A Longitudinal section showing a time series of cavitated vessels refilling in less than 4 hrs; B 3D reconstruction of four vessel lumen with water droplets forming on the vessel walls and growing over time to completely fill the embolized conduit. Beerling, D. Plant science: The hidden cost of transpiration. Nature , Brodersen, C. The dynamics of embolism repair in xylem: In vivo visualizations using high-resolution computed tomography Plant Physiology , Brodribb, T.
Water stress deforms tracheids peripheral to the leaf vein of a tropical conifer. Plant Physiology , Canadell, J. Maximum rooting depth of vegetation types at the global scale. Oecologia , Choat, B. Structure and function of bordered pits: New discoveries and impacts on whole-plant hydraulic function. New Phytologist , Chung, H. Absorption of water and "P through suberized and unsuberized roots of loblolly pine. Canadian Journal of Forest Research 5, Eapen, D. Hydrotropism: Root growth responses to water.
Trends in Plant Science 10, Hetherington, A. The role of stomata in sensing and driving environmental change. Holbrook, N. Vascular Transport in Plants. Javot, H. The role of aquaporins in root water uptake. Annals of Botany 90, Kramer, P. Water Relations of Plants and Soils. Seasonal variations in the proportions of suberized and unsuberized roots of trees in relation to the absorption of water. American Journal of Botany 53, MacFall, J. Observation of a water-depletion region surrounding loblolly pine roots by magnetic resonance imaging.
McCully, M. Roots in Soil: Unearthing the complexities of roots and their rhizospheres. McDowell, N. Mechanisms of plant survival and mortality during drought: Why do some plants survive while others succumb to drought? Nardini, A. Refilling embolized xylem conduits: Is it a matter of phloem unloading? Plant Science , Pittermann, J. Torus-margo pits help conifers compete with angiosperms. Science , Sack, L. Leaf hydraulics. Annual Review of Plant Biology 57, Schenk, H. Journal of Ecology 90, Sperry, J.
Mechanism of water-stress induced xylem embolism. Plant Physiology 88, Steudle, E. The cohesion-tension mechanism and the acquisition of water by plants roots. Transport of water in plants.
Environmental Control in Biology 40, Takahashi, H. Hydrotropism and its interaction with gravitropism in roots. Plant Soil , Tyree, M. The hydraulic architecture of trees and other woody plants. Phloem transports sucrose and amino acids up and down the plant. This is called translocation. In general, this happens between where these substances are made the sources and where they are used or stored the sinks.
This means, for example, that sucrose is transported:. In that respect, although the discovery of aquaporins Murata et al. Numerous tools have been developed to probe the mechanism underlying the passive transport of water in plants.
During the past two decades, the concept of passive water transport has been heatedly debated in the scientific community Zimmermann et al. In this review, we highlight the major experimental tools that have provided insight into sap flow through the xylem network. From the broader perspective of the Blue Revolution Pennisi, , understanding how water is transported from the soil through the intricate plant xylem network to the atmosphere can lead to innovative ways to optimize each drop of water in applied scientific fields such as molecular biology and agronomy, and in breeding programmes that seek to improve drought-resistance in crop plants.
Some industrial applications based on our understanding of microfluidics and nanofluidics have already started to emerge in the form of plant-inspired devices, such as synthetic trees Wheeler and Stroock, In recent reviews, Pittermann presented an integrated approach of the evolution of the plant vascular system, and Lucas et al. Two recent international meetings the 9th International Workshop on Sap Flow, , and the Third International Conference on Plant Vascular Biology, demonstrated that sap flow is an area of prolific and inspiring research.
However, there is no agreement as to which methods are best for examining sap flow or how the new results contribute to unravel sap flow dynamics in vascular plants.
In this review, we briefly retrace the scientific investigation of water transport in vascular plants, and evaluate basic concepts and theories in light of new experimental methods. Real-time imaging emerges as the most promising approach for integrating the xylem network structure and its multiple layers of regulation.
Our understanding of xylem hydraulic properties has evolved with the development of theoretical modelling and novel experimental tools to visualize the cross-sectional and three-dimensional structure of xylem.
Tracheary elements TEs are the elementary units of xylem. After a complex process of differentiation, TEs lose their nuclei and cell contents, leaving behind a central lumen surrounded by secondary cell walls, which together form tracheids or vessels Fukuda, The structural characteristics of tracheids in conifers and vessel elements in angiosperms have been well characterized using optical and electron microscopy.
The diameter of TEs varies from a few micrometres to a few hundred micrometres. Their association in series to form long-distance pathways can attain a few millimetres up to several metres. Torus-margo or pit membranes integrated in the secondary cell wall provide various levels of subcellular resistance to water flow Schulte and Castle, ; Hacke et al.
Although the structural characteristics of TEs are well established, our understanding of water flow dynamics is limited to the tissue or organ level. From a bottom-up perspective, water and minerals from the soil are absorbed through apoplastic and symplastic pathways into protoxylem vessels of the roots Passioura, Then, long-distance transport in the stem is generally attributed to large metaxylem vessels.
The vascular bundles in leaves become highly branched reducing the distance of most leaf cells to less than a few hundred micrometres from a vessel Fig. The mesophyll at the interface with air represents the highest resistance to water flow Cochard et al. Main characteristics of the xylem network.
Organization and characteristics of the xylem network: water flow throughout the plants depends on characteristics of the xylem in different organs. Water absorbed by the roots moves radially from small protoxylem vessels, which have high hydraulic resistance, to larger metaxylem vessels, with reduced hydraulic resistance.
In the stem, the number and organization of vessels vary along the height of the plant height. Packing and tapering functions can be used to characterize each level of organization. In the leaves, water travels through small xylem vessels. During transpiration, negative hydrostatic pressure is generated at the interface between mesophyll cells and air.
The overall resistance is determined by soil water potential, conducting vessels, transpiration rate, plant height, and gravity. In this physical conceptualization of the soil-plant-atmosphere continuum, the tension of the driving force of sap ascent continuously decreases in the direction of flow, and the pressure gradient is proportional to the evaporative flux density from the leaves Tyree, The xylem provides a low-resistance pathway for long-distance water movement by minimizing the pressure gradients required to transport water from the soil to the leaves Jeje and Zimmermann, The pipe model has contributed to the estimation of canopy-level parameters by incorporating variations in vessel size and number at the tissue and organ levels, and was also used to understand tree growth, resource allocation, and plant biomechanics Niklas et al.
However, the functionalities of the xylem network integrate different structural organization at the tissue and organ levels that cannot be supported by this simplified model. Hydraulic resistance is highly variable depending on the species and organ. Unravelling how water is collected from all the vessels in the roots, passes through the stem, and is distributed in the leaves requires an integrated functional approach at the whole-plant level Sperry, ; Loepfe et al.
At the tissue level, the hydraulic conductivity per unit of cross-sectional area generally defines efficiency. The constraints on the maximum diameter, length, and number of xylem vessels for a given cross-sectional area limit efficiency: this is related to a species-dependent limit on conduit frequency.
Such variation is due to the lower investment in mechanical strength in angiosperms, which rely on wood fibres, whereas conifer tracheids provide both transport and support functions. Conduit diameter and frequency are not the only factors determining efficiency of water flow, because the conductivity of conduits of a given diameter can also vary.
In angiosperms, simple or schalariform perforation plates and conduit end walls create differences in actual conductivity compared with the theoretical maximum set by the Hagen-Poiseuille equation. Lumen and end-wall resistance is relatively constant and flow resistance through pits does not increase with cavitation safety. Pit membrane porosity does not seem to be related to cavitation pressure Hacke et al.
Despite difference in size, the end wall resistance at a given diameter seems to be relatively similar between conifers and angiosperms. The presence of specialized structures, such as the torus-margo in conifers, greatly reduces the resistance of inter-tracheid water flow Pittermann et al. In summary, the most important structural features of the xylem at the cellular level are conduit diameter, length, wall features i. At the tissue level, inter-conduit pitting determined by the density and size of torus-margo or pit membrane and the number of conduits define the connectivity of the xylem network.
The two main functions of the xylem hydraulic network in vascular plants are i to supply water and minerals to all tissues and ii to provide mechanical support. In living organisms, similar functions are generally carried out by similar structures. A large diversity in xylem hydraulic architecture exists between organs and among species, and the initial structures are even modified during growth and development. Among seed plants, coniferous, diffuse-porous, and ring-porous trees have radically different xylem anatomy McCulloh et al.
Within angiosperms, the vascular bundles of dicot and monocot plants have distinct organizations that vary in different organs. These differences in organization are ultimately due to differences in conduit tapering and packing of similar elementary structures. However, the functional consequences of these distinct organizations are not well understood at either the conduit or the whole-organism level. The integration of organ-level variation in xylem architecture at the whole-plant level is essential for unravelling the mechanisms that maintain the integrity of water transport from roots to leaves.
The elementary elements of the system i. To integrate structural characteristics into functional roles, it requires determining how the dynamic hydraulic properties at the cellular level are incorporated into tissue and organ levels. For example, the hydraulic efficiency per conduit diameter and length is higher for conifer tracheids than for angiosperm vessel elements; however, the wider diameter and greater length of angiosperm vessels provide greater conductivity per xylem area.
In terms of the biophysical mechanisms underlying these processes, the major challenge is to understand how the trade-off between efficiency and safety is achieved at different levels of organization.
The hydraulic regulation attributed to the xylem is generally considered to depend on the specific organization in each organ; however, the respective contributions are difficult to integrate into the entire network Fig. In leaves, direct pressure-drop measurements confirmed that mesophyll cells are the major component of hydraulic resistance, even though the vascular system accounts for the longest distance Cochard et al. From the soil to the atmosphere, the relationship between hydraulic resistance and stomatal conductance is a key component Cruiziat et al.
When transpiration is high, maintaining the continuity of flow in individual vessels is seriously challenged owing to cavitation risks Cochard, However, cavitation can be reduced by the hydraulic capacitance of the xylem and the water storage capacity of each organ, and the network organization can also provide alternative pathways to avoid disruption of water flow Tyree and Ewers, ; Sperry et al. Ultimately, water transport and gas exchange in the leaves have a major physiological effect on the photosynthetic capacity of the plant Tyree and Ewers, The structural model of the hydraulic transport system proposed by West, Brown, and Enquist WBE model; has been widely used to explain the maintenance of a constant flow rate along the entire flow path.
In this model, it is assumed that plants minimize the effect of hydraulic resistance imposed by increasing height and total path-length conductance by tapering the xylem conduits. Plant size is related to the geometry of the branching architecture and metabolism. Based on the fact that all living organisms contain a transport system for aqueous materials, the plant vascular system should minimize the hydrodynamic resistance of nutrient transport, while maximizing the exchange surface with the environment Petit and Anfodillo, The ideas that i all plants adopt a universal architecture of the xylem transport system, and ii hydraulic efficiency is independent of plant height are very attractive.
Although a wide range of plants seemed to comply with these assumptions West et al. Despite controversies, the WBE model highlights the value of architectural modelling in simplifying plant diversity and stimulated prolific empirical research. Now, complementary models of the vascular system not only include a more realistic view of the hydraulic architecture Savage et al. Although the plant xylem is non-living tissue, there is an extraordinary degree of coordination between the hydraulic capacity and photosynthetic assimilation because both of these pathways intersect at stomata during the exchange of water and CO 2 at the leaf surface Brodribb, The rate of transpiration and gas exchange via stomata are limited by the xylem hydraulic system.
Packing and taper functions are the backbone of a robust framework for modelling network transport Sperry et al. Strength and storage requirements set a packing limit and influence the conducting capacity Zanne et al. Theoretically, a small number of wide conduits are more efficient than a large number of narrow ones.
This is reflected by the more efficient networks of ring-porous trees compared with conifers McCulloh et al. Without tapering of the xylem conduits, branches would have the highest conductivity in a tree.
In other words, tapering counterbalances the decline in conductance due to increasing path length, but maintaining similar conductivity requires an increase in the number of xylem vessels per unit cross-sectional area as conduits become narrower. The organization of the xylem network thus defines the functional trade-off between efficiency and safety in each organ.
As the water potential is lower at the plant apex, fewer pores in the pits near the apex would also restrict the spreading of embolisms. An optimal hydraulic structure would have conduits that decrease in size from the base to the apex defining tapering function. In parallel, the vulnerability to cavitation can be reduced by increasing conduit number defining the packing function.
Indeed, whole-plant carbon-use efficiency demands that conduit size decreases and conduit number increases simultaneously Lancashire and Ennos, ; Choat et al. The theoretical and conceptual bases of water transport and xylem hydraulic architecture have been examined by various experimental methods Fig. Technical reliability of new methodology is of prime importance in investigating the processes of water transport. Moreover, subsequent results are rarely cross-validated with those obtained using other methods.
A difficulty in making proper comparisons is that the measurement techniques do not address the same level of the xylem network. For instance, the technical limitations of new methods in measuring internal pressure or vulnerability to cavitation have sometimes resulted in a misunderstanding of the elementary processes and have given erroneous interpretations.
The invasive methods using excised tissues do not change the internal xylem structure, but water flow generated artificially in isolated leaves, stems or roots does not accurately reflect water flow in intact plants. Methods and instruments used to analyse sap flow in plants. Schematic representation of different methods used to measure sap flow velocity. In heat-based methods, heat sensors heat pulse velocity, heat field deformation, or thermal dissipation are installed radially into a segment.
In radioisotope or dye methods, tracers are injected into the xylem or uptaken from a cut segment. Methods used to measure negative pressure in the xylem. The observation scale and measurement target i. Simultaneous visualization of xylem structure and sap flow using magnetic resonance, neutron or synchrotron X-ray imaging methods. The temporal and spatial resolutions vary for each imaging method. Three categories of methods are currently available for investigating xylem sap flow: i continuous measurement of sap flow velocity to confirm the relationship between transpiration and water uptake ; ii internal pressure measurement to confirm that negative hydrostatic pressure is the main driving force of sap flow ; and iii visualization of sap flow through the xylem.
Experimental data obtained using these different methods were frequently not in agreement, because the scale of the xylem architecture examined from the whole-plant network to individual vessels generally differed. Futhermore, sap flow dynamics were not always measured with the same hydraulic parameters. Therefore, it is crucial to understand the advantages and limitations of different techniques to compare the characteristics of sap flow across different species.
Continuous sap-flow monitoring has been most commonly used to measure water flux through the stems and branches of trees, but the resolution is not sufficient for determining leaf-level responses to environmental changes. Flow monitoring techniques using tracers and histological sections enabled the identification of the water-conducting vessels of the xylem network and provided a snapshot of how they function under different environmental conditions. The injection of different dyes e.
Recently, a number of concerns have been raised in interpreting the results of dye injection Umebayashi et al. First, the type of dye and the method used for sample preparation greatly affect the distribution and diffusion of the dye through the xylem. Second, the diversity in plant size, and vessel size and organization do not generally allow extrapolation of the results obtained for a stem, root, or leaf sections to other organs.
Third, it is difficult to compare the results of studies conducted at the whole-plant level under various environmental conditions with those obtained from the isolated tissues.
Using improved preparation methods, stabilized dye can enable the identification of water-conducting vessels in trees at the cellular level Sano et al. Dye injection is a relatively easy technique, but it gives misleading interpretations about the functional water-conducting pathways if the procedures are not well defined and standardized Umebayashi et al. More accurate modelling of leaf and plant-level responses to abiotic stresses is essential to predict the canopy response to future climate change.
In forest ecosystems, water fluxes in trees can be monitored at the stem or leaf level Fig. Heat-balance and heat-pulse methods estimate whole-plant water flow using heat-based sensors Smith and Allen, In both cases, probes inserted into the stem of a tree generate heat that is used as a tracer.
The heat-balance method calculates the mass flow of sap in the stem from the amount of heat taken up by the moving sap stream. In the heat-pulse method short pulses of heat are applied, and the mass flow of sap is determined from the velocity of the heat pulses moving along the stem Cohen et al. The thermal dissipation method, which is based on the propagation of heat pulses and was initially developed by Huber and refined by Vieweg and Ziegler , is also widely used to estimate sap flow rates.
The direction of volume flow is derived from the asymmetry of thermal dissipation; however, reliable estimates of the sap-conducting surface area and size are essential to compare the deduced sap flow rates with the actual sap flow rates Green et al. One of the major limitations of theses techniques is that the inserted probes disrupt the sap stream, which alters the thermal homogeneity of the sapwood. Recently, mathematical corrections of sap velocity include effects due to heat-convection Vandegehuchte and Steppe, b or natural temperature gradients Lubczynski et al.
In ecophysiological studies, technically improved probes are now available for continuous sap flow measurements in trees Burgess et al. A sophisticated four-needle, heat-pulse sap flow probe even permits measurement of non-empirical sap flux density and water content Vandegehuchte and Steppe, a. Measurements of sap flow alone do not provide sufficient spatial resolution to evaluate the variations in xylem water transport properties.
The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional. This video provides an overview of the different processes that cause water to move throughout a plant use this link to watch this video on YouTube , if it does not play from the embedded video :.
The atmosphere to which the leaf is exposed drives transpiration, but also causes massive water loss from the plant. Up to 90 percent of the water taken up by roots may be lost through transpiration. Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface.
Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration.
Therefore, plants must maintain a balance between efficient photosynthesis and water loss. Plants have evolved over time to adapt to their local environment and reduce transpiration. Desert plant xerophytes and plants that grow on other plants epiphytes have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments mesophytes.
Aquatic plants hydrophytes also have their own set of anatomical and morphological leaf adaptations. Trichomes are specialized hair-like epidermal cells that secrete oils and substances. These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants. It is the faith that it is the privilege of man to learn to understand, and that this is his mission.
Organismal Biology. Skip to content. Water Transport in Plants: Xylem Learning Objectives Explain water potential and predict movement of water in plants by applying the principles of water potential Describe the effects of different environmental or soil conditions on the typical water potential gradient in plants Identify and describe the three pathways water and minerals can take from the root hair to the vascular tissue Explain the three hypotheses explaining water movement in plant xylem, and recognize which hypothesis explains the heights of plants beyond a few meters Water Transport from Roots to Shoots The information below was adapted from OpenStax Biology Water Potential Plants are phenomenal hydraulic engineers.
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