Root Hydraulic Properties Research Articles

Unique root hydraulic and mechanical properties support the resilience of grapevines adapted to the Atacama Desert.

Cortical lacunae caused by drought, especially observed in hybrids originating from Vitis rupestris, disrupt the connection between roots and soil. Yet, the physiological processes behind lacuna formation during drought and its consistency across Vitis species remain unclear. Here, we used a root pressure probe to investigate fine root hydraulic and mechanical properties, in the arid-adapted R-65 and drought-susceptible 101-14Mgt cultivars. We then performed P-V curves, root sap osmolality, and electrolyte leakage (EL) and used fluorescent light microscopy techniques. Only 101-14Mgt showed lacunae formation during drought due to its stiffer cortical tissue, unlike R-65. Lacunae resulted in a notable decline in root hydraulic conductivity during severe drought, with increased EL and root sap osmolality, indicating potential cellular damage. R-65 displayed different and xerophyte-like characteristics featuring a higher turgor loss point and decreased root capacitance, essential for maintaining root structural integrity in arid conditions. Our findings highlight lacuna formation is impacted by root tissue elasticity possibly linked to specific Vitis species favoring deeper rooting. In arid-adapted grapevines, hydraulic regulators such as reduced turgor loss point, and root capacitance could contribute to enhanced drought tolerance.

Mechanistically derived macroscopic root water uptake functions: The α and ω of root water uptake functions

AbstractWater uptake by plant roots is an important component of the soil water balance. Predicting to what extent potential transpiration from the canopy, that is, transpiration demand, can be met by supply of water from the soil through the root system is crucial to simulate the actual transpiration and assess vegetation water stress. In models that simulate the dynamics of vertical soil water content profiles as a function of water fluxes and soil water potential gradients, the root water uptake (RWU) distribution is represented by macroscopic sink terms. We present RWU functions that calculate sink terms based on a mechanistic model of water flow in the soil–root system. Based on soil–root hydraulics, we define α‐supply functions representing the maximal uptake by the root system from a certain soil depth when the root collar water potential equals the wilting point, ω‐supply factors representing the maximal supply from the entire root system, and a critical ωc factor representing the potential transpiration demand. These functions and factors are subsequently used to calculate RWU distributions directly from potential transpiration or demand and the soil water potentials. Unlike currently used approaches, which define α‐stress functions and ω factors representing ratios of actual uptake to uptake demand, the supply‐based formulations can be derived directly from soil and root hydraulic properties and can represent processes like root hydraulic redistribution and hydraulic lift.

Root hydraulic properties: An exploration of their variability across scales.

Root hydraulic properties are key physiological traits that determine the capacity of root systems to take up water, at a specific evaporative demand. They can strongly vary among species, cultivars or even within the same genotype, but a systematic analysis of their variation across plant functional types (PFTs) is still missing. Here, we reviewed published empirical studies on root hydraulic properties at the segment-, individual root-, or root system scale and determined its variability and the main factors contributing to it. This corresponded to a total of 241 published studies, comprising 213 species, including woody and herbaceous vegetation. We observed an extremely large range of variation (of orders of magnitude) in root hydraulic properties, but this was not caused by systematic differences among PFTs. Rather, the (combined) effect of factors such as root system age, driving force used for measurement, or stress treatments shaped the results. We found a significant decrease in root hydraulic properties under stress conditions (drought and aquaporin inhibition, p < .001) and a significant effect of the driving force used for measurement (hydrostatic or osmotic gradients, p < .001). Furthermore, whole root system conductance increased significantly with root system age across several crop species (p < .01), causing very large variation in the data (>2 orders of magnitude). Interestingly, this relationship showed an asymptotic shape, with a steep increase during the first days of growth and a flattening out at later stages of development. We confirmed this dynamic through simulations using a state-of-the-art computational model of water flow in the root system for a variety of crop species, suggesting common patterns across studies and species. These findings provide better understanding of the main causes of root hydraulic properties variations observed across empirical studies. They also open the door to better representation of hydraulic processes across multiple plant functional types and at large scales. All data collected in our analysis has been aggregated into an open access database (https://roothydraulic-properties.shinyapps.io/database/), fostering scientific exchange.

Contrasting Regulation of Phaseolus vulgaris Root Hydraulic Properties Under Drought and Saline Conditions by Three Arbuscular Mycorrhizal Fungal Species From Soils with Divergent Moisture Regime

PurposeArbuscular mycorrhizal fungi (AMF) may help plants to overcome abiotic stresses, in part by improving their water uptake capacity. However how different AMF isolated from different climatic regions regulate plant abiotic stress tolerance and water uptake capacity is barely studied. The aim of this study was to reveal how three AMF isolated from two Mediterranean climate locations contrasting in annual precipitation, modify bean (Phaseolus vulgaris L.) root hydraulic properties facing drought and salinity.MethodsRhizophagus intraradices (Ri) and Funneliformis mosseae (Fm) were isolated from a humid area, whereas Claroideoglomus etunicatum (Ce) was isolated from a dry location. All plants (inoculated or not) were subjected to four days of withholding water or salt treatment. Root hydraulic properties including root hydraulic conductivity and aquaporin expression and abundance were determined.ResultsAll three AMF isolate induced significant differences in plant physiology regardless their different mycorrhizal colonization extent. Drought treatment diminished root hydraulic conductivity and only Fm inoculated plants featured measurable amount of sap exudate. After salt irrigation, AMF inoculation counterbalanced the drop of root hydraulic conductivity. In such situation two AMF, Fm and Ce, presented lowered phosphorylated (Ser-283) PIP2 AQP amount. AQP gene expression highlighted the importance of PvPIP1;2 and PvPIP2;3 plasticity in plants facing osmotic stress. After drought treatment AMF species from the humid location, Ri and Fm, improved plant water status and Fm enhanced root hydraulic conductivity, whereas all AMF performed similarly after salt irrigation, enhancing stomatal conductance and root hydraulic conductivity.ConclusionUnder drought conditions, the AMF isolates from humid regions were the ones that most effectively improved plant water relations. However, under salt stress, all three AMF isolates exhibited similar behavior. Therefore, to some extent, the climatic origin of the AMF could have influenced the response of host plants to drought stress, suggesting that those originating from dry areas may not necessarily be the most efficient.

Combining root and soil hydraulics in macroscopic representations of root water uptake

AbstractPlant water uptake and plant and soil water status are important for the soil water balance and plant growth. They depend on atmospheric water demand and the accessibility of soil water to plant roots, which is in turn related to the hydraulic properties of the root system and the soil around root segments. We present a simulation model that describes water flow in the soil–plant system mechanistically considering both root and soil hydraulic properties. We developed an approach to upscale three‐dimensional (3D) flow in the soil toward root segments of a 3D root architecture to a model that considers one‐dimensional flow between horizontal soil layers and radial flow to root segments in that layer. The upscaled model couples upscaled linear flow equations in the root system with an analytical solution of the nonlinear radial flow equation between the soil and roots. The upscaled model avoids simplifying assumptions about root hydraulic properties and water potential drops near roots made in, respectively, soil‐ and root‐centered models. Xylem water potentials and soil–root interface potentials are explicitly simulated and show, respectively, large variations with depth and large deviations from bulk soil water potentials under dry soil conditions. Accounting for hydraulic gradients in the soil around root segments led to an earlier but slower reduction of transpiration during a drought period and a better plant water status with higher nighttime plant water potentials.

Co-Inoculation with Arbuscular Mycorrhizal Fungi and Dark Septate Endophytes under Drought Stress: Synergistic or Competitive Effects on Maize Growth, Photosynthesis, Root Hydraulic Properties and Aquaporins?

Arbuscular mycorrhizal fungi (AMF) and dark septate fungi (DSE) were simultaneously colonized in the root cells of maize. Single AMF and DSE symbiosis have been proven to improve the drought tolerance of maize. However, the effects of both fungi coexisting in maize roots under drought stress are not yet known. In this study, pot experiments of maize seedlings were conducted through four inoculation treatments (single AMF inoculation of Rhizophagus irregularis, single DSE inoculation of Exophiala pisciphila, co-inoculation of AMF + DSE and non-mycorrhizal inoculation) under well-watered (WW) and drought-stressed (DS) conditions. AMF and DSE colonization status, maize physiology and aquaporin gene expression in maize roots were investigated. The objective of this paper was to evaluate whether AMF and DSE had competitive, independent or synergistic effects on regulating the drought tolerance of maize. When maize seedlings of three inoculation treatments were subjected to drought stress, single AMF inoculation had the highest shoot and root dry weight, plant height, root length, osmotic root hydraulic conductivity and hydrostatic root hydraulic conductivity in maize seedlings. However, co-inoculation of AMF + DSE induced the highest stomatal conductance in maize leaves and the lowest H2O2 and O2•- concentration, membrane electrolyte leakage, intercellular CO2 concentration and gene expression level of ZmPIP1;1, ZmPIP1;2, ZmPIP2;1, ZmPIP2;5 and ZmPIP2;6. In addition, co-inoculation of AMF + DSE also obviously down-regulated the GintAQPF1 and GintAQPF2 expression in R. irregularis compared with single AMF inoculation treatment. Under DS stress, there were competitive relationships between AMF and DSE with regard to regulating mycorrhizal colonization, maize growth, root hydraulic conductivity and the gene expression of aquaporins in R. irregularis, but there were synergistic relationships with regard to regulating membrane electrolyte leakage, oxidative damage, photosynthesis and the aquaporin gene expression of maize seedlings. The obtained results improve our knowledge about how the mechanisms of AMF and DSE coexist, promoting the drought tolerance of host plants.

A mycorrhizal helper bacterium alleviates drought stress in mycorrhizal Helianthemum almeriense plants by regulating water relations and plant hormones

Complex interactions between mycorrhizal helper bacteria (MHBs) and mycorrhizal fungi have rarely been studied in the context of drought. Separately, both MHB inoculation and moderate drought stress have previously shown to enhance desert truffle mycorrhization in Helianthemum almeriense plants, but the combined effect of both of them has not been evaluated yet. The goal of this study is to determine whether mycorrhization between Helianthemum almeriense and the desert truffle Terfezia claveryi can be triggered by applying Pseudomonas mandelii #29 under drought stress and whether this could improve plant water-relations. To investigate this, we evaluated the effect of the combination of MHB inoculation with two different water regimes (well-watered and water-deficit) over the mycorrhizal development, root hydraulic properties, gene expression of AQPs and plant hormones. The results showed that presence of P. mandelii under drought stress caused a synergistic increase of fungal colonization and a higher nutrient uptake. Although drought stress decreased root hydraulic conductivity (Lpr) regardless of whether plants were MHB-inoculated or not, this decrease was buffered in MHB-inoculated plants, concomintantly with regulation of plant hormones, such as abscisic acid, and aquaporin gene expression. Our results indicate that drought-mediated tripartite interactions (P. mandelii #29 x T. claveryi x H. almeriense) are beneficial for the plant to cope with water stress.

Changes in root hydraulic conductivity in wheat (Triticum aestivum L.) in response to salt stress and day/night can best be explained through altered activity of aquaporins.

Salt stress reduces plant water flow during day and night. It is not known to which extent root hydraulic properties change in parallel. To test this idea, hydroponically grown wheat plants were grown at four levels of salt stress (50, 100, 150 and 200 mM NaCl) for 5-8d before harvest (d14-18) and subjected to a range of analyses to determine diurnal changes in hydraulic conductivity (Lp) at cell, root and plant level. Cell pressure probe analyses showed that the Lp of cortex cells was differentially affected by salt stress during day and night, and that the response to salt stress differed between the main axis of roots and lateral roots. The Aquaporin (AQP) inhibitor H2 O2 reduced Lp to a common, across treatments, level as observed in salt-stressed plants during the night. Analyses of transpiring plants and exuding root systems provided values of root Lp which were in the same range as values modeled based on cell-Lp. The results can best be explained through a change in root Lp in response to salt stress and day/night, which results from an altered activity of AQPs. qPCR gene expression analyses point to possible candidate AQP isoforms.

Field scale plant water relation of maize (Zea mays) under drought – impact of root hairs and soil texture

Background and aimsImpact of drought on crop growth depends on soil and root hydraulic properties that determine the access of plant roots to soil water. Root hairs may increase the accessible water pool but their effect depends on soil hydraulic properties and adaptions of root systems to drought. These adaptions are difficult to investigate in pot experiments that focus on juvenile plants.MethodsA wild-type and its root hairless mutant maize (Zea mays) were grown in the field in loam and sand substrates during two growing seasons with a large precipitation deficit. A comprehensive dataset of soil and plant properties and monitored variables were collected and interpreted using simulations with a mechanistic root water uptake model.ResultsTotal crop water use was similar in both soils and for both genotypes whereas shoot biomass was larger for the wild type than for the hairless mutant and did not differ between soils. Total final root length was larger in sand than in loam but did not differ between genotypes. Simulations showed that root systems of both genotypes and in both soils extracted all plant available soil water, which was similar for sand and loam, at a potential rate. Leaf water potentials were overestimated by the model, especially for the hairless mutant in sand substrate because the water potential drop in the rhizosphere was not considered.ConclusionsA direct effect of root hairs on water uptake was not observed but root hairs might influence leaf water potential dependent growth.

Oxidative stress impedes recovery of canola (Brassica napus) plants from waterlogging by inhibiting aquaporin-mediated root water transport

We exposed hydroponically grown canola plants for three days to root hypoxia (waterlogging) followed by re-aeration. Hypoxia decreased root hydraulic conductivity ( L pr ) and the apoplastic contribution to water transport, which impacted gas exchange and plant water relations. However, the most severe reduction of L pr was one day following re-aeration and it was accompanied by an increase in the contribution of apoplast to water flow. After one day of re-aeration, a sharp increase in ROS in roots was measured together with a decrease in transcript abundance of most BnPIP2 aquaporins in roots and leaves. A water permeability assay of these BnPIP2 s heterologously expressed in yeast confirmed that they are fast water transporters. All yeast strains displayed decreased water permeability with increased concentrations of applied H 2 O 2 and exogenous application of H 2 O 2 to plant roots resulted in a decrease of L pr . The yeast H 2 O 2 survival assay demonstrated that BnPIP1;2 and BnPIP1;3 facilitate H 2 O 2 transport and, therefore, the increase in gene expression of these aquaporins during re-aeration likely contributed to plant waterlogging recovery. A gradual recovery of L pr following re-aeration was accompanied by up-regulation of BnPIP s in roots and leaves and the activation of antioxidant enzymes in roots. Net photosynthesis, transpiration rates, and shoot water contents remained depressed one day after re-aeration but recovered over time. Collectively, the results demonstrate that oxidative burst had a decisive impact on modulating the hydraulic recovery of plants upon re-aeration. Since plant injury from root hypoxia involved both the hypoxic event and the ROS burst that occurred soon after root aeration, the ability of plants to recover from waterlogging was partly determined by their ability to handle oxidative stress. • Both root hypoxia and oxidative burst during re-aeration of canola plants had a severe impact on root hydraulic properties and gas exchange. • Increases in gene expression of BnPIPs in hypoxic and reaerated roots likely helped facilitate oxygen and hydrogen peroxide transport. • Sincehypoxia injury involves the hypoxic event and oxidative burst following aeration, waterlogging recovery requires resistance to both stresses.

Quantification of root water uptake and redistribution using neutron imaging: a review and future directions.

Quantifying root water uptake is essential to understanding plant water use and responses to different environmental conditions. However, non-destructive measurement of water transport and related hydraulics in the soil-root system remains a challenge. Neutron imaging, with its high sensitivity to hydrogen, has become an unparalleled tool to visualize and quantify root water uptake in vivo. In combination with isotopes (e.g., deuterated water) and a diffusion-convection model, root water uptake and hydraulic redistribution in root and soil can be quantified. Here, we review recent advances in utilizing neutron imaging to visualize and quantify root water uptake, hydraulic redistribution in roots and soil, and root hydraulic properties of different plant species. Under uniform soil moisture distributions, neutron radiographic studies have shown that water uptake was not uniform along the root and depended on both root type and age. For both tap (e.g., lupine [Lupinus albus L.]) and fibrous (e.g., maize [Zea mays L.]) root systems, water was mainly taken up through lateral roots. In mature maize, the location of water uptake shifted from seminal roots and their laterals to crown/nodal roots and their laterals. Under non-uniform soil moisture distributions, part of the water taken up during the daytime maintained the growth of crown/nodal roots in the upper, drier soil layers. Ultra-fast neutron tomography provides new insights into 3D water movement in soil and roots. We discuss the limitations of using neutron imaging and propose future directions to utilize neutron imaging to advance our understanding of root water uptake and soil-root interactions.

Differential Hydraulic Properties and Primary Metabolism in Fine Root of Avocado Trees Rootstocks.

Avocados (Persea americana Mill.) are one of the crops with the highest water footprints in Chile and the production is at risk due to severe and frequent droughts. The current production is mostly based on sexually (seed) propagated rootstocks, while clonally propagated rootstocks are on the rise. In a recent study, we found differences in aerial, root growth and water use efficiency between trees grown on these two different rootstocks under controlled continuous fertigation and environmental conditions. In this study, we further describe possible mechanisms which drive the differences. Avocado cv. “Hass” grafted on “Dusa” (D, clonally propagated) and “Mexicola” (M, sexually propagated) rootstocks and different root segments (3, 5 and 8 cm from root tip) were investigated using a combination of hydraulic measurements and polar metabolite (GC-MS) techniques. The results show significant differences in root hydraulic properties, indicating that “Mexicola” fine roots have higher water uptake capacity. The polar metabolites analysis revealed 13 compounds significantly different between rootstocks while nine were found significantly different among root segments. Principal component analysis (PCA) revealed differences between rootstocks and root segments. The data presented here highlight the importance of considering key physiological knowledge in avocado rootstocks breeding programs to be better prepared for future challenging environmental conditions.

Root hydraulic phenotypes impacting water uptake in drying soils.

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.

Exposure to high nitrogen triggered a genotype-dependent modulation of cell and root hydraulics, which can involve aquaporin regulation.

Root nitrogen acquisition has been proposed to be regulated by mass flow, a process by which water flow brings nutrients to the root surface, depending on a concerted regulation of the root hydraulic properties and stomatal conductance. As aquaporins play an important role in regulating transcellular water flow, we aimed at evaluating the short-term effect of high nitrogen (HN) availability on the dynamics of hydraulic parameters at both the root and cell level and the regulation of aquaporins. The effect of short-term HN (8 mM NO3 - ) treatment was investigated on 12 diverse 15-day-old maize genotypes. Root exposure to HN triggered a rapid (<4 h) increase in the root hydraulic conductivity (Lpr ) in seven genotypes while no Lpr variation was recorded for the others, allowing the separation of the genotypes into two groups (HN-responsive and HN-nonresponsive). A remarkable correlation between Lpr and the cortex cell hydraulic conductivity (Lpc ) was observed. However, while differences in gas exchange parameters were also observed, the variations were genotype-specific and not always correlated with the root hydraulic parameters. We then investigated whether HN-induced Lpr variations were linked to the activity and regulation of plasma membrane PIP aquaporins. While some changes in PIP mRNA levels were detected, this was not correlated with the protein levels. On the other hand, the rapid variation in Lpr observed in the B73 genotype was correlated with the PIP protein abundance in the plasma membrane, highlighting PIP posttranslational mechanisms in the short-term regulation of root hydraulic parameters in response to HN treatment.

Investigating Soil-Root Interactions with the Numerical Model R-SWMS.

In this chapter, we present the Root and Soil Water Movement and Solute transport model R-SWMS, which can be used to simulate flow and transport in the soil-plant system. The equations describing water flow in soil-root systems are presented and numerical solutions are provided. An application of R-SWMS is then briefly discussed, in which we combine in vivo and in silico experiments in order to decrypt water flow in the soil-root domain. More precisely, light transmission imaging experiments were conducted to generate data that can serve as input for the R-SWMS model. These data include the root system architecture, the soil hydraulic properties and the environmental conditions (initial soil water content and boundary conditions, BC). Root hydraulic properties were not acquired experimentally, but set to theoretical values found in the literature. In order to validate the results obtained by the model, the simulated and experimental water content distributions were compared. The model was then used to estimate variables that were not experimentally accessible, such as the actual root water uptake distribution and xylem water potential.

Combining cross-section images and modeling tools to create high-resolution root system hydraulic atlases in Zea mays.

Root hydraulic properties play a central role in the global water cycle, in agricultural systems productivity, and in ecosystem survival as they impact the canopy water supply. However, the existing experimental methods to quantify root hydraulic conductivities, such as the root pressure probing, are particularly challenging, and their applicability to thin roots and small root segments is limited. Therefore, there is a gap in methods enabling easy estimations of root hydraulic conductivities in diverse root types. Here, we present a new pipeline to quickly estimate root hydraulic conductivities across different root types, at high resolution along root axes. Shortly, free-hand root cross-sections were used to extract a selected number of key anatomical traits. We used these traits to parametrize the Generator of Root Anatomy in R (GRANAR) model to simulate root anatomical networks. Finally, we used these generated anatomical networks within the Model of Explicit Cross-section Hydraulic Anatomy (MECHA) to compute an estimation of the root axial and radial hydraulic conductivities (k x and k r , respectively). Using this combination of anatomical data and computational models, we were able to create a root hydraulic conductivity atlas at the root system level, for 14-day-old pot-grown Zea mays (maize) plants of the var. B73. The altas highlights the significant functional variations along and between different root types. For instance, predicted variations of radial conductivity along the root axis were strongly dependent on the maturation stage of hydrophobic barriers. The same was also true for the maturation rates of the metaxylem vessels. Differences in anatomical traits along and across root types generated substantial variations in radial and axial conductivities estimated with our novel approach. Our methodological pipeline combines anatomical data and computational models to turn root cross-section images into a detailed hydraulic atlas. It is an inexpensive, fast, and easily applicable investigation tool for root hydraulics that complements existing complex experimental methods. It opens the way to high-throughput studies on the functional importance of root types in plant hydraulics, especially if combined with novel phenotyping techniques such as laser ablation tomography.

Mechanistic modeling of pesticide uptake with a 3D plant architecture model

Meaningful assessment of pesticide fate in soils and plants is based on fate models that represent all relevant processes. With mechanistic models, these processes can be simulated based on soil, substance, and plant properties. We present a mechanistic model that simulates pesticide uptake from soil and investigate how it is influenced, depending on the governing uptake process, by root and substance properties and by distributions of the substance and water in the soil profile. A new root solute uptake model based on a lumped version of the Trapp model (Trapp, 2000) was implemented in a coupled version of R-SWMS-ParTrace models for 3-D water flow and solute transport in soil and root systems. Solute uptake was modeled as two individual processes: advection with the transpiration stream and diffusion through the root membrane. We set up the model for a FOCUS scenario used in the European Union (EU) for pesticide registration. Considering a single vertical root and advective uptake only, the root hydraulic properties could be defined so that water and substance uptake and substance fate in soil showed a good agreement with the results of the 1D PEARL model, one of the reference models used in the EU for pesticide registration. Simulations with a complex root system and using root hydraulic parameters reported in the literature predicted larger water uptake from the upper root zone, leading to larger pesticide uptake when pesticides are concentrated in the upper root zone. Dilution of root water concentrations at the top root zone with water with low pesticide concentration taken up from the bottom of the root zone leads to larger uptake of solute when uptake was simulated as a diffusive process. This illustrates the importance of modeling uptake mechanistically and considering root and solute physical and chemical properties, especially when root-zone pesticide concentrations are non-uniform.

Root hydraulics adjustment is governed by a dominant cell-to-cell pathway in Beta vulgaris seedlings exposed to salt stress

Soil salinity reduces root hydraulic conductivity (Lpr) of several plant species. However, how cellular signaling and root hydraulic properties are linked in plants that can cope with water restriction remains unclear. In this work, we exposed the halotolerant species red beet (Beta vulgaris) to increasing concentrations of NaCl to determine the components that might be critical to sustaining the capacity to adjust root hydraulics. Our strategy was to use both hydraulic and cellular approaches in hydroponically grown seedlings during the first osmotic phase of salt stress. Interestingly, Lpr presented a bimodal profile response apart from the magnitude of the imposed salt stress. As well as Lpr, the PIP2-aquaporin profile follows an unphosphorylated/phosphorylated pattern when increasing NaCl concentration while PIP1 aquaporins remain constant. Lpr also shows high sensitivity to cycloheximide. In low NaCl concentrations, Lpr was high and 70 % of its capacity could be attributed to the CHX-inhibited cell-to-cell pathway. More interestingly, roots can maintain a constant spontaneous exudated flow that is independent of the applied NaCl concentration. In conclusion, Beta vulgaris root hydraulic adjustment completely lies in a dominant cell-to-cell pathway that contributes to satisfying plant water demands.

Normalization criteria determine the interpretation of nitrogen effects on the root hydraulics of pine seedlings.

Plant hydraulics is key for plant survival and growth because it is linked to gas exchange and drought resistance. Although the environment influences plant hydraulics, there is no clear consensus on the effect of nitrogen (N) supply, which may be, in part, due to different hydraulic conductance normalization criteria and studied species. The objective of this study was to compare the variation of root hydraulic properties using several normalization criteria in four pine species in response to three contrasting N fertilization regimes. We studied four closely related, yet ecologically distinct species: Pinus nigra J.F. Arnold, Pinus pinaster Ait., Pinus pinea L. and Pinus halepensis Mill. Root hydraulic conductance (Kh) was measured with a high-pressure flow meter, and values were normalized by total leaf area (leaf specific conductance, Kl), xylem cross-section area (xylem specific conductance, Ks), total root area (root specific conductance, Kr) and the area of fine roots (fine root specific conductance, Kfr). Controlling for organ size differences allowed comparison of the hydraulic efficiency of roots to supply or absorb water among fertilization treatments and species. The effect of N on the root hydraulic efficiency depended on the normalization criteria. Increasing N availability reduced Kl and Ks, but increased Kh, Kr and especially Kfr. The positive effect of N on Kr and Kfr was positively related to seedling relative growth rate and was also consistent with published results at the interspecific level, whereby plant hydraulics is positively linked to photosynthesis and transpiration rate and fast growth. In contrast, normalization by leaf area and xylem cross-sectional area (Kl and Ks) reflected opposite responses to Kr and Kfr. This indicates that the normalization criteria determine the interpretation of the effect of N on plant hydraulics, which can limit species and treatment comparisons.

Techniques to Determine the Effects of Jasmonates on Root Hydraulic Conductivity.

Plants subjected to drought and saline stress conditions suffer from tissue dehydration. Such dehydration is caused by the imbalance between root water uptake by roots and water loss by transpiration. Therefore, determination of root hydraulic properties is crucial to understand plant water balance. Root hydraulic conductivity (L) can be used to estimate root water transport capacity. L depends on root architecture (length and diameter of the root and proliferation of secondary roots), radial water transport pathway (root xylem vessels, plasmodesmata, apoplastic space, caspary bands), and on intrinsic membrane permeability to water (aquaporins, water membrane protein channels). Different methods have been developed to measure L, such as Pressure Chamber, Free Exudation, High-Pressure Flowmeter (HPFM), and Root Pressure Probe (RPP). In this chapter, we will focus on Pressure Chamber, Free Exudation, and HPFM methods which have been used to determine the effect of jasmonates (JA) on root hydraulic conductivity.