Xylem: A Plant's Water-Conducting System
- Discuss the concept of water potential and use it to make predictions about the movement of water in plants.
- Explain how the normal water potential gradient in plants is affected by variations in environmental or soil conditions.
- Water and minerals can travel along three different routes from the root hairs to the vascular tissue.
- Recall which of the three hypotheses regarding water transport in plant xylem accounts for plant heights greater than a few meters, and explain how this observation was made.
The following data comes courtesy of OpenStax Biology 30.5.
A plant's vascular system, which includes its roots, stems, and leaves, carries water, nutrients, and photosynthates from the soil to its various organs. This transport is primarily facilitated by the phloem and xylem tissues. How plants move water and nutrients is affected by water potential, evapotranspiration, and stomatal regulation. The energetics of water potential is essential to comprehending the functioning of these processes.
Hydrological systems in plants are marvels of design. Plants can transport water to the top of a 116-meter-tall tree using only the laws of physics and the simple manipulation of potential energy. Hydraulic power in plants can be used to create forces strong enough to crack rocks and crack pavement. Plants are able to do this due to water potential.
The coastal redwood (Sequoia sempervirens) is the world's tallest tree at nearly 116 meters in height. Homeowners and city maintenance crews are often frustrated by the fact that plant roots can easily generate enough force to (b) buckle and break concrete sidewalks. Bernt Rostad gets credit for adapting an existing work, and Pedestrians Educating Drivers on Safety, Inc. gets credit for adapting yet another existing work. (Photo courtesy of OpenStax Biology.)
Specifically, water potential is a measurement of the energy required to move water from one system to another. A water sample's potential is measured against the potential energy of pure water (at standard atmospheric pressure and temperature). The energy potential of water is denoted by the Greek letter psi (psi) and is given in pressure units. megapascals (MPa) Although it contains a lot of potential energy, the potential of pure water (pure H2O) is assigned a value of zero. The water potential of a plant's root, stem, or leaf is expressed as a percentage of the water potential of pure H2O.
Together, the effects of a solute's concentration (s) and pressure (p) are accounted for in the water potential:
Ψ system = Ψs Ψp
when s is the solute potential and p is the pressure potential Increasing solute addition reduces water potential, while decreasing solute addition raises water potential. The water potential rises as pressure is applied, and falls as pressure is removed (a vacuum is created).
To achieve equilibrium, water always moves from high- to low-potential areas. When the system is in equilibrium, there is no water potential difference between the two sides. Through a process known as transpiration, water moves from the soil to the air at various points along the plant, beginning at the soil. This movement of water requires a certain arrangement of the soil, root, stem, leaf, and atmosphere.
As an example, let's think about plant cells in terms of solutes and pressure potentials:
- Potential of solutions () s When solutes are present, water potential (s.w.p., also known as osmotic potential) is negative in a plant cell but is equal to zero in pure water. The high solute content of the cytoplasm causes the plant cell's internal water potential to be more negative than that of pure water. Osmosis is the process by which water moves from the soil into a plant's root cells due to the difference in water potential between the two environments. Osmotic potential is another name for solute potential. S is a metabolic variable that can be adjusted by plant cells by either adding or removing solute molecules.
Possible pressure ( Ψ ) (also known as turgor potential) can be either positive or negative. As the pressure rises (compresses), Ψ p, and vacuum pressure is reduced. Ψ p Turgor pressure is the result of positive pressure inside a cell that is contained by the cell wall. Maximum pressure potentials have been measured at 1. 5.15 MPa in a properly watered plant A Ψ p of 1 Pressures of 5 MPa are equivalent to 210 psi; for reference, most car tires should be inflated to between 30 and 34 psi. One plant's ability to Ψ due to its power of manipulation, Ψ s, and through osmosis When the solute concentration in the cytoplasm of a plant cell rises, Ψ , osmosis will draw water into the cell, and s will go down. Ψ Inflation rate p is expected to rise Ψ As a result of stomatal opening and closing, plants can also exert indirect control over p. These pores, called stomates, let water vapour escape from the leaf and help the plant conserve Ψ p and Ψ water flow from the petiole into the leaf by raising the water potential difference between the water in the leaf and the petiole.
Because of the difference in water potential between the two aqueous systems, water will flow from the region with the higher water potential to the region with the lower water potential until equilibrium is reached. Total water potential on both ends of the tube is affected by solute concentration (s) and pressure (p). The water flows because of the pressure gradient between the left and right sides of the tube. Image courtesy of OpenStax Biology.
The turgor pressure effect can be seen in the wilting and subsequent recovery of plant leaves after being watered. The process of transpiration (approximately Ψ p = 0 MPa at the point of wilting) and replenished by root absorption.
Plants wilt when the total water potential (a) outside the cells is less than inside. By maintaining its erect posture, a plant maintains turgor pressure (p) when (b) the total water potential outside the plant cells is greater than inside. (Photo courtesy of OpenStax Biology; Victor M. ) Vicente Selvas
(Start watching at 5:05 for the highlights) This video gives a brief introduction to water potential, covering both solute and pressure potential.
This video also explains how plants use water potential manipulation to take in water and how minerals are transported to the plant tissues via the root system:
Once water (and minerals) are inside the root, the negative water potential continues to drive movement; The soil's entropy is much greater than that of the root, and the ground tissue's cortex has a greater entropy than the root's stele, where the vascular tissue is located. After being taken in by a root hair, water can travel one of three ways through the plant's ground tissue before entering the xylem.
- the symplast, where sym stands for same or shared, and plastid for cytoplasm. In this route, water and minerals travel from one plant cell to another via plasmodesmata, which are physical connections between plant cells.
- water travels through channels in the plasma membranes of plant cells, from one cell to the next, and then to the xylem. This is known as the transmembrane pathway.
- the apoplast: the prefix "a" indicates location outside of the cell. This route is used by water and dissolved minerals to reach plant cells without crossing the plasma membrane of the cell.
By Jackacon and Smartse's vectorization - the apoplast and symplast pathways animated gif, CC0, https://commons.wikimedia.org/wiki/Image: wikimedia org/w/index php curid=12063412
In contrast to water and minerals entering a cell via the plasma membrane, which have already been "filtered" as they move through water and other channels within the plasma membrane, water and minerals entering a cell via the apoplast do not encounter a filtering step until they reach a layer of cells known as the cytoskeleton. the endodermis, located between the root's outermost layer of ground tissue and the root's vascular tissue (also known as the stele). Endodermis is unique to roots and acts as a filter for substances entering the root's vascular system. The endodermal cells' walls contain a waxy substance called suberin. The Casparian strip is a waxy region between endodermal cells that prevents water and solutes from diffusing between the cells. Thus, the endodermis filters out harmful substances and pathogens while allowing the root to absorb only what it needs.
The following picture was uploaded after the IKE went live:
Both symplastic and apoplastic water transport pathways are used. Kelvinsong, Author of this Work, CC BY-SA 3.0 Zero, Commons. wikimedia org/w/index php curid=25917225
A dicot root's cross section is characterized by a central X-shaped structure. Many xylem cells form the X shape. Between the X-shaped cambium and the cortex, phloem cells The xylem and phloem are surrounded by a ring of cells called the pericycle. Endodermis describes the pericycle's outermost layer. The pericycle is surrounded by an abundance of cortex tissue. The epidermis, a thin layer of cells, covers and protects the cortex. The monocot root resembles a dicot root, but its center is filled with pith rather than cork. Around the pith, phloem cells cluster to make a ring. Phloem contains spherical clusters of xylem cells that are arranged symmetrically around the pith. The dicot root has an identical outer pericycle, endodermis, cortex, and epidermis. Photo: OpenStax College of Biology
Anti-Gravity Water Movement
Because there is no "pump" in a plant's vascular tissue to propel water against gravity, how does water get transported up a plant? There are three potential explanations for how water can move upwards in a plant against gravity. Tall trees can only be explained by one of these hypotheses, but they are not mutually exclusive and each contribute to the movement of water in a plant.
- Hydraulic pressure from the roots forces the liquid up.
- Water travels up the xylem via capillary action.
- Water is drawn up the xylem by cohesion-tension forces.
We're going to take these in order of discussion.
Water absorbed by the roots from the soil creates a positive pressure that drives the plant to grow. Due to the roots' low solute potential (lower than the soil's), water is drawn into the plant from the soil via osmosis. has more to do with the roots than the soil) We take in this If there is more moisture near the roots, root phloem, forcing water to rise Due to intense pressure from below, leaves may start to "guttate," or exude water droplets through their stomata. Root pressure, however, is insufficient to transport water up the height of a tall tree because it can only overcome gravity over a short distance.
When a liquid is contained in a thin tube (capillary), it will rise against the force of gravity. These three characteristics of water are responsible for capillarity:
- Tension at the water’s surface is caused by the fact that hydrogen bonds between water molecules are more powerful at the air–water interface than they are between water molecules themselves.
- Molecular attraction between "unlike" molecules is called adhesion. Adhesion in xylem occurs between water molecules and the molecules of the xylem cell walls.
- Molecular "like"-molecular attraction, or cohesion Cohesion in water results from hydrogen bonds between water molecules.
Capillary action is effective within a vertical stem for about 1 meter, but this is not enough to transport water up a tall tree.
This video gives a quick rundown of the fundamental aspects of water that make this kind of transport possible:
Water movement in vascular plants is best explained by the cohesion-tension hypothesis. Cohesion-tension combines capillary action with transpiration, or the evaporation of water through the stomata of plants. Water movement in xylem is ultimately driven by transpiration. For the cohesion-tension model to function, it is necessary that
- The release of water vapor into the atmosphere is called transpiration and is caused by the opening of stomata, which allow for gas exchange during photosynthesis. Negative pressure (also called tension or suction) is created within a leaf as a result of transpiration, which causes the water meniscus to become deeper.
- In the same way that sucking on a straw causes water to move upward, transpiration creates tension in the plant xylem that "pulls" water upward.
- More water molecules fill the void in the xylem due to cohesion (water sticking to each other) as the uppermost water is drawn toward the stomata.
Here's how it actually happens: on the subcellular level within the leaf, water dripping off the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. As oxygen and carbon dioxide must be exchanged during photosynthesis, the leaf has many large intercellular air spaces. The thin film of water that covers the surface of the mesophyll cells is reduced as the wet cell wall is exposed to the leaf's internal air space. As a result, the pull on the water in the xylem vessels increases, and the tension on the water in the mesophyll cells increases. There is a structural adaptation in the xylem vessels and tracheids that allows the plant to withstand significant pressure fluctuations. Like the rings on a vacuum cleaner hose, which keep the hose open no matter how much suction is being sucked through it, rings in the vessels keep the tubular shape of the vessels even when subjected to high pressure. The cavitation risk is mitigated by reducing the number and size of gas bubbles thanks to small perforations between vessel elements. When air bubbles form in the xylem, they create a blockage in the sap flow from the roots to the leaves, a phenomenon known as an embolism. There will be more cavitation events and higher tension forces required to pull water up a taller tree. The resulting embolisms can clog xylem vessels in larger trees, rendering them useless.
Cohesion-tension theory is demonstrated for sap ascent. Whenever water evaporates from mesophyll cells, a negative water potential gradient is created, which forces water to move up the plant, from the roots, via the xylem. Biological Science OpenStax Image
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Transpiration is driven by the atmosphere to which the leaf is exposed, but this also results in a dramatic loss of water from the plant. Transpiration can account for the loss of up to 90% of the water absorbed by the roots.
A waxy coating can be found on the leaves. cuticle on the surface that blocks evaporation Therefore, the opening and closing of stomata on the leaf surface is the primary means by which transpiration is regulated. Light intensity and quality, leaf water status, and carbon dioxide concentrations are just a few of the environmental cues that cause the stomata to open and close, which are surrounded by two specialized cells called guard cells. The stomata in a leaf must open so that carbon dioxide and oxygen from the air can diffuse into the leaf for photosynthesis and respiration to occur. However, when stomata are open, transpiration increases because water vapor is released into the environment. Plants must strike a balance between photosynthesis and transpiration to survive.
Plants have evolved to better suit their environments, allowing them to exhale less water vapor. It's difficult for xerophytes (desert plants) and epiphytes (plants that grow on other plants) to get enough water to survive. The cuticle of these plants is typically much thicker and waxier than the cuticle of mesophytes, which thrive in more moderate, well-watered environments. Hydrophytes, or plants that thrive in water, have evolved unique leaf anatomy and shape.
Trichomes and stomata that are recessed below the leaf's surface are common characteristics of xerophytes and epiphytes. Epidermal cells called trichomes secrete oils and substances like hair. Changes like these reduce transpiration by blocking airflow through the stomatal pore. Some of these plant species also have multiple epidermal layers.
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