Therefore, plants have developed an effective system to absorb, translocate, store and utilize water. To understand water transport in plants, one first needs to understand the plants' plumbing. Plants contain a vast network of conduits, which consists of xylem and phloem tissues. This pathway of water and nutrient transport can be compared with the vascular system that transports blood throughout the human body.
Like the vascular system in people, the xylem and phloem tissues extend throughout the plant. These conducting tissues start in the roots and transect up through the trunks of trees, branching off into the branches and then branching even further into every leaf. Phloem tissue is responsible for translocating nutrients and sugars carbohydrates , which are produced by the leaves, to areas of the plant that are metabolically active requiring sugars for energy and growth.
The xylem is also composed of elongated cells. Once the cells are formed, they die. But the cell walls still remain intact, and serve as an excellent pipeline to transport water from the roots to the leaves. A single tree will have many xylem tissues, or elements, extending up through the tree. Each typical xylem vessel may only be several microns in diameter. The main driving force of water uptake and transport into a plant is transpiration of water from leaves.
Transpiration is the process of water evaporation through specialized openings in the leaves, called stomates. The evaporation creates a negative water vapor pressure develops in the surrounding cells of the leaf.
Once this happens, water is pulled into the leaf from the vascular tissue, the xylem, to replace the water that has transpired from the leaf. This pulling of water, or tension, that occurs in the xylem of the leaf, will extend all the way down through the rest of the xylem column of the tree and into the xylem of the roots due to the cohesive forces holding together the water molecules along the sides of the xylem tubing.
Remember, the xylem is a continuous water column that extends from the leaf to the roots. Finally, the negative water pressure that occurs in the roots will result in an increase of water uptake from the soil.
The loss of water from a leaf negative water pressure, or a vacuum is comparable to placing suction to the end of a straw. If the vacuum or suction thus created is great enough, water will rise up through the straw. If you had a very large diameter straw, you would need more suction to lift the water. Likewise, if you had a very narrow straw, less suction would be required.
This correlation occurs as a result of the cohesive nature of water along the sides of the straw the sides of the xylem. Because of the narrow diameter of the xylem tubing, the degree of water tension, vacuum required to drive water up through the xylem can be easily attained through normal transpiration rates that often occur in leaves. He offers the following answer to this oft-asked question: "Once inside the cells of the root, water enters into a system of interconnected cells that make up the wood of the tree and extend from the roots through the stem and branches and into the leaves.
The scientific name for wood tissue is xylem; it consists of a few different kinds of cells. The cells that conduct water along with dissolved mineral nutrients are long and narrow and are no longer alive when they function in water transport. Some of them have open holes at their tops and bottoms and are stacked more or less like concrete sewer pipes. Other cells taper at their ends and have no complete holes. All have pits in their cell walls, however, through which water can pass.
Water moves from one cell to the next when there is a pressure difference between the two. It might seem possible that living cells in the roots could generate high pressure in the root cells, and to a limited extent this process does occur. But common experience tells us that water within the wood is not under positive pressure--in fact, it is under negative pressure, or suction.
To convince yourself of this, consider what happens when a tree is cut or when a hole is drilled into the stem. If there were positive pressure in the stem, you would expect a stream of water to come out, which rarely happens. Each water molecule has both positive and negative electrically charged parts. As a result, water molecules tend to stick to one another; that adhesion is why water forms rounded droplets on a smooth surface and does not spread out into a completely flat film.
As one water molecule evaporates through a pore in a leaf, it exerts a small pull on adjacent water molecules, reducing the pressure in the water-conducting cells of the leaf and drawing water from adjacent cells. This chain of water molecules extends all the way from the leaves down to the roots and even extends out from the roots into the soil. So the simple answer to the question about what propels water from the roots to the leaves is that the sun's energy does it: heat from the sun causes the water to evaporate, setting the water chain in motion.
Old growth redwoods, such as these giants from Rockefeller Forest in California's Humboldt Redwoods State Park, reach heights of meters or more. To evolve into tall, self-supporting land plants, trees had to develop the ability to transport water from a supply in the soil to the crown--a vertical distance that is in some cases meters or more the height of a story building. To understand this evolutionary achievement requires an awareness of wood structure, some of the biological processes occurring within trees and the physical properties of water.
Water and other materials necessary for biological activity in trees are transported throughout the stem and branches in thin, hollow tubes in the xylem, or wood tissue.
These tubes are called vessel elements in hardwood or deciduous trees those that lose their leaves in the fall , and tracheids in softwood or coniferous trees those that retain the bulk of their most recently produced foliage over the winter. There are three hypotheses that explain the movement of water up a plant against gravity. These hypotheses are not mutually exclusive, and each contribute to movement of water in a plant, but only one can explain the height of tall trees:.
Root pressure relies on positive pressure that forms in the roots as water moves into the roots from the soil. In extreme circumstances, root pressure results in guttation , or secretion of water droplets from stomata in the leaves. However, root pressure can only move water against gravity by a few meters, so it is not strong enough to move water up the height of a tall tree. Capillary action or capillarity is the tendency of a liquid to move up against gravity when confined within a narrow tube capillary.
Capillarity occurs due to three properties of water:. On its own, capillarity can work well within a vertical stem for up to approximately 1 meter, so it is not strong enough to move water up a tall tree. This video provides an overview of the important properties of water that facilitate this movement:.
The c ohesion-tension hypothesis is the most widely-accepted model for movement of water in vascular plants. Cohesion-tension essentially combines the process of capillary action with transpiration , or the evaporation of water from the plant stomata. Transpiration is ultimately the main driver of water movement in xylem. The cohesion-tension model works like this:. Here is a bit more detail on how this process works: Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall.
The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells. This decrease creates a greater tension on the water in the mesophyll cells, thereby increasing the pull on the water in the xylem vessels.
The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure.
Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap. 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. Patterns of water movement in forest trees. Botanical Gazette , — Physiology of trees. Modelling the hydrodynamic resistance of bordered pits. Journal of Experimental Botany 53 , — Analysis of HRCT-derived xylem network reveals reverse flow in some vessels.
Journal of Theoretical Biology , — Hydraulic characteristics of water-refilling process in excised roots of Arabidopsis. Embolism resistance as a key mechanism to understand adaptive plant strategies. Current Opinion in Plant Biology 16 , — Estimating volume flow-rates through xylem conduits. American Journal of Botany 82 , — The relevance of xylem network structure for plant hydraulic efficiency and safety.
Novel, cyclic heat dissipation method for the correction of natural temperature gradients in sap flow measurements. Part 1. Theory and application. Tree Physiology 32 , — The plant vascular system: evolution, development and functions. Journal of Integrative Plant Biology 55 , — Structural determinants of water permeation through aquaporin Moving water well: comparing hydraulic efficiency in twigs and trunks of coniferous, ring-porous, and diffuse-porous saplings from temperate and tropical forests.
Cryo-scanning electron microscopy CSEM in the advancement of functional plant biology. Morphological and anatomical applications. Functional Plant Biology 36 , 97 — Water transport in trees: current perspectives, new insights and some controversies. Environmental and Experimental Botany 45 , — Comparative measurements of xylem pressure in transpiring and non-transpiring leaves by means of the pressure chamber and the xylem pressure probe.
Journal of Experimental Botany 49 , — Milburn JA. Sap ascent in vascular plants: challengers to the cohesion theory ignore the significance of immature xylem and the recycling of munch water. Annals of Botany 78 , — Plant biomechanics: an overview and prospectus.
Oda Y Hasezawa S. Cytoskeletal organization during xylem cell differentiation. Journal of Plant Research , — Three-dimensional xylem networks and phyllode properties of co-occurring Acacia. Passioura JB. Water Transport in and to roots.
Passioura JB Munns R. Hydraulic resistance of plants. II Effects of rooting medium, and time of day, in barley and lupin. Australian Journal of Plant Physiology 11 , — Pennisi E. Plant genetics: the blue revolution, drop by drop, gene by gene. Science , — Petit G Anfodillo T. Plant physiology in theory and practice: An analysis of the WBE model for vascular plants. Journal of Theoretical Biology , 1 — 4.
Pittermann J. The evolution of water transport in plants: an integrated approach. Geobiology 8 , — Torus-margo pits help conifers compete with angiosperms. Science , Sustained and significant negative water-pressure in xylem. Renner O. Flora , — The hydraulic limitation hypothesis revisited. Leaf palmate venation and vascular redundancy confer tolerance of hydraulic disruption. Visualizing water-conduction pathways of living trees: selection of dyes and tissue preparation methods.
Tree Physiology 25 , — Hydraulic trade-offs and space filling enable better predictions of vascular structure and function in plants. Intact plant magnetic resonance imaging to study dynamics in long-distance sap flow and flow-conducting surface area.
Sap pressure in vascular plants: negative hydrostatic pressure can be measured in plants. Water flow through vessel perforation plates--the effects of plate angle and thickness for Liriodendron tulipifera. Journal of Experimental Botany 44 , — Water Flow in vessels with simple or compound perforation plates. Annals of Botany 64 , — Decline of leaf hydraulic conductance with dehydration: relationship to leaf size and venation architecture. A quantitative analysis of plant form. Pipe model theory.
Basic analysis. Japanese Journal of Ecology 14 , — Measurement of sap flow in plant stems. Journal of Experimental Botany 47 , — Toward a systems approach to understanding plant cell walls. Sperry JS. Evolution of water transport and xylem structure. Hydraulic consequences of vessel evolution in angiosperms. International Journal of Plant Sciences , — Safety and efficiency conflicts in hydraulic architecture: scaling from tissues to trees.
Xylem tension affects growth-induced water potential and daily elongation of maize leaves. Comparison of water potentials measured by in situ psychrometry and pressure chamber in morphologically different species.
Plant Physiology 74 , — Tyree MT. The Cohesion-tension theory of sap ascent: current controversies. Journal of Experimental Botany 48 , — Plant hydraulics: The ascent of water.
The hydraulic architecture of trees and other woody plants. Vulnerability of xylem to cavitation and embolism. Xylem structure and the ascent of sap. Berlin : Springer-Verlag. Optimal conditions for visualizing water-conducting pathways in a living tree by the dye injection method. Tree Physiology 27 , — Vandegehuchte MW Steppe K. Use of the correct heat conduction—convection equation as basis for heat-pulse sap flow methods in anisotropic wood.
Journal of Experimental Botany 63 , — Water transport in plants as a caternary process. Discussion of the Faraday Society 3 , — Vieweg GH Ziegler H. Thermoelektrische registrierung der geschwindigkeit des transpirationsstromes. Berichte der Deutsch Botanischen Gesellschaft 73 , — A species-level model for metabolic scaling of trees II. Testing in a ring- and diffuse-porous species. Functional Ecology 26 , — Neutron imaging reveals internal plant water dynamics.
Plant and Soil , — Water ascent in plants: do ongoing controversies have a sound basis? Trends in Plant Science 4 , — The essentials of direct xylem pressure measurement. Direct measurement of xylem pressure in leaves of intact maize plants.
A test of the cohesion-tension theory taking hydraulic architecture into consideration. A general model for the structure and allometry of plant vascular systems. The transpiration of water at negative pressures in a synthetic tree. Cutting xylem under tension or supersaturated with gas can generate PLC and the appearance of rapid recovery from embolism.
Plant Cell Environment 36 , — Xylem flow and its driving forces in a tropical liana: concomitant flow-sensitive NMR imaging and pressure probe measurements. Plant Biology 2 , — High-contrast three-dimensional imaging of the Arabidopsis leaf enables the analysis of cell dimensions in the epidermis and mesophyll.
Plant Methods 6 , Angiosperm wood structure: Global patterns in vessel anatomy and their relation to wood density and potential conductivity.
American Journal of Botany 97 , — Going beyond histology. Synchrotron micro-computed tomography as a methodology for biological tissue characterization: from tissue morphology to individual cells. Journal of The Royal Society Interface 7 , 49 — Thermal dissipation probe measurements of sap flow in the xylem of trees documenting dynamic relations to variable transpiration given by instantaneous weather changes and the activities of a mistletoe xylem parasite.
Trees 23 , — A novel, non-invasive, online-monitoring, versatile and easy plant-based probe for measuring leaf water status. Zimmermann MH. New York : Springer-Verlag. Trees: Structure and Function.
Xylem water transport: is the available evidence consistent with the cohesion theory? Water ascent in tall trees: does evolution of land plants rely on a highly metastable state? Diurnal variation in xylem hydraulic conductivity in white ash Fraxinus americana L. Trends in Plant Science 14 , — Oxford University Press is a department of the University of Oxford.
It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract. Linking xylem structural components and their functions.
Functionality of the xylem network: bottleneck for efficiency or smart design for safety? What is the appropriate approach to investigate the regulation of sap flow dynamics?
Toward real-time imaging of flow dynamics in the xylem network. Future directions. Investigating water transport through the xylem network in vascular plants. Box E-mail: hk. Oxford Academic. Joonghyuk Park. Ildoo Hwang. Select Format Select format. Permissions Icon Permissions. Abstract Our understanding of physical and physiological mechanisms depends on the development of advanced technologies and tools to prove or re-evaluate established theories, and test new hypotheses.
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