The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled.
The atmosphere to which the leaf is exposed drives transpiration, but it 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. 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.
Spatial variations in xylem structure and hydraulic properties have to be compared with the actual patterns of in vivo water flow dynamics. Measuring the sap flow i. A reliable interpretation of instrumentation outputs requires an integrated understanding of both the structural complexity and technical limits of each measurement method. In particular, the velocity or pressure measurements should be evaluated with respect to the hydraulic architecture of the xylem network.
Tension measured using pressure bombs and xylem pressure probes were only in accordance for non-transpiring leaves and differed considerably for transpiring leaves Melcher et al. The deviations were later attributed to technical limitations, as the range of sensitivity of the initially developed pressure probes was below 0.
Pressure probes can now be used to measure negative pressures; however, theoretical values of up to —10MPa cannot be verified. The existence of negative hydrostatic pressure is no longer a question. Meanwhile, how this pressure is transmitted through the xylem network requires a better understanding of the relationship between changes in pressure and network architecture.
A lack of consistency between results obtained using tracer dyes and probes called into question which velocity component each method measures. Flow velocities obtained from heat-pulse or particle-type tracers, such as radioisotopes, probably differ owing to the way in which axial and radial flow components are measured. Vessels involved in the flow and the total lumen area are generally not known and it is technically difficult to insert the glass tip of a pressure probe into a vessel without causing cavitation Heine and Farr, ; Dye et al.
In a tropical forest canopy, axial long-distance flow and transport of radial water were affected by the internal water-exchange capacity and the transpiration stream James et al. An inverse relationship between the internal water-exchange capacity and the specific hydraulic conductivity confirmed a trade-off between transport efficiency and water storage.
By combining the thermal-dissipation technique with infrared gas analysis, sap flow and transpiration could be measured simultaneously Ziegler et al. Since the formulation of the CTT, multiple instruments and techniques have been developed to measure the negative pressure in xylem vessels. It rapidly became a reference tool to measure negative hydrostatic pressures in excised leaves.
Despite initial disagreement between the results obtained from the pressure bomb, in situ psychrometry Turner et al. Later on, cell pressure probes developed by Balling and Zimmermann gave access to in vivo measurements of pressure in individual xylem vessels Pockman et al. Measurements of xylem pressures, leaf balancing pressures, transpiration rates, and leaf hydraulic properties are now possible; however, the reasons behind the large variations in pressure obtained using different techniques need further investigation.
Better integration of the hydraulic regulation at each level of organization of the xylem network should thus be the next step Fig. How is water from individual vessels in the roots transmitted to a network of vessels in the stem? How is long-distance water transport redistributed to vessels in the leaves? How is each level of hydraulic regulation coordinated at the whole-plant level? Visualization of in vivo water flow dynamics using magnetic resonance imaging MRI and synchrotron X-ray imaging provided the first tools for examining flow regulation and a specific level of structural organization.
In particular, it is now possible to visualize the functionality of individual xylem vessels under different environmental conditions. Nuclear magnetic resonance NMR or magnetic resonance imaging MRI is the least invasive method to investigate sap flow, and provide spatially and temporally resolved information on sap flow at the level of membrane, cell-to-cell, and long-distance transport Witsuba et al.
Relative differences in flow volume in different vascular bundles suggested that each vascular bundle is under different tension. Also, root pressure can be estimated non-destructively by taking continuous measurement of sap flow and variations in root segments of different stem diameter and integrating this information with a mechanistic flow and storage model De Swaef et al.
Numerous studies examine sap flow as a combination of flow velocity and pressure measurements under different environmental conditions Witsuba et al. A wide range of devices are available to measure pressure and flow at different scales: mobile MRI systems for outdoor tree measurements Kimura et al.
Now, measurements of sap flow velocity, xylem pressure at the level of individual vessels and in-vivo real-time visualization are required to completely unravel the dynamics of sap flow regulation in the xylem network.
Real-time imaging methods, such as synchrotron X-ray imaging, have recently revealed that radial flow of water can occur during refilling of dehydrated xylem vessels in monocot leaves Kim and Lee, and in the roots of Arabidopsis plants Lee et al.
Embolism of xylem vessels reduces hydraulic conductivity, and the percentage loss of conductivity PLC is used to estimate cavitation and embolism repair Zwieniecki and Holbrook, For a long time, the experimental research was focused on trying to identify how frequently embolism occurred and how it could be repaired, especially in trees.
The refilling of embolized vessels is not explained by thermodynamic laws Holbrook and Zwieniecki, However, the latest comparison of different methods used to measure the PLC showed that embolism repair is largely due to technical artefacts Wheeler et al.
The ability to limit embolism occurrence is a major component of hydraulic safety and the frequent cavitation reported in earlier studies was due to erroneous interpretations. In particular, inappropriate dehydration methods to generate vulnerability curves led to an overestimation of the vulnerability to cavitation Cochard et al.
Nonetheless, the ability of plants to refill embolized vessels during transpiration cannot be neglected and the biophysical mechanisms that enable plants to do so remain to be elucidated Zwieniecki and Holbrook, Synchrotron X-ray computed tomography is an extremely promising method to visualize and quantify refilling dynamics Brodersen et al.
In grapevines, water influx in the embolized vessels has been attributed to adjacent vessels or the surrounding living tissue. These advances in imaging techniques provide sufficient spatial and temporal resolution to visualize axial, radial, and reverse flow Lee et al.
Although such methods cannot be used on trees due to limitations in sample size and field of view, the experimental results obtained from model plants can be integrated into a broader framework to understand the hydraulic regulation of active water flow. If refilling under tension is indeed a physical process, we need to re-evaluate the reality of this phenomenon and identify the source of the driving force that draws water into embolized vessels, localize the origin of this water, and determine how embolized and functional vessels are hydraulically compartmentalized Holbrook and Zwieniecki, Real-time, high-resolution imaging methods are ideal for visualizing dynamic processes such as embolism repair Brodersen et al.
A multitude of tools and methods are now available to study water transport from the level of individual xylem vessels to the whole plant. It is crucial to consolidate our current knowledge in order to guide future research on plant water transport in the most relevant directions.
Whereas plant physiologists are the ones who better understand the complexity of this transport system, they need support from physicists to validate the results obtained with new methods.
Molecular biologists should also play a key role in incorporating the role of aquaporins in regulating plant water transport, especially in the roots and leaves. Ecologists, agronomists, and breeders can benefit tremendously by including the basic processes of water transport in their modelling and selection approaches. Currently, it is difficult to attribute structural characteristics of the xylem network to specific functions related to efficiency or safety.
Developing new tools and methods that connect flow and structure at different scales is probably the most promising approach for gaining new insight into hydraulic regulation along the transpiration stream. Using a combination of structural and functional methods, it is now possible to distinguish between water-conducting and non-functional vessels. However, given the diversity of plant hydraulic architecture and dimensions, the same methods cannot be applied to all plants.
Advanced high-resolution imaging methods such as MRI, synchrotron X-ray imaging, and neutron-based imaging, now allows the analysis of flow dynamics at the organ level, as reported for rice leaves, grapevine stems, or Arabidopsis roots Kim and Lee, ; Brodersen, ; Lee et al.
The next major step will be to reconstitute a realistic 3D map of the hydraulic network of the whole organism starting with small model plants, such as Arabidopsis. At the subcellular level, the combination of scanning electron microscopy nano-scale and macroscopic techniques will enable investigations of the relationship between cell wall characteristics and the xylem network McCully et al.
Atomic force microscopy will provide information about the surface chemistry of xylem cell walls. Confocal microscopy of leaves can provide insight into the relationship between leaf water dynamics and transpiration Botha et al. On the other hand, portable devices such as portable MRI are being developed to measure sap flow under real-field conditions. Infrared imaging techniques can provide a detailed map of surface temperatures and promote insight into water distribution, evaporation, ice formation, and sap flow.
The development of enhanced computing power will also give rise to more realistic models and simulations of sap flow. Transport of water and minerals is at the centre of all metabolic processes in plants, yet many variables and parameters related to this transport are unknown.
In a broader perspective, a functional framework of the xylem network that integrates water flow dynamics at various levels of organizations can lead the development of bio-inspired technologies based on sap flow in plants.
For decades, research on water transport in plants has hinged on a reference theory. To move forward, the research should now focus on unravelling how water transport through the xylem network is regulated using ingenious combinations of advanced techniques that probe the structure-function relationships of this fascinating transport system.
The cohesion-tension theory. New Phytologist , — Google Scholar. Revisiting the evolutionary origin of allometric metabolic scaling in biology. Functional Ecology 22 , — Balling A Zimmermann U. Which of the following statements is false? The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled.
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 Figure. 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. Plants need an energy source to grow. In seeds and bulbs, food is stored in polymers such as starch that are converted by metabolic processes into sucrose for newly developing plants. Once green shoots and leaves are growing, plants are able to produce their own food by photosynthesizing. The products of photosynthesis are called photosynthates, which are usually in the form of simple sugars such as sucrose.
Structures that produce photosynthates for the growing plant are referred to as sources. Sugars produced in sources, such as leaves, need to be delivered to growing parts of the plant via the phloem in a process called translocation. The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks. The products from the source are usually translocated to the nearest sink through the phloem.
For example, the highest leaves will send photosynthates upward to the growing shoot tip, whereas lower leaves will direct photosynthates downward to the roots. Intermediate leaves will send products in both directions, unlike the flow in the xylem, which is always unidirectional soil to leaf to atmosphere.
The pattern of photosynthate flow changes as the plant grows and develops. Photosynthates are directed primarily to the roots early on, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development.
They are also directed to tubers for storage. Photosynthates, such as sucrose, are produced in the mesophyll cells of photosynthesizing leaves. From there they are translocated through the phloem to where they are used or stored. Mesophyll cells are connected by cytoplasmic channels called plasmodesmata. Photosynthates move through these channels to reach phloem sieve-tube elements STEs in the vascular bundles.
From the mesophyll cells, the photosynthates are loaded into the phloem STEs. The sucrose is actively transported against its concentration gradient a process requiring ATP into the phloem cells using the electrochemical potential of the proton gradient.
Phloem STEs have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap. Takahashi, H. Hydrotropism and its interaction with gravitropism in roots. Plant Soil , Tyree, M. The hydraulic architecture of trees and other woody plants. Vulnerability of xylem to cavitation and embolism.
Xylem Structure and the Ascent of Sap. Wheeler, T. The transpiration of water at negative pressures in a synthetic tree. Wullschleger, S. A review of whole-plant water use studies in trees. Tree Physiology 18, Homeostatic Processes for Thermoregulation. Physiological Ecology Introduction. Physiological Optima and Critical Limits. Avian Egg Coloration and Visual Ecology. The Ecology of Photosynthetic Pathways. Global Treeline Position.
Allometry: The Study of Biological Scaling. Extreme Cold Hardiness in Ectotherms. Plant-Soil Interactions: Nutrient Uptake. Water Uptake and Transport in Vascular Plants. McElrone U. Citation: McElrone, A. Nature Education Knowledge 4 5 How does water move through plants to get to the top of tall trees? Here we describe the pathways and mechanisms driving water uptake and transport through plants, and causes of flow disruption.
Aa Aa Aa. Water is the most limiting abiotic non-living factor to plant growth and productivity, and a principal determinant of vegetation distributions worldwide. Since antiquity, humans have recognized plants' thirst for water as evidenced by the existence of irrigation systems at the beginning of recorded history.
Water's importance to plants stems from its central role in growth and photosynthesis, and the distribution of organic and inorganic molecules. The remainder passes through plants directly into the atmosphere, a process referred to as transpiration. The amount of water lost via transpiration can be incredibly high; a single irrigated corn plant growing in Kansas can use L of water during a typical summer, while some large rainforest trees can use nearly L of water in a single day!
From the Soil into the Plant. Through the Plant into the Atmosphere. Water flows more efficiently through some parts of the plant than others. For example, water absorbed by roots must cross several cell layers before entering the specialized water transport tissue referred to as xylem Figure 4. These cell layers act as a filtration system in the root and have a much greater resistance to water flow than the xylem, where transport occurs in open tubes. Imagine the difference between pushing water through numerous coffee filters versus a garden hose.
Mechanism Driving Water Movement in Plants. Unlike animals, plants lack a metabolically active pump like the heart to move fluid in their vascular system. Instead, water movement is passively driven by pressure and chemical potential gradients.
The bulk of water absorbed and transported through plants is moved by negative pressure generated by the evaporation of water from the leaves i. This system is able to function because water is "cohesive" — it sticks to itself through forces generated by hydrogen bonding. These hydrogen bonds allow water columns in the plant to sustain substantial tension up to 30 MPa when water is contained in the minute capillaries found in plants , and helps explain how water can be transported to tree canopies m above the soil surface.
The tension part of the C-T mechanism is generated by transpiration. Evaporation inside the leaves occurs predominantly from damp cell wall surfaces surrounded by a network of air spaces. Menisci form at this air-water interface Figure 4 , where apoplastic water contained in the cell wall capillaries is exposed to the air of the sub-stomatal cavity. Driven by the sun's energy to break the hydrogen bonds between molecules, water evaporates from menisci, and the surface tension at this interface pulls water molecules to replace those lost to evaporation.
This force is transmitted along the continuous water columns down to the roots, where it causes an influx of water from the soil. Disruption of Water Movement. Fixing the Problem. References and Recommended Reading Agrios, G.
Plant Pathology. Plant Physiology , Canadell, J. Zimmerman, M. Berlin, Germany: Springer-Verlag, Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel.
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