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Water transport in plants as a catenary process
... Under non-limited water condition and according to the constancy of water flow through the plant [34], almost the whole volumetric water flux density taken up by root is transpired by leaf and in turn the same volume of water is loaded into xylem. Hence, variation in the Jw calculated through plant transpiration is a reliable proxy for volumetric water uptake from soil and loaded into xylem. ...
... To test the influence of Jw on R, a group of trees was bagged to bring the air surrounding the canopy close to the saturation point and in turn the VPD close to zero. The bagging method has been documented to effectively reduce VPD at plant or branch and fruit scale by reducing RH with negligible impact on air temperature [30,34,40]. ...
... air temperature [30,34,40]. ...
Research on organic electrochemical transistor (OECT) based sensors to monitor in vivo plant traits such as xylem sap concentration is attracting attention for their potential application in precision agriculture. Fabrication and electronic aspects of OECT have been the subject of extensive research while its characterization within the plant water relation context deserves further efforts. This study tested the hypothesis that the response (R) of an OECT (bioristor) implanted in the trunk of olive trees is inversely proportional to the water flux density flowing through the plant (Jw). This study also examined the influence on R of vapor pressure deficit (VPD) as coupled/uncoupled with light. R was hourly recorded in potted olive trees for a 10-day period concomitantly with Jw (weight loss method). A subgroup of trees was bagged in order to reduce VPD and in turn Jw, and other trees were located in a walk-in chamber where VPD and light were independently managed. R was tightly sensitive to diurnal oscillation of Jw and at negligible values of Jw (late afternoon and night) R increased. The bioristor was not sensitive to the VPD per se unless a light source was coupled to trigger Jw. This study preliminarily examined the suitability of bioristor to estimate the mean daily nutrients accumulation rate (Ca, K) in leaves comparing chemical and sensor-based procedures showing a good agreement between them opening new perspective towards the application of OECT sensor in precision agricultural cropping systems.
... Far from being a temporal gradient of "better models" but rather a diverse palette, each of the cited approaches currently has a niche determined by the compromise between its specificity and simplicity. On the one hand, in applications at the plant scale and beyond, radial water transport across root tissues is generally considered as driven by pressure gradients across a single hydraulic conductance, as already done by van den Honert (1948). Examples of this implementation comprise newly implemented plant hydraulic modules in terrestrial biosphere models (TBMs) (Agee et al., A c c e p t e d M a n u s c r i p t 2021; Bisht & Riley, 2019;Kennedy et al., 2019;Sulis et al., 2019) but also recent functionalstructural plant models (FSPMs) (Bouda et al., 2018;Javaux et al., 2008;Postma et al., 2017;Roose & Fowler, 2004). ...
... As such, the total resistance of the system tot R equals the sum of all resistances: Cowan (1965) complemented the approach of van den Honert by adding the soil as three additional compartments: (1) the sub-soil, (2) the top-soil (i.e., the plant rooting zone) and (3) the soil in direct vicinity of roots (rhizosphere). The work of Cowan (1965) and van den Honert (1948), essentially comes down to a series connection of (simplifications of) the water flow processes described in sections 3.1 to 3.4. In the study of Cowan (1965), a capacitor was added in the model to cope with non-steady state, but only in the bulk soil compartments. ...
Water potential explains water transport in the Soil-Plant-Atmosphere Continuum (SPAC), and is gaining interest as connecting variable between ‘pedo-, bio- and atmosphere’. It is primarily used to simulate hydraulics in the SPAC, and is thus essential for studying drought effects. Recent implementations of hydraulics in large-scale Terrestrial Biosphere Models (TBMs) improved their performance under water-limited conditions, while hydraulic features of recent detailed Functional-Structural Plant Models (FSPMs) open new possibilities for dissecting complex traits for drought tolerance. These developments in models across scales deserve a critical appraisal to evaluate its potential for wider use in FSPMs, but also in crop systems models (CSMs), where hydraulics are currently still absent. After refreshing the physical basis, we first address models where water potential is primarily used for describing water transport along the transpiration pathway from the soil to the leaves, through the roots, the xylem and the leaf mesophyll. Then, we highlight models for three ecophysiological processes, which have well-recognised links to water potential: phloem transport, stomatal conductance and organ growth. We identify water potential as the bridge between soil, root and shoot models, as the physiological variable integrating below- and above-ground abiotic drivers, but also as the link between water status and growth. Models making these connections enable identifying crucial traits for ecosystem resilience to drought and for breeding towards improved drought tolerance in crops. Including hydraulics often increases model complexity, and thus requires experimental data on soil and plant hydraulics. Nevertheless, modelling hydraulics is insightful at different scales (FSPMs, CSMs and TBMs).
... A lot of theoretical and experimental studies have been carried out on the hydrodynamic mechanism of water transmission in plants. In 1948, Van den Honert compared the hydraulic resistance of plant tissue with the resistance in Ohm's law and laid the foundation for quantitative analysis of plant water transport mechanism to a certain extent 23,24 . Experimental exploration has also been carried out by researchers with the help of artificial trees 25,26 , dyeing 27,28 , heat tracking 29-31 , pressure measuring 32-34 , D 2 O tracer 35,36 etc. X-ray phase-contrast imaging was employed to track the change of gas-water interface and a model of embolism repair was provided 21,37 . ...
Cuttage is a common plant cultivation method, and the key to its survival is the restoration of water refilling, which remains unclear up to now. We report 3D dynamic imaging of water refilling of cuttage without resorting to any contrast agent. Hydrodynamics of the refilled water flow over time reveals the existence of a unit mass force field with a gradient along the refilling direction, which means that cutting plants also have a gradient force field to drive the recovery of water refilling, as predicted by Cohesion-Tension theory in normal plants. We found that force fields of different functional regions are isolated and independently distributed, which is conducive to ensure the safety of water transmission. At the same time, we also found that there is a so-called "inchworm effect" in the mass force field, which contributes to the force transfer inside the cutting through local force accumulation. Results of this paper demonstrate that the developed method for the measurement of mass force field in-vivo is applicable to help decipher the mechanism of plant water refilling.
... Flow in the soil-plant system has been represented as a passive catenary process that can be represented as a flow driven by potential differences across a network of resistances (or their inverse, conductances) (van den Honert, 1948). First network models considered resistances to flow from the bulk soil to soil-root interfaces, to radial flow into root segments, and to axial flow along root segments toward the leaves (Cowan, 1965). ...
Plant 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.
... By contrast, Ψ leaf measurements do not have this limitation because the plant is affected by the Ψ soil across its entire root system and is thought to equilibrate at night to the highest Ψ soil according to the root density distribution [3]. Mechanistic models describing water flow in the soil-plant-atmosphere continuum are usually analyzed by means of Ohm's law [4]. Under the assumption that stomatal closure at night prohibits transpiration and that there was ample time for hydraulic equilibrium to be established, predawn leaf water potential (Ψ pd ) has been proposed as a proxy for Ψ soil [5]. ...
Background:
In plant water relations research, predawn leaf water potential (Ψpd) is often used as a proxy for soil water potential (Ψsoil), without testing the underlying assumptions that nighttime transpiration is negligible and that enough time has passed for a hydrostatic equilibrium to be established. The goal of this research was to test the assumption Ψpd = Ψsoil for field-grown grapevines.
Results:
A field trial was conducted with 30 different cultivars of wine grapes grown in a single vineyard in arid southeastern Washington, USA, for two years. The Ψpd and the volumetric soil water content (θv) under each sampled plant were measured multiple times during several dry-down cycles. The results show that in wet soil (Ψsoil > - 0.14 MPa or relative extractable water content, θe > 0.36), Ψpd was significantly lower than Ψsoil for all 30 cultivars. Under dry soil conditions (Ψsoil < - 0.14 MPa or θe < 0.36) Ψpd lined up better with Ψsoil. There were differences between cultivars, but these were not consistent over the years.
Conclusion:
These results suggest that for wet soils Ψpd of grapevines cannot be used as a proxy for Ψsoil, while the Ψpd = Ψsoil assumption may hold for dry soils.
... Trees, and other plants, can absorb water across the surfaces of their leaves when wet or under nearly saturated vapor conditions (Rundel et al. 1982, Guzmán-Delgado et al. 19399170, ja, Downloaded Dawson & Goldsmith 2018, Schreel & Steppe 2019a, 2020a. Water is pulled into leaves down the gradient in potential energy (called water potential) that exists between liquid water outside the leaf and the leaf interior, and slowed by hydraulic resistance imposed by leaf structure (van den Honert 1948, Berry et al. 2019, Guzmán-Delgado et al. 2018, Chin et al. 2022a). Access to above ground water sources can improve plant water status and carbon balance, repair drought and freeze-thaw induced damage such as buckled tracheids and embolisms, extend photosynthetic time, support turgor-driven cell expansion, as well as provide water to dry soils (Burgess & Dawson 2004, Eller et al. 2013, Steppe et al. 2018, Binks et al. 2019, Boanares et al. 2020, Chin et al. 2022b). ...
Analysis of functional traits is a cornerstone of ecology, yet individual traits seldom explain useful amounts of variation in species distribution or climatic tolerance, and their functional significance is rarely validated experimentally. Multivariate suites of interacting traits could build an understanding of ecological processes and improve our ability to make sound predictions of species success in our rapidly changing world. We use foliar water uptake capacity as a case study because it is increasingly considered to be a key functional trait in plant ecology due to its importance for stress‐tolerance physiology. However, the traits behind the trait, that is, the features of leaves that determine variation in foliar water uptake rates, have not been assembled into a widely applicable framework for uptake prediction. Focusing on trees, we investigated relationships among 25 structural traits, leaf osmotic potential (a source of free energy to draw water into leaves), and foliar water uptake in 10 diverse angiosperm and conifer species. We identified consistent, multitrait “uptake syndromes” for both angiosperm and conifer trees, with differences in key traits revealing suspected differences in the water entry route between these two clades and an evolutionarily significant divergence in the function of homologous structures. A literature review of uptake‐associated functional traits, which largely documents similar univariate relationships, provides additional support for our proposed “uptake syndrome.” Importantly, more than half of shared traits had opposite‐direction influences on the capacity of leaves to absorb water in angiosperms and conifers. Taxonomically targeted multivariate trait syndromes provide a useful tool for trait selection in ecological research, while highlighting the importance of micro‐traits and the physiological verification of their function for advancing trait‐based ecology.
... By contrast, Ψ leaf measurements do not have this limitation because the plant is affected by the Ψ soil across its entire root system and is thought to equilibrate at night to the highest Ψ soil according to the root density distribution [3]. Models describing water ow in the soil-plantatmosphere continuum are usually analyzed by means of Ohm's law [4]. Under the assumption that stomatal closure at night prohibits transpiration and that there was ample time for hydraulic equilibrium to be established, predawn leaf water potential (Ψ pd ) has been proposed as a proxy for Ψ soil [5]. ...
Background
In plant water relations research, predawn leaf water potential (Ψpd) is often used as a proxy for soil water potential (Ψsoil), without testing the underlying assumptions that nighttime transpiration is negligible and that enough time has passed for a hydrostatic equilibrium to be established. The goal of this research was to test the assumption that Ψpd = Ψsoil for field-grown grapevines.
Results
A field trial was conducted with 30 different varieties of wine grapes grown in a single vineyard in arid southeastern Washington, USA, for two years. The Ψpd and the volumetric soil water content (θv) under each sampled plant were measured multiple times during several dry-down cycles. The results show that in wet soil (θv > 0.146 m³ m− 3), Ψpd was significantly lower than Ψsoil for all 30 varieties. Under drought conditions (θv < 0.105 m³ m− 3) Ψpd lined up better with Ψsoil. There were differences between varieties, but these were not consistent over the years.
Conclusion
These results suggest that for wet soils Ψpd of grapevines cannot be used as a proxy for Ψsoil, while the Ψpd = Ψsoil assumption holds for dry soils.
... Sensational or not, the analogy of plants as power grids is in fact a well-established quantitative approach. Much understanding of plant hydraulics arose from the classical comparison of the soil-plant-atmosphere continuum to an electrical circuit, where flow paths are resistors and water potential gradients are voltages, subject to the application of the analogy to Ohm's Law ( Van den Honert, 1948;Tyree and Zimmermann, 2002): ...
We present unresolved questions in plant abiotic stress biology as posed by 15 research groups with expertise spanning eco-physiology to cell and molecular biology. Common themes of these questions include the need to better understand how plants detect water availability, temperature, salinity, and rising CO2 levels; how environmental signals interface with endogenous signaling and development (e.g. circadian clock, flowering time); and how this integrated signaling controls downstream responses (e.g. stomatal regulation, proline metabolism, growth versus defense balance). The plasma membrane comes up frequently as a site of key signaling and transport events (e.g. mechanosensing and lipid-derived signaling, aquaporins). Adaptation to water extremes and rising CO2 affects hydraulic architecture and transpiration, as well as root and shoot growth and morphology, in ways not fully understood. Environmental adaptation involves tradeoffs that limit ecological distribution and crop resilience in the face of changing and increasingly unpredictable environments. Exploration of plant diversity within and among species can help us know which of these tradeoffs represent fundamental limits and which ones can be circumvented by bringing new trait combinations together. Better defining what constitutes beneficial stress resistance in different contexts and making connections between genes and phenotypes, and between laboratory and field observations, are overarching challenges.
... Release of stored water to the transpiration stream in response to decreasing water potentials is another means of 395 Water movement through the soil-plant-atmosphere-continuum (SPAC) has been characterized using an analogue of Ohm's Law (2,8). Elfving et al. (5) found that transpirational flux (F) was a function of the difference between leaf (T'j) and soil (T's) water potentials divided by the soil-to-leaf resistance (r) when measured under steady state conditions. ...
Greenhouse-grown potted apple trees ( Malus domestica Borkh. cv. Empire) on Malling-Merton (MM) 111 rootstocks displayed much lower water potentials at the end of the season than did similar trees grown outdoors when measured under the same soil moisture and evaporative demand conditions. Differences in water potentials over a wide range of soil moistures were due to the much higher plant and root resistances and the lower leaf capacitances of the greenhouse-grown trees as calculated by using electric analogue circuits. Root resistance differences could not be explained by changes in feeder root percentages or root:shoot ratios.
... Concerning the whole root scale, one of the foundation was the assimilation of water transport to a catenary process (a network of resistances in series or in parallel) and the analogy to the Ohm's law, where the water transport is directly proportional to a potential difference (Honert 1948). This view was developed further by Doussan and his colleagues in 1998 (Doussan et al. 1998a(Doussan et al. , 1998b. ...
Background
Root water transport, which critically contributes to the plant water status and thereby plant productivity, has been the object of extensive experimental and theoretical studies. However, root systems represent an intricate assembly of cells in complex architectures, including many tissues at distinct developmental stages. Our comprehension of where and how molecular actors integrate their function in order to provide the root with its hydraulic properties is therefore still limited.
Scope
Based on current literature and prospective discussions, this review addresses how root water transport can be experimentally measured, what is known about the underlying molecular actors, and how elementary water transport processes are scaled up in numerical/mathematical models.
Conclusions
The theoretical framework and experimental procedures on root water transport that are in use today have been established a few decades ago. However, recent years have seen the appearance of new techniques and models with enhanced resolution, down to a portion of root or to the tissue level. These advances pave the way for a better comprehension of the dynamics of water uptake by roots in the soil.
... Today, the accepted theory, known as cohesion tension theory (Dixon & Joly, 1894), envisions a water column within the xylem resembling a 'chain' of interconnected water molecules. This chain is anchored to the parenchyma tissue of a leaf on one end and to the root-water on the other (Konrad et al., 2019;Van den Honert, 1948). For every water molecule lost to the atmosphere from leaves, the entire chain must be pulled up a distance of one water molecule thereby increasing the tension on the chain (Konrad et al., 2019). ...
Water inside plants forms a continuous chain from water in soils to the water evaporating from leaf surfaces. Failures in this chain result in reduced transpiration and photosynthesis and are caused by soil drying and/or cavitation‐induced xylem embolism. Xylem embolism and plant hydraulic failure share several analogies to “catastrophe theory” in dynamical systems. These catastrophes are often represented in the physiological and ecological literature as tipping points when control variables exogenous (e.g., soil water potential) or endogenous (e.g., leaf water potential) to the plant are allowed to vary on time scales much longer than time scales associated with cavitation events. Here, plant hydraulics viewed from the perspective of catastrophes at multiple spatial scales is considered with attention to bubble expansion within a xylem conduit, organ‐scale vulnerability to embolism, and whole‐plant biomass as a proxy for transpiration and hydraulic function. The hydraulic safety‐efficiency tradeoff, hydraulic segmentation and maximum plant transpiration are examined using this framework. Underlying mechanisms for hydraulic failure at fine scales such as pit membranes and cell‐wall mechanics, intermediate scales such as xylem network properties and at larger scales such as soil‐tree hydraulic pathways are discussed. Understudied areas in plant hydraulics are also flagged where progress is urgently needed. This article is protected by copyright. All rights reserved.
... columns are fractured and a gaseous bubble, referred to as embolism, forms preventing water movement (Cochard et al., 2013). A simple model proposed by Van Den Honert (1948) demonstrates the water and sap transportation mechanism: ...
... Today, the accepted theory, known as cohesion tension theory (Dixon & Joly, 1894), envisions a water column within the xylem resembling a 'chain' of interconnected water molecules. This chain is anchored to the parenchyma tissue of a leaf on one end and to the root-water on the other (Konrad et al., 2019;Van den Honert, 1948). For every water molecule lost to the atmosphere from leaves, the entire chain must be pulled up a distance of one water molecule thereby increasing the tension on the chain (Konrad et al., 2019). ...
Water inside plants forms a continuous chain from water in soils to the water evaporating from leaf surfaces. Failures in this chain result in reduced transpiration and photosynthesis and these failures are caused by soil drying and/or cavitation-induced xylem embolism. Xylem embolism and plant hydraulic failure share a number of analogies to “catastrophe theory” in dynamical systems. These catastrophes are often represented in the physiological and ecological literature as tipping points or alternative stable states when control variables exogenous (e.g. soil water potential) or endogenous (e.g. leaf water potential) to the plant are allowed to slowly vary. Here, plant hydraulics viewed from the perspective of catastrophes at multiple spatial scales is considered with attention to bubble expansion (i.e. cavitation), organ-scale vulnerability to embolism, and whole-plant biomass as a proxy for transpiration and hydraulic function. The hydraulic safety-efficiency tradeoff, hydraulic segmentation and maximum plant transpiration are examined using this framework. Underlying mechanisms for hydraulic failure at very fine scales such as pit membranes, intermediate scales such as xylem network properties and at larger scales such as soil-tree hydraulic pathways are discussed. Lacunarity areas in plant hydraulics are also flagged where progress is urgently needed.
... à l'interface entre une branche et une feuille). Une analogie entre le fonctionnement hydraulique des plantes et la loi d'Ohm a été proposée, permettant de mieux caractériser la dynamique du transport de l'eau ( Van den Honert, 1948). Cette formulation relativement simple, originellement appliquée à la description d'un système électrique, se retrouve dans la loi de Darcy qui décrit le flux d'eau dans un milieu poreux (Whitehead et al., 1984) et postule que le flux d'eau à travers une plante (F) est proportionnel au gradient de potentiel hydrique et inversement proportionnel à la résistance hydrodynamique, tel que : ...
Les forêts tropicales humides jouent un rôle clé dans les cycles biogéochimiques à l’échelle globale. Les évènements de sécheresse saisonniers entrainent des modifications dans le fonctionnement de ces forêts. L’augmentation de la fréquence et de l’intensité des évènements de sécheresse de forte intensité à l’échelle de l’Amazonie entraine déjà un changement dans la composition des communautés d’arbres, mais notre capacité à prédire leur réponse future dépend en partie de nos connaissances des mécanismes physiologiques leur permettant de résister à la sécheresse. Cette thèse explore la diversité interspécifique des mécanismes de résistance à la sécheresse des arbres en forêt tropicale humide de Guyane et vise à améliorer les connaissances actuelles. Elle montre que les espèces d’arbres de canopée en forêt naturelle peuvent avoir une grande résistance à l’embolie des tiges et qu’une majorité des espèces ont un grand niveau de sureté hydraulique en saison sèche, avec une grande variabilité interspécifique. Cependant, pour la majorité d’entre eux, une diminution de la disponibilité en eau du sol entraine une diminution de leur densité de flux de sève en saison sèche, et une partie de cette sensibilité peut être expliquée par des mécanismes physiologiques liés à des stratégies de résistance à la sécheresse. Les mécanismes physiologiques sous-jacents à ces stratégies varient fortement entre espèces pour de jeunes arbres. Certaines bénéficient d’une fermeture stomatique précoce et d’une faible conductance minimum, alors que d’autres ont une plus grande résistance à l’embolie des tiges et un plus fort degré de segmentation de vulnérabilité, parfois associés à une grande tolérance à l’embolie des tiges. Lors d’une sécheresse de forte intensité, nous avons observé une forte réduction des teneurs en carbohydrates, ce qui souligne l’interdépendance entre le fonctionnement hydraulique et carboné des espèces. Cependant, le principal processus physiologique causant la mort semble être la défaillance hydraulique. Cette forte variabilité interspécifique amène à postuler que les populations d’arbres de forêt tropicale humide pourraient répondre de manière contrastée à une intensification des épisodes de sécheresse, ce qui pourrait avoir des conséquences sur la composition des communautés d’arbres en Guyane.
... Gradmann (1928) first recognized that for an isothermal system, water potential is a gradient from soil to plant to atmosphere, where flow is proportional to the resistance of the component parts. His work was largely overlooked for twenty years until van den Honert (1948) reintroduced and refined it. Van den Honert described plant water transport as a catenary process, a chain where the soil-root interface and leaf-atmosphere interfaces formed the terminal ends of the soil-plant-atmospheric continuum (SPAC). ...
Shifting patterns of precipitation and rising temperatures have highlighted forest vulnerability to heat- and drought-induced stress. For systems that face water-limitation, either from short-term, seasonal dry periods or longer-term droughts, plasticity of root system function establishes the ability of individuals to meet atmospheric demand and maintain physiological function. This functional plasticity is determined by an individual’s intrinsic properties and their interactions within the community and environment. Given limitations to in situ measurement, improved model representation of below-ground structural and functional complexity has provided means for exploring these ecophysiological feedbacks between drying soil and trees across biomes. This research addresses individual, ecosystem, and basin scale responses to water limitation by examining (i) the role of below-ground structure and ecophysiological controls on water uptake across functional gradients (i.e., low diversity vs. high diversity ecosystems); (ii) identifying and expanding the utility of novel proxies of hydraulic function; and (iii) exploring the feasibility of monitoring drought response at large scales using a parsimonious model of surface energy partitioning. Modeled root water uptake from both temperate and tropical systems highlight that independent of functional strategy, root lateral interactions at the tree scale directly impact the depth distribution of water uptake and plant hydraulic status. A newly developed index of root system interaction provides an amenable axis with which to explore the tradeoffs between structural investment and resource acquisition. Laboratory and field analysis show that conventional technologies used to measure sap flow velocity may contain hidden information regarding a tree’s hydraulic state. This low frequency signal may also serve well as a proxy for below-ground response to the drying soil, providing valuable validation for future modeling efforts. Finally, the feasibility of hourly, basin scale estimates of the land-surface energy budget partition are tested. The Maximum Entropy Production model is successfully applied to the Amazon River Basin, a highly complex region prone to strong seasonal droughts, elucidating avenues of future research needed to more fully link ecosystem and hydrologic processes. The methodologies developed and expanded in this work provide new avenues for assessing tree-scale water fluxes and hydraulic state, providing a means for observing and testing hypotheses related to ecophysiological response across spatiotemporal scales.
... plant water uptake and transpiration rates) within the soil-plant-atmosphere system (SPAS), the saturation level and therefore the hydraulic potential, in units of pressure, needs to be observed. Van den Honert in 1948 [2] was among the first to recognize the analogy of current flow by Ohm's law with the water movement processes along SPAS. Thus, the hydraulic potential differences between single compartments in SPAS (e.g. ...
... Water flow inside plant tissues is analogous to river streamflow. As rivers flow from high to low elevations, the plant transpiration stream flows along a cascade of water potentials -or free energy status -that are high (in soil) to low (in roots, then even lower in leaves); a theory untouched for more than seven decades (Van Den Honert, 1948). Metaphorically, water flows "downhill" of gradients of summed pressure and gravitational potentials across non-selective tissues, such as xylem vessels, the part of vascular tissues made of dead cells emptied from their protoplasts. ...
With global warming, climate zones are projected to shift poleward, and the frequency and intensity of droughts to increase, driving threats to crop production and ecosystems. Plant hydraulic traits play major roles in coping with such droughts, and process-based plant hydraulics (water flowing along decreasing pressure Ψ p or total water potential Ψ tot gradients ) has newly been implemented in land surface models.
An enigma reported for the past 35 years is the observation of water flowing along increasing water potential gradients across roots. By combining the most advanced modelling tool from the emerging field of plant micro-hydrology with pioneering cell solute mapping data, we found that the current paradigm of water flow across roots of all vascular plants is incomplete: it lacks the impact of solute concentration (and thus negative osmotic potential Ψ o ) gradients across living cells . This gradient acts as a water pump as it reduces water tension without loading solutes in plant vasculature (xylem). Importantly, water tension adjustments in roots may have large impacts in leaves due to the tension-cavitation feedback along stems.
Here, we mathematically demonstrate the water pumping mechanism by solving water flow equations analytically on a triple-cell system. Then we show that the simplistic upscaled equations hold in 2- and 3-D maize, grapevine and Arabidopsis complex hydraulic anatomies, and that water may flow “uphill” of water potential gradients toward xylem as observed experimentally.
Besides its contribution to the fundamental understanding of plant water relations, this study lays new foundations for future multidisciplinary research encompassing plant physiology and ecohydrology, and has the ambition to mathematically capture a keystone process for the accurate forecasting of plant water status in crop models and LSMs.
Graphical Abstract
Highlights
- We provide a scale-consistent solution of water flow equations across root tissues
- Symplasmic osmotic potential gradients are missing in the current theory of root water uptake
- The model solves the empirical enigma of root water uptake uphill of water potential gradients
... principle of potential difference by flow resistance is motivated from the field of electricity by Ohm's law. Van den Honert in 1948 was one of the first realizing and showing this connecting concept(Van den Honert, 1948; ...
The vegetation optical depth (VOD) parameter contains information on plant water content and biomass, and can be estimated alongside soil moisture from currently operating satellite radiometer missions, such as SMOS (ESA) and SMAP (NASA). The estimation of water fluxes, such as plant water uptake (PWU) and transpiration rate (TR), from these Earth system parameters (VOD, soil moisture) requires assessing potential (suction tension) gradients of water and flow resistances in the soil, the vegetation and the atmosphere, yet it remains an elusive challenge especially on global scale. Here, we used a field-scale experiment to test mechanistic models for the estimation of seasonal water fluxes (PWU and TR) of a winter wheat stand including measurements of soil moisture, VOD, and relative air humidity (RH) under a controlled environment. We utilized microwave L-band observations from a tower-based radiometer to estimate VOD of a wheat stand during the 2017 growing season at the Selhausen laboratory in Germany. From VOD, we first extracted the gravimetric moisture of vegetation and then determined subsequently the relative water content (RWC) and the vegetation water potential (VWP) of the wheat field. Although the relative water content could directly be estimated from VOD, our results indicate this may be problematic for the phenological phases, when rapid biomass and plant structure development take place in the wheat canopy. The water uptake from the soil to the wheat plants was estimated from the difference between the soil and vegetation potentials divided by flow resistance from soil into wheat plants. The transpiration rate from the wheat plants into the atmosphere was obtained from the difference between the vegetation and atmosphere potentials divided by flow resistances from plants to the atmosphere. For this, the required soil matric potential (SMP), the vapor pressure deficit and the flow resistances were obtained from on-site observations of soil, plant and atmosphere and simple mechanistic models. This pathfinder study shows that the L-band microwave radiation contains valuable information on vegetation water status that enables the estimation of water dynamics (up to fluxes) from the soil via wheat plants into the atmosphere, when combined with additional information of soil and atmosphere water content. Still, assumptions when estimating the vegetation water potential from relative water content as well as when estimating the water flow resistances between soil, wheat plants and atmosphere had to be made. Moreover, validation of water flux estimates for assessing their absolute accuracy could not be performed due to a lack of in situ PWU and TR measurements. Nonetheless, our estimates of water status, potentials and fluxes show the expected temporal dynamics and intercompare reasonably well in absolute terms, providing confidence in further developing the proposed approach. Our findings support that passive microwave remote sensing techniques allow for the estimation of vegetation water dynamics next to traditionally measured stand-scale or plot-scale techniques. This might shed light on the potential capabilities of monitoring water dynamics in the soil-plant-atmosphere system using wide-area, remote sensing-based Earth observation data.
... A short history of the simulation of water flow from the soil through the root system to the plant starts with representing the flow using electrical analogies (e.g. van den Honert, 1948). Based on these analogies, Alm et al. (1992) developed a numerical model of water transport for an unbranched root. ...
Root architectural (RSA) models have become important tools in root research and plant phenotyping for studying root traits, processes, and interactions with the environment. The models have been used to simulate how various root traits and processes influence water and nutrient uptake. At a more technical level, they have been used to develop phenotyping technology, particularly for testing algorithms for segmenting roots. To compute these quantitative estimates regarding plant nutrition and root functioning, much development occurred in the last decade increasing the complexity of the models. This chapter describes first the application of the models to questions in plant biology, breeding, and agronomy, and second the development of the models. It concludes with a small outlook suggesting that models need benchmarking and validation and that new developments are likely to include better descriptions of root plasticity responses and focus on biological interactions among (soil) organisms, including mycorrhizal fungi.
Modern plant physiological theory stipulates that the resistance to water movement from plants to the atmosphere is overwhelmingly dominated by stomata. This conception necessitates a corollary assumption—that the air spaces in leaves must be nearly saturated with water vapour; that is, with a relative humidity that does not decline materially below unity. As this idea became progressively engrained in scientific discourse and textbooks over the last century, observations inconsistent with this corollary assumption were occasionally reported. Yet, evidence of unsaturation gained little traction, with acceptance of the prevailing framework motivated by three considerations: (1) leaf water potentials measured by either thermocouple psychrometry or the Scholander pressure chamber are largely consistent with the framework; (2) being able to assume near saturation of intercellular air spaces was transformational to leaf gas exchange analysis; and (3) there has been no obvious mechanism to explain a variable, liquid‐phase resistance in the leaf mesophyll. Here, we review the evidence that refutes the assumption of universal, near saturation of air spaces in leaves. Refining the prevailing paradigm with respect to this assumption provides opportunities for identifying and developing mechanisms for increased plant productivity in the face of increasing evaporative demand imposed by global climate change.
A new high-pressure flowmeter has been developed and used to estimate hydraulic resistance in whole shoots of woody plants. It is based on the perfusion of pressurized water in the plant, and the simultaneous measurement of water flow (introduced volume of water per unit of time). This device was first calibrated, and then used to estimate the hydraulic resistance of stems and leaves in 1-yr seedlings of broadleaf Quercus species (Q. rubra, Q. cerris, Q. velutina, Q. petraea, Q. frainetto, Q. pyrenaica). The measured values are reliable and similar to those reported in the literature, but taking into account that this study dealt with seedlings. We have found significant differences between species and groups of species according to their hydraulic resistance values for stems and leaves. Those species whose seedlings showed lower leaf hydraulic resistance values can rehydrate their leaves more quickly and survive in environments with certain water stress (e.g., Q. pyrenaica). The high leaf hydraulic resistance of other species indicate that they can not resist these levels of water stress (e.g, Q. rubra). This study provided new insight to the importance of leaf hydraulic resistance as an adaptative trait to face water stress. Hydraulic resistance variability must be studied in depth as an additional mechanism within a wide array of morphological and ecophysiological adaptations that provide tree species a greater drought tolerance.
Key message
The high-wood-density species displays greater water limitation tolerance, as it maintains leaf transpiration under drought conditions.
Abstract
The relationship between environmental conditions and plant hydraulic safety is essential to understand species’ strategies to minimize damage to their hydraulic structure yet maintain function. In the Brazilian semi-arid, the relationships between rainfall seasonality, hydraulic conductivity, wood density, stomatal conductance, and phenology in different species still needs to be clarified. To better understand these relationships, we selected two deciduous trees species with contrasting wood density: (1) Commiphora leptophloeos (Mart.) J.B. Gillett (low wood density) and (2) Cenostigma pyramidale (Tul.) E. Gagnon & G. P. Lewis (high wood density) from the Caatinga dry forest of northeast Brazil. We tracked monthly measurements of whole-tree hydraulic conductivity, leaf stomatal conductance, leaf transpiration rate, xylem water potential, and phenology. We found that the low-wood-density species had a higher whole-tree hydraulic conductivity and an early leaf flush and fall. In addition, lower leaf transpiration rate and higher water storage capacity maintained high xylem water potential and stomatal conductance values, especially in the rainy season. On the other hand, the high-wood-density species had a lower whole-tree hydraulic conductivity and higher leaf transpiration rate, even during the dry season. These results point to the divergent hydraulic strategies employed by each species, further suggesting opposing hydraulic safety pathways during drought.
Understanding plant water use patterns is crucial for comprehending the dynamics of the soil–plant‐atmosphere continuum and evaluating the adaptability of plants across diverse ecosystems. However, there remains a gap in our comprehension of non‐halophyte plants' water uptake patterns and driving factors in temperate coastal regions. For this reason, we used locust trees (a widely planted non‐halophyte tree species in northern China) as a study subject. We collected water isotope data (δ ² H and δ ¹⁸ O) for locust trees xylem and soil over two consecutive growing seasons. The MixSIAR model was used along with five distinct sets of input data (single isotopes, uncorrected dual isotopes, and corrected dual isotopes incorporating δ ² H data obtained by soil water line or cryogenic vacuum distillation methods) to infer water utilization patterns. The results indicated that locust trees primarily absorb shallow soil water (0–20 cm, 29.4% ± 16.9%) and deep soil water (120–180 cm, 24.7% ± 5.8%). Pearson's correlation analysis revealed the key driving factors behind water uptake patterns were vegetation transpiration and soil salinity. Remarkably, the build up of salts in the lower soil layer (60–120 cm) hinders the absorption of water by plants. To prevent high salt concentrations from affecting water uptake in non‐halophyte plants, we recommend implementing sufficient irrigation from March to April each year to meet the water needs of plant growth and regulate the accumulation of salts in various soil layers. This study reveals the dynamic water utilization strategy of non‐halophyte plants in temperate coastal regions, offering valuable information for water resources management.
A minuscule fraction of the Earth's paleobiological diversity is preserved in the geological record as fossils. What plant remnants have withstood taphonomic filtering, fragmentation, and alteration in their journey to become part of the fossil record provide unique information on how plants functioned in paleo‐ecosystems through their traits. Plant traits are measurable morphological, anatomical, physiological, biochemical, or phenological characteristics that potentially affect their environment and fitness. Here, we review the rich literature of paleobotany, through the lens of contemporary trait‐based ecology, to evaluate which well‐established extant plant traits hold the greatest promise for application to fossils. In particular, we focus on fossil plant functional traits, those measurable properties of leaf, stem, reproductive, or whole plant fossils that offer insights into the functioning of the plant when alive. The limitations of a trait‐based approach in paleobotany are considerable. However, in our critical assessment of over 30 extant traits we present an initial, semi‐quantitative ranking of 26 paleo‐functional traits based on taphonomic and methodological criteria on the potential of those traits to impact Earth system processes, and for that impact to be quantifiable. We demonstrate how valuable inferences on paleo‐ecosystem processes (pollination biology, herbivory), past nutrient cycles, paleobiogeography, paleo‐demography (life history), and Earth system history can be derived through the application of paleo‐functional traits to fossil plants.
The water status of the living tissue in leaves between the xylem and stomata (outside xylem zone - OXZ) plays a critical role for plant function and global mass and energy balance but has remained largely inaccessible. We resolve the local water relations of OXZ tissue using a nanogel reporter of water potential (ψ), AquaDust, that enables an in-situ, non-destructive measurement of both ψ of xylem and highly localized ψ at the terminus of transpiration in the OXZ. Working in maize (Zea mays L.), these localized measurements reveal gradients in the OXZ that are several fold larger than those based on conventional methods and values of ψ in the mesophyll apoplast well below the macroscopic turgor loss potential. We find a strong loss of hydraulic conductance in both the bundle sheath and the mesophyll with decreasing xylem potential but not with evaporative demand. Our measurements suggest the OXZ plays an active role in regulating the transpiration path, and our methods provide the means to study this phenomenon.
Foliar water uptake can recharge water storage tissue and enable greater hydration than through access to soil water alone; however, few studies have explored the role of the bark in facilitating water uptake. We investigated pathways and dynamics of bark water uptake (BWU) in stems of the mangrove Avicennia marina. We provide novel evidence that specific entry points control dynamics of water uptake through the outer bark surface. Furthermore, using a fluorescent symplastic tracer dye we provide the first evidence that lenticels on the outer bark surface facilitate BWU, thus increasing stem water content by up to 3.7%. X‐ray micro‐computed tomography showed that BWU was sufficient to cause measurable swelling of stem tissue layers increasing whole stem cross‐sectional area by 0.83 mm2 or 2.8%, implicating it as a contributor to the diel patterns of water storage recharge that buffer xylem water potential and maintain hydration of living tissue.
The Jarvis‐type model, which incorporates stress functions, is commonly used to describe the physiological behaviour of stomatal response in various vegetation species. However, the model has been criticized for its empirically formulated multiplicative equation, which may not accurately capture the mutual impact of intercorrelated stress factors, for example, vapour pressure deficit (VPD) and air temperature ( T a ). This study proposed a modified Jarvis model that introduces reduction factors in the stress functions of VPD and T a to provide the description of canopy conductance. We used sap flow data from a beech forest in the mid‐latitude region of Centre Europe to inversely estimate the canopy conductance with optimized stress functions. Our findings reveal that two recommended parameterization strategies for general deciduous broadleaf forest (DBF) significantly overestimated the transpiration rate, with a maximum value of ~2 mm/day on rainless days. This suggested that the beech forest exhibited a distinct stomatal response compared to the general DBF category. By applying boundary line analysis to fit the parameters, both the unmodified and modified Jarvis models provided better simulations of transpiration, with relatively high Nash‐Sutcliffe Efficiency (NSE) values of 0.75 and 0.77, respectively. These results indicated that modelling transpiration can be improved by refining the parameterization of canopy conductance, particularly for vegetation species with unique stomatal behaviours that deviated from the characteristics of their general vegetation type. The modified Jarvis model offers a more accurate description of canopy conductance and enhances the modelling of transpiration in vegetated areas, especially under dry environment conditions with relatively high VPD.
Associated with climate change, the frequency, duration, and intensity of heatwaves are increasing in most of the key wine regions worldwide. Depending on timing, intensity, and duration, heatwaves can impact grapevine yield and berry composition, with implications for wine quality. To overcome these negative effects, two types of mitigation practices have been proposed (i) to enhance transpiration and (ii) to reduce the radiation load on the canopy. Here we use a biophysical model to quantify the impact of these practices on canopy gas exchange, vine water status, and leaf temperature (T l). Model validation was performed in a commercial vineyard. Modelled T l from 14 to 43 • C, and transpiration, from 0.1 to 5.4 mm d − 1 , aligned around the identity line with measurements in field-grown vines; the RMSD was 2.6 ºC for temperature and 0.96 mm day − 1 for transpiration. Trellis system and row orientation modulate T l. A sprawling single wire trellis with an EW orientation maintained the canopy around 1ºC cooler than a Vertical Shoot Positioned canopy with NS for the same range of total fraction of soil available water (TFAW). Although irrigation before a heatwave is a recommended practice, maximum transpi-ration can be sustained even when TFAW is reduced, limiting the heat dampening effect of irrigation. Alternatively , canopy cooling can be achieved through Kaolin application, the installation of shade cloth placement, or canopy trimming. Shade cloth produced a greater cooling than Kaolin in all the simulated scenarios; however, T l differences between them varied. Trimming reduced T l from 2 ºC to almost 8 ºC compared to its non-trimmed counterpart. Our analysis presents new insights to design heat wave mitigation strategies and supports agro-nomically meaningful definitions of heat waves that include not only temperature, but also wind, VPD, and radiation load as these factors influence crop physiology under heat stress.
Tomato is the second most important vegetable crop after potato and a widely studied model organism. Apart from real-life measurements, mathematical models that translate experimental data into meaningful information for production optimization, are becoming more essential, especially regarding the current challenges of climate change and increasing energy costs. The objective of this research is to develop a tomato plant growth model incorporating water potential, a connecting variable of physiological processes of the soil-plant-atmosphere continuum. Two experiments at different sites, measuring plant growth from planting for several months, were conducted to measure leaves and trusses (nodes) appearance rate, stem elongation, leaf area, organs’ fresh and dry mass, whole-plant transpiration and stem water potential. Climate (temperature, relative humidity, light radiation) was monitored at different vertical positions. The collected data were used for parameter estimation and validation of the integrated model coupling a reduced TOMGRO model for plant growth, a transpiration model based on the adaptation of the Penman-Monteith equation, and a water transport model using an analogue of the electric current. By using a compartment structure, the functional-structural plant model is simplified while preserving structural information. The top-bottom modelling approach was able to precisely simulate long-term plant growth. Furthermore, the model accounts for microclimate and allows custom-defined grower treatments as discrete steps in simulation, calculating water potential differences through the plant structure, as observed from experimental trails. The main aim for the future is to use modeled variables (such as water potential) as an input for the fruit growth and ripening model. Thanks to the modularity the model can be easily extended by additional organs (e.g., roots) or processes (e.g., photosynthesis). The fruit model incorporating the above plant model will provide more precise results in terms of understanding physiological processes, prediction and optimization.
Water acquisition via the root system of woody species is a key factor governing plant physiology. In order to compare the impact of water acquisition on the hydraulic and photosynthetic characteristics of different-sized Populus euphratic, which is a desert riparian tree species, we quantified leaf hydraulic conductance (KL), stomatal conductance (gs), net photosynthetic rate (PN), predawn and midday leaf water potential (Ψ), and the stem δ18O of the saplings and mature trees. The results showed that the saplings had a lower predawn leaf water potential (Ψpd) and soil-to-leaf water potential gradient (ΔΨ) and a higher KL than the mature trees but had a similar gs and PN to the mature trees. In arid zones, probably due to root limitation, the saplings were more likely to use unreliable topsoil water (<80 cm), whereas the mature trees typically uptake reliable deep soil water (>80 cm) and groundwater due to having deeper root systems. The unreliability of the water supply might make saplings hold a higher hydraulic conductance to ensure that the water is transported efficiently to the leaves and to satisfy their transpiration need. In contrast, the mature trees, which uptake the more reliable deeper water resources, had a relatively low leaf-specific hydraulic conductance because of the increased path length versus the saplings. However, adult trees can maintain stomatal conductance by upregulating ΔΨ, thereby facilitating their ability to maintain a carbon assimilation rate similar to that of the saplings. This regulating behavior benefits mature trees’ net carbon gain, but it comes at the expense of an increased risk of hydraulic failure. These results imply that the top priority for saplings should be to maintain hydraulic system functioning, whereas, for mature trees, the priority is to assure stable net carbon gain for their growth.
During stem elongation, wheat (Triticum aestivum) increases its stem carbohydrate content before anthesis as a reserve for grain filling. Hydraulic functioning during this mobilization process is not well understood, and contradictory results exist on the direct effect of drought on carbohydrate mobilization.
In a dedicated experiment, wheat plants were subjected to drought stress during carbohydrate mobilization. Measurements, important to better understand stem physiology, showed some unexpected patterns that could not be explained by our current knowledge on water transport. Traditional water flow and storage models failed to properly describe the drought response in wheat stems during carbohydrate mobilization. To explain the measured patterns, hypotheses were formulated and integrated in a dedicated model for wheat.
The new mechanistic model simulates two hypothetical water storage compartments: one where water is quickly exchanged with the xylem and one that contains the carbohydrate storage. Water exchange between these compartments is turgor‐driven. The model was able to simulate the measured increase in stored carbohydrate concentrations with a decrease in water content and stem diameter.
Calibration of the model showed the importance of turgor‐driven apoplastic water flow during carbohydrate mobilization. This resulted in an increase in stem hydraulic capacitance, which became more important under drought stress.
In recent years, research interest in plant water uptake strategies has significantly grown in many disciplines such as hydrology, plant ecology and ecophysiology. Quantitative modelling approaches to estimate plant water uptake and the spatio-temporal dynamics significantly advanced from different disciplines across scales. Despite this progress, major limitations, i.e. to predict plant water uptake under drought or it´s impact at large-scales remain. These are less attributed to limitations in process understanding, but rather to a lack of implementation of cross-disciplinary insights in plant water uptake model structure. The main goal of this review is to highlight how the 4 dominant model approaches, e.g. Feddes approach, hydrodynamic approach, optimality and statistical approaches, can be and have been used to create interdisciplinary hybrid models enabeling a holistic system understanding that e.g. embeds plant water uptake plasticity into a broader conceptual view of soil-plant feedbacks of water, nutrient and carbon cycling or reflects observed drought responses of plant-soil feedbacks and their dynamics under e.g. drought. Specifically, we provide examples of how integration of Bayesian and hydrodynamic approaches might overcome challenges in interpreting plant water uptake related to e.g. different travel and residence times of different plant water sources or trade-offs between root system optimization to forage for water and nutrients during different seasons and phenological stages.
Background
Recent reports of extreme levels of undersaturation in the internal leaf air spaces have called into question one of the foundational assumptions of leaf gas exchange analysis, that leaf air spaces are effectively saturated with water vapor at leaf surface temperature. Historically, inferring the biophysical states controlling assimilation and transpiration from the fluxes directly measured by gas exchange systems has presented a number of challenges, including: 1) a mismatch in scales between the area of flux measurement, the biochemical cellular scale, and the meso-scale introduced by the localization of the fluxes to stomatal pores; 2) the inaccessibility of the internal states of CO2 and water vapor required to define conductances; and 3) uncertainties about the pathways these internal fluxes travel. In response, plant physiologists have adopted a set of simplifying assumptions that define phenomenological concepts such as stomatal and mesophyll conductances.
Scope
Investigators have long been concerned that a failure of basic assumptions could be distorting our understanding of these phenomenological conductances, and the biophysical states inside leaves. Here we review these assumptions and historical efforts to test them. We then explore whether artifacts in analysis arising from the averaging of fluxes over macroscopic leaf areas could provide alternative explanations for some part, if not all, of reported extreme states of undersaturation.
Conclusions
Spatial heterogeneities can, in some cases, create the appearance of undersaturation in the internal air spaces of leaves. Further refinement of experimental approaches will be required to separate undersaturation from the effects of spatial variations in fluxes or conductances. Novel combinations of current and emerging technologies hold promise for meeting this challenge.
The perfect storm. That is the term used by Gerald C. Nelson to describe the triple challenge of increasing food production while adapting to climate change and reducing the environmental impact of agricultural systems. Nowadays, conventional farming systems are showing some limitations, such as low resources use efficiency and poor ecosystems services that appear to be associated to the loss of plant diversity and perenniality in crop rotations. In addition, water, the most important yield limiting factor worldwide, will increasingly restrict food production in the future due to rainfall shortage and increase in human consumption. In such context, perennial crops, with denser and deeper root system could use resources in deep soil layers that are logically inaccessible to crops with shallower root system. The goal of this thesis was therefore to investigate the root growth and water uptake capacity of intermediate wheatgrass (Kernza®) and alfalfa, two deep rooted perennial crops, under field conditions and at great soil depth (i.e. 1.0-2.5 m).
Maintaining hydraulic continuity along the soil-plant-atmosphere continuum is a prerequisite for deep water uptake. At the plant level, hydraulic conductivity depends on complex anatomical and physiological processes among which the root system constitutes the second largest resistance to water flow. Therefore, in depth characterisation of root and xylem anatomy was done to understand the hydraulic properties of the crop root systems, with a focus on their evolution with soil depth. Crops were grown in the field, rhizoboxes, mesocosms and solution culture to take into account the variability of root type and soil depth as well as growing environment. For both crops, axial hydraulic conductance decreased with soil depth and along individual root segment. Alfalfa roots had greater axial hydraulic conductance in comparison to intermediate wheatgrass roots, especially at depth. Root and xylem anatomy were highly variable across crop species, root types and growing environments. In parallel, a combination of imaging and sensor technology, stable isotope techniques and a modelling approach was used to study root growth and water uptake under field conditions during the 2018-2019 seasons. Both crops presented roots down to 2.0 m soil depth that were active in terms of water uptake. Alfalfa had greater root length at depth and absorbed twice as much water below 1 m soil depth, than intermediate wheatgrass. For both crops, model simulations predicted that water uptake in deep soil layers (i.e. 1.5 – 2.0 m) increase (i.e.>30%) under dry conditions.
This thesis brings insights into the understudied field of root growth and water uptake at great soil depth. Particular efforts were put in understanding the environmental and agricultural contexts in which deep root growth, deep water uptake
The vegetation optical depth (VOD) variable contains information on plant water content and biomass. It can be estimated alongside soil moisture from currently operating satellite radiometer missions, such as SMOS (ESA) and SMAP (NASA). The estimation of water fluxes, such as plant water uptake (PWU) and transpiration rate (TR), from these earth system parameters (VOD, soil moisture) requires assessing water potential gradients and flow resistances in the soil, the vegetation and the atmosphere. Yet water flux estimation remains an elusive challenge especially on a global scale. In this concept study, we conduct a field-scale experiment to test mechanistic models for the estimation of seasonal water fluxes (PWU and TR) of a winter wheat stand using measurements of soil moisture, VOD, and relative air humidity (RH) in a controlled environment. We utilize microwave L-band observations from a tower-based radiometer to estimate VOD of a wheat stand during the 2017 growing season at the Selhausen test site in Germany. From VOD, we first extract the gravimetric moisture of vegetation and then determine the relative water content (RWC) and vegetation water potential (VWP) of the wheat field. Although the relative water content could be directly estimated from VOD, our results indicate this may be challenging for the phenological phases, when rapid biomass and plant structure development take place within the wheat canopy. We estimate water uptake from the soil to the wheat plants from the difference between the soil and vegetation potentials divided by the flow resistance from soil into wheat plants. The TR from the wheat plants into the atmosphere was obtained from the difference between the vegetation and atmosphere water potentials divided by the flow resistances from plants to the atmosphere. For this, the required soil matric potential (SMP), the vapor pressure deficit (VPD), and the flow resistances were obtained from on-site observations of soil, plant, and atmosphere together with simple mechanistic models. This pathfinder study shows that the L-band microwave radiation contains valuable information on vegetation water status that enables the estimation of water dynamics (up to fluxes) from the soil via wheat plants into the atmosphere, when combined with additional information of soil and atmosphere water content. Still, assumptions have to be made when estimating the vegetation water potential from relative water content as well as the water flow resistances between soil, wheat plants, and atmosphere. Moreover, direct validation of water flux estimates for the assessment of their absolute accuracy could not be performed due to a lack of in situ PWU and TR measurements. Nonetheless, our estimates of water status, potentials, and fluxes show the expected temporal dynamics, known from the literature, and intercompare reasonably well in absolute terms with independent TR estimates of the NASA ECOSTRESS mission, which relies on a Priestly–Taylor type of retrieval model. Our findings support that passive microwave remote-sensing techniques qualify for the estimation of vegetation water dynamics next to traditionally measured stand-scale or plot-scale techniques. They might shed light on future capabilities of monitoring water dynamics in the soil–plant–atmosphere system including wide-area, remote-sensing-based earth observation data.
Modelling the water transport along the soil–plant–atmosphere continuum is fundamental to estimating and predicting transpiration fluxes. A Finite-difference Ecosystem-scale Tree Crown Hydrodynamics model (FETCH3) for the water fluxes across the soil–plant–atmosphere continuum is presented here. The model combines the water transport pathways into one vertical dimension, and assumes that the water flow through the soil, roots, and above-ground xylem can be approximated as flow in porous media. This results in a system of three partial differential equations, resembling the Richardson–Richards equation, describing the transport of water through the plant system and with additional terms representing sinks and sources for the transfer of water from the soil to the roots and from the leaves to the atmosphere. The numerical scheme, developed in Python 3, was tested against exact analytical solutions for steady state and transient conditions using simplified but realistic model parameterizations. The model was also used to simulate a previously published case study, where observed transpiration rates were available, to evaluate model performance. With the same model setup as the published case study, FETCH3 results were in agreement with observations. Through a rigorous coupling of soil, root xylem, and stem xylem, FETCH3 can account for variable water capacitance, while conserving mass and the continuity of the water potential between these three layers. FETCH3 provides a ready-to-use open access numerical model for the simulation of water fluxes across the soil–plant–atmosphere continuum.
To prevent rise of water table above a critical level, so as to avoid waterlogging and salinization of rootzone, attempts are usually made to lower it by installing drainage system wherein saline groundwater is pumped out and disposed of. Conventional surface and subsurface drainage are not always feasible due to cost factors, topographic constraints, clogging of drains and problems associated with disposal of salt-loaded drainage effluents which many times also contain toxic elements. It requires fossil energy to operate pumps and cannot be adopted by individual farmer.
Plant stems show reversible diurnal fluctuation and irreversible growth, both related to plant water status. The reversible stem diameter shrinkage and swelling are caused by a depletion and refilling of the plant’s internal water storage pools, while irreversible growth occurs when turgor pressure exceeds a certain threshold value. For this reason, stem diameter measurements provide a useful tool to assess plant water status. In this study, the use of continuous stem diameter measurements to detect atmospheric and soil drought stress in Ficus benjamina L. was explored by assessing the deviation between measured and simulated stem diameter variations using a mechanistic stem diameter model with moving window calibration. Sap flow, either directly measured or simulated with measurements of the microclimate, was used as input to the model. Both atmospheric stress by a high vapor pressure deficit and drought stress by a reduced soil water content were detected, with the latter 2 to 3 days earlier than detection based on maximum daily shrinkage and daily growth. Therefore, comparing stem diameter measurements with model simulations shows great potential for irrigation scheduling in horticulture.
Leaf hydraulic networks play an important role not only in fluid transport but also in maintaining whole-plant water status through transient environmental changes in soil-based water supply or air humidity. Both water potential and hydraulic resistance vary spatially throughout the leaf transport network, consisting of xylem, stomata and water-storage cells, and portions of the leaf areas far from the leaf base can be disproportionately disadvantaged under water stress. Besides the suppression of transpiration and reduction of water loss caused by stomatal closure, the leaf capacitance of water storage, which can also vary locally, is thought to be crucial for the maintenance of leaf water status. In order to study the fluid dynamics in these networks, we develop a spatially explicit, capacitive model which is able to capture the local spatiotemporal changes of water potential and flow rate in monocotyledonous and dicotyledonous leaves. In electrical-circuit analogs described by Ohm's law, we implement linear capacitors imitating water storage, and we present both analytical calculations of a uniform one-dimensional model and numerical simulation methods for general spatially explicit network models, and their relation to conventional lumped-element models. Calculation and simulation results are shown for the uniform model, which mimics key properties of a monocotyledonous grass leaf. We illustrate water status of a well-watered leaf, and the lowering of water potential and transpiration rate caused by excised water source or reduced air humidity. We show that the time scales of these changes under water stress are hugely affected by leaf capacitance and resistances to capacitors, in addition to stomatal resistance. Through this modeling of a grass leaf, we confirm the presence of uneven water distribution over leaf area, and also discuss the importance of considering the spatial variation of leaf hydraulic traits in plant biology.
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Functional structural plant models of tree crops are useful tools that were introduced more than two decades ago. They can represent the growth and development of a plant through the in silico simulation of the 3D architecture in connection with physiological processes. In tree crops, physiological processes such as photosynthesis, carbon allocation, and growth are usually integrated into these models although other functions such as water and nutrient uptake are often disregarded. The implementation of the 3D architecture involves different techniques such as L-system frameworks, pipe model concepts, and Markovian models to simulate branching processes, bud fates, and elongation of stems based on the production of metamers. The simulation of root architecture is still a challenge for researchers due to a limited amount of information and experimental issues in dealing with roots because root development is not based on the production of metamers. This review aims to focus on functional-structural models of fruit tree crops, highlighting their physiological components. The potential and limits of these tools are reviewed to point out the topics that still need more attention.
With the rapid development of society, the soil and water environments in many countries are suffering from severe pollution. Pollutants in different phases will eventually gather into the soil and water environments, and a series of migrations and transformations will take place at ecohydrological interfaces with water flow. However, it is still not clear how ecohydrological interfaces affect the migration and the transformation of pollutants. Therefore, this paper summarizes the physical, ecological, and biogeochemical characteristics of ecohydrological interfaces on the basis of introducing the development history of ecohydrology and the concept of ecohydrological interfaces. The effects of ecohydrological interfaces on the migration and transformation of heavy metals, organic pollutants, and carbon‑nitrogen‑phosphorus (C-N-P) pollutants are emphasized. Lastly, the prospects of applying ecohydrological interfaces for the removal of pollutants from the soil and water environment are put forward, including strengthening the ability to monitor and simulate ecohydrological systems at micro and macro scales, enhancing interdisciplinary research, and identifying main influencing factors that can provide theoretical basis and technical support.
Hydraulic conductance is recognized as a major determinant of gas exchange and productivity. However, whether this also applies to seedlings, a critically important stage for vegetation regeneration, has been largely unknown.
We analyzed the hydraulic and stomatal conductance of leaves and shoots for 6‐wk‐old Acer pseudoplatanus seedlings emerging in different lowland and treeline habitats and under glasshouse conditions, respectively, as well as on 9‐, 15‐ and 18‐wk‐old plants, and related findings to leaf and xylem anatomical traits.
Treeline seedlings had higher leaf area‐specific shoot hydraulic conductance (Kshoot‐L), and stomatal conductance (gs), associated with wider xylem conduits, lower leaf area and higher stomatal density than lowland and glasshouse‐grown plants. Across the first 18 wk of development, seedlings increased four‐fold in absolute shoot hydraulic conductance (Kshoot) and declined by half in Kshoot‐L, with correlated shifts in xylem and leaf anatomy. Distal leaves had higher leaf hydraulic conductance (Kleaf) and gs compared to basal leaves.
Seedlings show strong variation across growth environments and ontogenetic shifts in hydraulic and anatomical parameters. Across growth sites, ontogenetic stages and leaf orders, gs was tightly correlated with Kshoot‐L and Kleaf, balancing hydraulic supply with demand for the earliest stages of seedling establishment.
Global and regional monitoring of drought are becoming an active research subject during the last decades. In the Middle East and North Africa (MENA) region drought episodes highly control water availability and the functioning of both forested and cultivated ecosystems. The ecohydrologic approach represents a relatively new trend in the holistic assessment of these limited water resources ecosystems as it explains the equilibrium between the components of the soil-vegetation-climate complex. On the other hand, during the last two decades, the models used in the assessment of drought causes and manifestations combine more and more indicators from multi-sensors satellite images. In the present study, the ecohydrology concepts and their methodological basis are presented, and an overview of biophysical and energetic variables derived from remote sensing data at the regional scale are exposed. A general review of the use of remote sensing in ecohydrology during the last two decades is also addressed as well as various methods using satellite images in the ecohydrologic modelling. These methods are divided into two major groups: the direct use of remote sensing in drought and humidity assessment, and the integration of satellite images with other data in water balance models for hydric stress assessment. The ecohydrological modelling integrating remote sensing data makes use of different types of models such as statistical, empirical or physical based models. The availability of free time series satellite images such as MODIS sensors since the year 2000 had allowed the exploration of various models for drought assessment where multispectral images are combined to derive drought indicators of the vegetation in a Tunisian Mediterranean ecosystem. Finally, data quality of time-series images and their calibration and correction are discussed to highlight the required processing for convenient use of these data in drought monitoring at the regional scale.
Different from leaf photosynthesis that depends on stomatal uptake of CO2, stem green tissue mainly uses internal CO2 for re-assimilation. Research attention has moved to its specific effect on xylem hydraulic function, however, the effect of stem corticular photosynthesis on whole-plant water transport and leaf photosynthesis during drought stress of varying intensities remains unexplored. To fill this knowledge gap, we designed stem shading treatments on Eucalyptus urophylla trees in the field and its seedlings in the greenhouse with soil water-controlled experiment. Sap flow density (Js), whole-plant hydraulic conductance (Kplant), stem tissue fluorescence parameters and stem non-structural carbohydrates (NSC) were measured. Stem shading treatments caused the decrease of Kplant and Js of E. urophylla trees, more importantly, the increase of Kplant and Js with the recovery of corticular photosynthesis after shading removal was observed in the field experiment. The main effect of shading treatment on stem NSC concentration of E. urophylla seedlings is a decrease, which was not influenced by soil water conditions. Moreover, the shading effects on leaf gas exchange vary with drought intensity, under severe drought stress, stem shading caused a significant decline of stomatal conductance (gs,) and leaf maximum net photosynthesis (Amax) for E. urophylla seedlings. These findings suggested that stem corticular photosynthesis plays a compensatory role in maintaining whole-plant hydraulic conductance integrity and water transport of E. urophylla. The remarkable negative impact of stem shading on leaf gas exchange implying a possible interconnection between leaf and stem photosynthesis, which highlights the potential significance of stem photosynthesis in drought stress resistance under changing climate regimes.
This research work took into account two extracting areas that were used in the construction of the road between Pirayú
and Yaguarón in Paraguay and describes the situation in the area taking into account the current related legislation. Signs of
degradation have been observed, such as erosion in progress, compacted soil, scarce vegetative repopulation and lack of organic
matter necessary to regenerate the flora, among others. Includes an analysis of the techniques applied in similar cases of degradation
in other countries and those techniques that show more compatibility with the conditions of the areas under study have been selected;
alternative rehabilitation techniques in road bank areas and surrounding surfaces are also recommended. The goal is that the selected
areas will blend in with the landscape and will resume similar conditions to the surrounding landscape in a medium term. The most
compatible techniques that were found were the reshaping of road banks in the degraded areas and the insertion of herbal species for
the proposal for the implementation of bioengineering and soil improvements including the introduction of hardwood species after
the insertion of herbal species.
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