uptake for beech and spruce under drying conditions.

Soil is considered the largest sink of microplastics (MP) in terrestrial ecosystems. Among the expected effects of MP as hydrophobic surface addition is the likelihood that MP enhances soil water repellency. So, crucial for MP fate in soils is the interaction between MP and water. If MP is translocated by water flow and, vice versa, MP impacts water flow, to what extent? Water flow on the pore scale will be impacted with feedbacks on transport and retention of MP. However, we don’t know the extent of and conditions under which MP are transported through porous media and, if deposited, how they interplay with soil water dynamics. We hypothesize that: (i) isolated MP are displaced and translocated by air-water interfaces and (ii) local accumulation of MP is facilitated by bypassing water flow. To approach this question, neutron and x-ray imaging of MP and water in soils was utilized.Dual neutron and x-ray imaging at the beamlines ICON (Paul-Scherrer-Institute) during repeated wetting-drying cycles was applied to trace MP-water interactions in aluminum cylinders filled with sand (0.7-1.2 mm) and MP (PET, 20-75 µm) in gravimetric contents of 0.35, 1.05 and 2.10%. The contents refer to static contact angle estimations of the mixtures resembling < 90°, 90° and > 90°. First, simultaneous neutron and x-ray tomography captured the initial dry MP configuration in samples. Subsequently, neutron radiographies of deuterated water flow through the sample of 1 ml min-1 were recorded for 200s. After drying, repeated tomography gave insights into MP translocation.Neutron and x-ray imaging results showed that regions of major MP content are water repellent. Water flow bypasses and MP is mainly retained. Resultant air entrapments lead to reduced water contents. In regions of minor MP content water can infiltrate. Here, the air-water interface collects isolated MP and shifts their distribution towards an enhanced accumulation.Extrapolation of these results to natural soil systems suggests that vertical transport of MP can be limited especially at hotspots of high MP contents. Water bypasses here. This might limit the water dependent degradation processes of MP due to reductions in hydrolysis, coating and colonization by microorganisms even elongating the process of natural attenuation.

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

At a critical soil water content (θcrit), terrestrial ecosystem fluxes at the soil-vegetation-atmosphere interface transition from energy into water limitation. Understanding and predicting soil, plant and atmospheric mechanisms that control θcrit are central to interpreting and predicting impacts of drought on ecosystems, including the associated feedbacks to carbon and hydrological cycle. Thanks to the existing monitoring networks, θcrit can now be estimated globally across soils, biomes and climates. However, the mechanisms and key parameters that explain θcrit as a result of soil-, plant-, and climate-interaction remain elusive. Here, we show that the soil hydraulic conductivity function determines mean and variability of θcrit. The underlying concept to calculate θcrit assumes that soil moisture limitation of transpiration is triggered by a loss in soil hydraulic conductivity around the roots. Taking soil-specific hydraulic properties into account, our soil-plant hydraulic model predicts the observed mean and variance of θcrit as a function of soil textural classes. In coarse textured soils, θcrit is small due to the lower absolute soil hydraulic conductivity and its steeper decline with soil drying compared to fine textured soils. The increasing variability of θcrit in fine-textured soils is explained by (i) the wide range of hydraulic conductivity values for similar soil textures as a result of soil structure formation and (ii) by the higher sensitivity to plant traits and climate for soils with less steep hydraulic conductivity curves (i.e., loamy soils). The corresponding critical soil matric potential (hcrit) is also soil texture specific, and it covers a broad range of values, from values close to field capacity in sandy soils (hcrit ca. -100 hPa) to values close to the wilting point in clay soils (hcrit ca. – 1 MPa). The model implies that climate change has a smaller effect on θcrit in sandy soils, suggesting that soil texture modulates climate effects on water use and photosynthesis globally. Overall, our results prove the prominent role of soil hydraulic conductivity for water limitation of ecosystem fluxes and for plants’ potential to adjust to water limitations subject to alterations due to climate change.

Plant roots and bacteria release in soils polymeric blends of substances which alter the physics of soil water flow and support life in soils. They adsorb water, decrease the surface tension of the soil solution and increase its viscosity, and, more generally, they change the soil solution into a non-Newtonian liquid with viscoelastic properties. A theory of drying of polymer solutions in porous media is missing and it is needed to understand the feedback between physics of porous media and life in soils. It was observed that during drying polymer solutions are deposited as thin surfaces spanning multiple pores and that these depositions are associated with a decrease in evaporation rate. Here, we provide a physical explanation of surface formation. The modeling framework includes Darcian flow across the polymeric network driven by a gradient in water potential. As the polymer dries and air invades the pore space the polymer network is stretched. The stretching causes a stress in the polymer network that alters the relation between water potential and polymer concentration: the more stretched is the polymer network the smaller is the spacing between the polymers at a given water potential, and the lower is the permeability of the network. The model predicts that at a critical point during evaporation there is an asymptotic increase in polymer concentration at the gas-gel interface corresponding to the deposition of solid-brittle interfaces. The onset of this glass transition depends on flow rate and pore size, with earlier deposition for fast high evaporative fluxes and small pores. The model explains why evaporation is suppressed much earlier and more significantly when the polymer solution dries into a porous medium, in comparison to the case when the polymer solution is free to dry outside a porous medium.

As safety mechanism during droughts, plants close their stomata to reduce transpiration losses and prevent excessively negative water potentials. Although the coordination between stomatal closure, xylem vulnerability and leaf traits has been extensively investigated, the role of soil hydraulic limitation remains elusive. Here, we test the hypothesis that stomatal closure is triggered by the loss of soil hydraulic conductivity in the root zone. As the soil hydraulic conductivity is a function of soil texture, we further hypothesize that stomatal closure, and more precisely the relation between stomatal conductance and leaf water potential, are soil texture dependent. An alternative hypothesis is that the shoot to root ratio adapts to the specific soil conditions, and it is lower in soils with low hydraulic conductivity. We compared three field sites in an inner alpine valley in Switzerland (yearly rainfall of 600 mm) with the same oak tree species (Quercus pubescens) and varying soil textures. We used the model of Carminati and Javaux (2020), which predicts that the decline in transpiration rate is controlled by the soil hydraulic conductivity, and, consequently, it is more abrupt in 1) coarse textured soils compared to fine textured soils and 2) in plants with high shoot to root ratio.To test these working hypotheses, stem water status and soil matric potential were measured at various depths of three oak sites. The soil matric potentials were then linked to the hydraulic conductivity and soil water content using a new pedotransfer function (PTF) for forest soils, which overcomes the limits of existing PTFs that were trained for arable soils. A simple water balance model based on changes in water content (deduced from PTF and measured potentials) was used to calculate transpiration rates and was compared with sap flow measurements conducted on two trees per site. Leaf water potentials were estimated from dendrometers after calibration with pressure chamber measurements of leaves. The sap flow measurements correlate well with estimated transpiration rates (R2=0.7).Comparisons between sites show a similar decrease in stomatal conductance at all three sites during drought, regardless of soil texture. Rather, our data suggest the trees hydraulically adapt to the local soil texture by adjusting their leaf area index and root length density to the local water demand and supply. In coarse-textured soils, oak had a low leaf area index, reducing their water demand, and high fine root length density, increasing their supply. In conclusion, our study suggests a hydraulic adaptation of trees to their local soil texture by adapting their “shoot to root ratio” as quantified here by leaf area index and root density.

Salinity and soil drying are expected to induce salt accumulation at the root–soil interface of transpiring plants. However, the consequences of this on the relationship between transpiration rate (E) and leaf xylem water potential (ψleaf-x) are yet to be quantified. Here, we used a noninvasive root pressure chamber to measure the E(ψleaf-x) relationship of tomato (Solanum lycopersicum L.) treated with (saline) or without 100-mM NaCl (nonsaline conditions). The results were reproduced and interpreted with a soil–plant hydraulic model. Under nonsaline conditions, the E(ψleaf-x) relationship became progressively more nonlinear as the soil dried (θ ≤ 0.13 cm3 cm−3, ψsoil = −0.08 MPa or less). Under saline conditions, plants exhibited an earlier nonlinearity in the E(ψleaf-x) relationship (θ ≤ 0.15 cm3 cm−3, ψsoil = −0.05 MPa or less). During soil drying, salinity induced a more negative ψleaf-x at predawn, reduced transpiration rate, and caused a reduction in root hydraulic conductance (from 1.48 × 10−6 to 1.30 × 10−6 cm3 s−1 hPa−1). The model suggested that the marked nonlinearity was caused by salt accumulation at the root surface and the consequential osmotic gradients. In dry soil, most water potential dissipation occurred in the bulk soil and rhizosphere rather than inside the plant. Under saline-dry conditions, the loss in osmotic potential at the root surface was the preeminent component of the total dissipation. The physical model of water flow and solute transport supports the hypothesis that a buildup of osmotic potential at the root–soil interface causes a large drop in ψleaf-x and limits transpiration rate under drought and salinity.

The efficiency-safety tradeoff has been thoroughly investigated in plants, especially concerning their capacity to transport water and avoid embolism. Stomatal regulation is a vital plant behaviour to respond to soil and atmospheric water limitation. Recently, a stomatal efficiency-safety tradeoff was reported where plants with higher maximum stomatal conductance (gmax) exhibited greater sensitivity to stomatal closure during soil drying, that is, less negative leaf water potential at 50% gmax (ψgs50). However, the underlying mechanism of this gmax-ψgs50 tradeoff remains unknown. Here, we utilized a soil-plant hydraulic model, in which stomatal closure is triggered by nonlinearity in soil-plant hydraulics, to investigate such tradeoff. Our simulations show that increasing gmax is aligned with less negative ψgs50. Plants with higher gmax (also higher transpiration) require larger quantities of water to be moved across the rhizosphere, which results in a precipitous decrease in water potential at the soil-root interface, and therefore in the leaves. We demonstrated that the gmax-ψgs50 tradeoff can be predicted based on soil-plant hydraulics, and is impacted by plant hydraulic properties, such as plant hydraulic conductance, active root length and embolism resistance. We conclude that plants may therefore adjust their growth and/or their hydraulic properties to adapt to contrasting habitats and climate conditions.

In drying soils, root water uptake is limited by the low soil hydraulic conductance. The magnitude of this conductance drop and its temporal dynamics are function of soil texture, soil water status, root hydraulic architecture, atmospheric demand and canopy conductance. Under dry climates, in order to survive, plants can adapt their carbon allocation by maximizing their root:shoot surface ratio, thereby decreasing their transpiration surface while increasing their root surface.Thanks to a simple soil-plant hydraulic model, we show that soil hydraulic conductivity controls the minimum root:shoot surface ratio. A meta-analysis of shoot:root surface ratio is combined with a database of soil hydraulic properties to demonstrate how the minimum root:shoot surface value changes with soil conductivity across soil textural classes for dry biomes. We discuss the mechanisms by which plants can control their carbon allocation in such conditions and investigate the sensitivity of this minimum root:shoot surface ratio to future shifts in evaporative demand.

Addition of microplastics (MP) to soil has the potential to increase soil water repellency. However, coating of MP with soil abundant substances e.g., iron compounds, can reduce this effect. Here, we tested if pre-coating or in situ coating of MP with ferrihydrite (Fh) reduces soil water repellency. We applied hotspots of pristine and coated MP (20-75 mu m, PS and PET) to sand and imaged capillary rise via neutron radiography. Capillary rise experiments in wetting-drying cycles were conducted using water and Fh suspension. Pristine MP hotspots were not wettable. Capillary rise of water into coated MP hotspots differed in wettability depending on polymer type. While coated PS was still non-wettable, water imbibed into the coated PET hotspot. Capillary rise of Fh suspensions in wetting and drying cycles also showed varying results depending on polymer type. MP hotspots were still non-wettable and local water content increased only marginally. Our results indicate that Fh coating of MP changes MP surface wettability depending on polymer type and therefore counteracts the hydrophobic properties of pristine MP. However, MP coating is likely to be slowed down by the initial hydrophobicity of pristine MP. Dynamics of MP coating and increasing wettability are key factors for biotic and abiotic degradation processes., Environmental Science: Processes & Impacts, 25 (6), ISSN:2050-7887, ISSN:2050-7895

Background and Aims Stomatal regulation allows plants to promptly respond to water stress. However, our understanding of the impact of above and belowground hydraulic traits on stomatal regulation remains incomplete. The objective of this study was to investigate how key plant hydraulic traits impact transpiration of maize during soil drying. We hypothesize that the stomatal response to soil drying is related to a loss in soil hydraulic conductivity at the root–soil interface, which in turn depends on plant hydraulic traits. Methods We investigate the response of 48 contrasting maize (Zea mays) genotypes to soil drying, utilizing a novel phenotyping facility. In this context, we measure the relationship between leaf water potential, soil water potential, soil water content and transpiration, as well as root, rhizosphere and aboveground plant traits. Key Results Genotypes differed in their responsiveness to soil drying. The critical soil water potential at which plants started decreasing transpiration was related to a combination of above and belowground traits: genotypes with a higher maximum transpiration and plant hydraulic conductance as well as a smaller root and rhizosphere system closed stomata at less negative soil water potentials. Conclusions Our results demonstrate the importance of belowground hydraulics for stomatal regulation and hence drought responsiveness during soil drying. Furthermore, this finding supports the hypothesis that stomata start to close when soil hydraulic conductivity drops at the root–soil interface.

Aims Although soil water deficit is the primary constraint on transpiration globally, the mechanisms by which soil drying and soil properties impact transpiration and stomatal regulation remain elusive. This work aimed to investigate how soil textures and vapor pressure deficit (VPD) impact the relationship between transpiration rate, canopy conductance, and leaf water potential of maize (Zea mays L.) during soil drying. We hypothesize that the decrease in soil–plant hydraulic conductance (Ksp) triggers stomatal closure and the latter is soil specific. Methods Plants were grown in two contrasting soil textures (sand and loam) and exposed to two consecutive VPD levels (1.8 and 2.8 kPa). We measured transpiration rate, canopy conductance, soil and leaf water potentials during soil drying. Results Transpiration rate decreased at higher soil matric potential in sand than in loam at both VPD levels. In sand, high VPD generated a steeper drop in canopy conductance with decreasing leaf water potential. The decrease in canopy conductance was well correlated with the drop in Ksp, which was significantly affected by soil texture. Conclusions Our results demonstrated that variations in canopy conductance were not simply a function of leaf water potential but largely affected by soil hydraulics. These results reinforce a model of stomatal closure driven by a loss in soil hydraulic conductivity. Further studies will determine if soil-specific stomatal regulation exists among species.

Understanding stomatal regulation during drought is essential to correctly predict vegetation-atmosphere fluxes. Stomatal optimization models posit that stomata maximize the carbon gain relative to a penalty caused by water loss, such as xylem cavitation. However, a mechanism that allows the stomata to behave optimally is unknown. Here, we introduce a model of stomatal regulation that results in similar stomatal behaviour without presupposing an optimality principle. By contrast, the proposed model explains stomatal closure based on a well-known component of stomatal regulation: abscisic acid (ABA). The ABA level depends on its production rate, which is assumed to increase with declining leaf water potential, and on its degradation rate, which is assumed to increase with assimilation rate. Our model predicts that stomata open until the ratio of leaf water potential to assimilation rate, proportional to ABA level, is at a minimum. As a prerequisite, the model simulates soil-plant hydraulics and leaf photosynthesis under varying environmental conditions. The model predicts that in wet soils and at low vapour pressure deficit (VPD), when there is no water limitation, stomatal closure is controlled by the relationship between photosynthesis and stomatal conductance. In dry soils or at high VPD, when the soil hydraulic conductivity limits the water supply, stomatal closure is triggered by the sharp decline in leaf water potential as transpiration rate increases. Being adaptive to changing soil and atmospheric conditions, the proposed model can explain how plants are enabled to avoid critical water potentials during drought for varying soil properties and atmospheric conditions., Ecohydrology, 15 (5), ISSN:1936-0592, ISSN:1936-0584

Pathways of Microplastic (MP) into ecosystems are manifold and range from agricultural mulching practices to atmospheric deposition with soil being considered the largest sink of MP in terrestrial ecosystems. Once deposited there, MP is posing a hydrophobic surface addition. Former experiments showed that pristine MP can cause lower water saturation of pore spaces and so change the liquid configuration within a porous network. If water cannot reach MP, biotic degradation might be hindered. However, in natural soil systems MP can be coated over time by soil abundant substances e.g., iron compounds with the potential effect of decreasing their hydrophobicity. We hypothesize that: 1) ferrihydrite pre-coated MP shows reduced hydrophobicity; 2) in-situ wetting and drying cycles with ferrihydrite leads to partial coating of MP.We tested these hypotheses by applying hotspots of MP, pre-coated and pristine, to sand in rectangular columns and performed neutron imaging during capillary rise. Neutron imaging allowed for visualizing and quantifying liquid dynamics and configuration. Water was used for the pre-coated MP (n=6) variants and ferrihydrite suspension (100 mg L-1) in three wetting and drying cycles for the pristine MP (n=6) variants. The utilized MP are polystyrene (PS, 20-75 µm) and polyethylene terephthalate (PET, 20-75 µm). The grain size of sand was 0.7-1.2 mm. Pre-coating was achieved by shaking the raw material for 3 h in a 100 mg L-1 ferrihydrite suspension and subsequent drying in a sieve supported by a vacuum pump.Capillary rise of water into pristine MP variants exhibited zero water saturation at the hotspot and water movement around the MP aggregation was observed. Capillary rise of water into pre-coated MP variants differ in result by polymer type. While pre-coated PS is still hydrophobic, the pore space of pre-coated PET was completely water saturated. The rising water accelerated towards the hotspot due to its lower matric potential compared to sand.Capillary rise of ferrihydrite suspension in wetting and drying cycles also showed varying results according to polymer type. While there is no effect on water saturation on PS in the hotspot after three wetting cycles, PET exhibits a slightly higher water saturation during the second wetting but stagnating in the third.Our results suggest that ferrihydrite coating, being only one of numerous potential coating agents, can bond to MP and change its surface polarity. Differences in completeness of coating can be explained by inherent chemical and physical properties of different polymer types. But once hydrophilic, completely, or only part of the surface, water flow induced colonization and migration of microorganisms and their enzymes can proceed and biotic degradation can take place. The open question lies within the time frame necessary to overcome MP’s inherent hydrophobicity.

Stomatal closure allows plants to promptly respond to water shortage. Although the coordination between stomatal regulation, leaf and xylem hydraulics has been extensively investigated, the impact of below-ground hydraulics on stomatal regulation remains unknown.We used a novel root pressure chamber to measure, during soil drying, the relation between transpiration rate (E) and leaf xylem water pressure (ψleaf-x) in tomato shoots grafted onto two contrasting rootstocks, a long and a short one. In parallel, we also measured the E(ψleaf-x) relation without pressurization. A soil–plant hydraulic model was used to reproduce the measurements. We hypothesize that (1) stomata close when the E(ψleaf-x) relation becomes non-linear and (2) non-linearity occurs at higher soil water contents and lower transpiration rates in short-rooted plants.The E(ψleaf-x) relation was linear in wet conditions and became non-linear as the soil dried. Changing below-ground traits (i.e. root system) significantly affected the E(ψleaf-x) relation during soil drying. Plants with shorter root systems required larger gradients in soil water pressure to sustain the same transpiration rate and exhibited an earlier non-linearity and stomatal closure.We conclude that, during soil drying, stomatal regulation is controlled by below-ground hydraulics in a predictable way. The model suggests that the loss of hydraulic conductivity occurred in soil. These results prove that stomatal regulation is intimately tied to root and soil hydraulic conductances.

Limited water supply is one of the largest impediments to food production worldwide in the light of climate change and increasing food demand. Stomatal regulation allows plants to promptly react to water stress and regulate water use. Although the coordination between stomatal closure and aboveground hydraulics has extensively been studied, our understanding of the impact of belowground hydraulics on stomatal regulation remains, as yet, incomplete. The overall objective of this study was to investigate the impact of belowground hydraulic conductivity as affected by differences in expressions of root and rhizosphere traits on the water use regulation of different maize genotypes.We have utilized a novel phenotyping facility to investigate the response of a selection of 48 maize (Zea mays L.) genotypes exhibiting different root and rhizosphere traits to soil drying. We measured the relation between leaf water potential, soil water potential, soil water content and transpiration rate, as well as root and rhizosphere traits (e.g. root length, rhizosheath mass) between genotypes. Our hypothesis is that stomatal response to soil drying is related to a loss in soil hydraulic conductivity and that key root and rhizosphere hydraulic traits affect such relation.We found that the genotypes differed in their responsiveness to drought and that such differences were related to belowground hydraulic traits. The critical soil water content at which plants started to decrease transpiration was related to a combination of plant- and rhizosphere traits (namely plant hydraulic conductance, maximum transpiration rate, root length and rhizosheath mass). Genotypes with a higher maximum transpiration rate and a higher plant hydraulic conductance and a smaller root system closed stomata in wetter soil conditions, meaning earlier in the drying process. This finding is explained by a soil-plant hydraulic model that assumes that stomata start to close when the soil hydraulic conductance of the soil-plant continuum starts to decline. Those findings stress the importance of belowground hydraulic properties on stomatal regulation and thereby drought responsiveness.

Evapotranspiration (ET) modeling is central to resolving water and energy balances and linking the terrestrial water and carbon cycles. An important challenge remains how to partition the ET flux into transpiration (T) and soil evaporation (E). We expand the surface evaporation capacitor model of Or and Lehmann (2019) by considering concurrent root water uptake. The original capacitor model simulates surface evaporation (focusing on stage-1) and internal redistribution following rainfall events. The thickness of the evaporation-active soil layer is defined by an intrinsic soil property termed the evaporation characteristic length (deduced from soil water characteristics and hydraulic conductivity functions). The modified model considers water extraction by plant roots from the capacitor layer as well as water redistributed into deeper layers (sheltered from soil evaporation but accessible for root water uptake). Depending on the amount of water leaking below the capacitor depth, vegetation can take up this natural storage at rates limited by the hydraulic properties of the rhizosphere. To model evapotranspiration and its partitioning at the global scale, we use spatial information on (i) maximum root depth and (ii) soil hydraulic properties defining the depth of the capacitor layer. We assess the performance of the ET capacitor model in comparison with results from available land surface models and estimates based on remote sensing products.

Although the impact of root hairs (RHs) in nutrients uptake is well documented, their role in water uptake and drought tolerance remains controversial. Maize wild type and its hair-defective mutant (rth3) were grown in two contrasting soil textures (sand and loam). We used a novel root pressure chamber to measure the relation between transpiration rate (E) and leaf xylem water potential (ψleaf_x) during soil drying. The hypotheses were: 1) RHs extend the root-soil contact and reduce the decline in ψleaf_x at high E in dry soils; 2) the impact of RHs is more pronounced in sand; and 3) mutants partly compensate for the lack of RHs by producing longer and/or thicker roots. The ψleaf_x(E) relation was linear in wet conditions and became nonlinear as the soils dried. The nonlinearity of the relation occurred more abruptly and at less negative matric potentials in sand (ca. -10 kPa) than in loam (ca. -100 kPa). At slightly more negative soil matric potentials, soil hydraulic conductance became smaller than root hydraulic conductance in both soils. Both genotypes exhibited ca. 1.7 times longer roots in loam, but 1.6 times thicker roots in sand. No differences were observed in the ψleaf_x(E) relation and active root length between the two genotypes. Root hairs had no contribution to soil-plant hydraulics in maize in both sand and loam. These results suggest that the role of root hairs cannot be easily generalized across species and the response of root hydraulics to soil drying is remarkably affected by soil textures.

The occurrence of drought is likely to increase and intensify as a result of climate change, which poses a great challenge to agriculture. It is thus crucial to enhance agronomic resilience to secure food and feed production. Roots and root functioning as well as the interplay of roots with the surrounding soil, the rhizosphere, plays a key role in water acquisition of plants. Investigating rhizosphere traits is hence promising to shed light on future crops that better adapt to drought stress. A great strength of this study is the screening of various varieties which is facilitated by the high-throughput phenotyping method. It allows a wider coverage of traits and especially the genetic and phenetic diversities preserved in landraces.Maize (Zea mays L.), being one of the major cereal crops worldwide, was selected as the plant of study. A total of 38 varieties, which encompasses hybrid varieties, open pollinated varieties, and landraces, were screened in the “Moving Fields”, a greenhouse equipped with the high-throughput phenotyping facility in the Bavarian State Research Center for Agriculture. Maize plants were grown in mesocosms filled with loamy soil. Plants were exposed to two water treatments, well-watered and drought-stressed, during vegetative stem extension stage. Dynamic plant development was captured by continuous image acquisition. A visible light (RGB) camera was used to document the size and architecture of shoots and roots, while a chlorophyll fluorescence camera recorded the metabolic activity of shoots.Using shoot images, we compared variety-specific plant growth curves under well-watered and drought-stressed conditions to highlight the growth strategy of plants towards drought stress. The results reveal differences in growth inhibition during drought across varieties. In addition, differences in shoot and root dry weights are found between landraces and modern varieties. More analyses are in progress in search of rhizosphere traits and their influences on agronomic resilience.

Mucilage is a gelatinous high-molecular-weight substance produced by almost all plants, serving numerous functions for plants and soil. To date, research has mainly focused on the hydraulic and physical functions of mucilage in the rhizosphere. Studies on the relevance of mucilage as a microbial habitat are scarce. Microbial research has largely focused on extracellular polymeric substances (EPS), gelatinous high-molecular-weight substances produced by microorganisms. In soil, EPS support the establishment of microbial assemblages by providing a moist environment, a protective barrier, and serving as carbon and nutrient sources. Our analyses show that mucilage shares the physical and chemical properties of EPS. Mucilage covers large extents of the rhizosphere and could function similarly to the biofilm matrix. Our laboratory and theoretical analyses largely confirmed similar viscosity and surface tension as important physical properties and polysaccharide, protein, neutral monosaccharide, and uronic acid composition as major chemical properties. Our study suggests that mucilage provides functions of EPS required for biofilm formation. Mucilage offers a protected habitat optimized for nutrient mobilization and provides carbon and nutrients. We suggest that the function of mucilage as a biofilm matrix and enabler of high rhizo-microbial abundance and activity has been strongly underestimated, and should be considered as an essential component of conceptual models of the rhizosphere.

Soil drying is a limiting factor for crop production worldwide. Yet, it is not clear how soil drying impacts water uptake across different soils, species, and root phenotypes. Here we ask (1) what root phenotypes improve the water use from drying soils? and (2) what root hydraulic properties impact water flow across the soil-plant continuum? The main objective is to propose a hydraulic framework to investigate the interplay between soil and root hydraulic properties on water uptake. We collected highly resolved data on transpiration, leaf and soil water potential across 11 crops and 10 contrasting soil textures. In drying soils, the drop in water potential at the soil-root interface resulted in a rapid decrease in soil hydraulic conductance, especially at higher transpiration rates. The analysis reveals that water uptake was limited by soil within a wide range of soil water potential (-6 to -1000 kPa), depending on both soil textures and root hydraulic phenotypes. We propose that a root phenotype with low root hydraulic conductance, long roots and/or long and dense root hairs postpones soil limitation in drying soils. The consequence of these root phenotypes on crop water use is discussed., Plant, Cell & Environment, 45 (3), ISSN:0140-7791, ISSN:1365-3040