3.3 Movement of Water and Minerals in the Roots
3.4 Movement of Water Against Gravity
3.5 Control of Transpiration
3_Water-Movement-in-Xylem
Water Movement in Xylem
Overview
Photohound, Public domain, via Wikimedia Commons
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Introduction
Learning Objectives
- Identify the components of water potential.
- Define transpiration.
- Explain the cohesion tension theory of xylem transport.
Key Terms
adhesion - the attraction between water molecules and other molecules
apoplastic movement - the movement of water along with dissolved ions in the space between the cell wall and plasma membrane
capillary rise - the tendency of a liquid to move up against gravity when confine within a narrow tube
cation exchange capacity - the measure of the total amount of changeable positive ions that a soil can hold
central vacuole - large plant cell organelle that regulates the cell's storage compartment, holds water and plays a significant role in cell growth as the site of macromolecule degradation
cohesion tension theory - is the hypothesis that explains the movement of water up the xylem vessels using the physical properties of water and the process of transpiration
electronegativity - the ability of a compound or particle to attract electrons
evaporation - the process by which water changes from a liquid to a gas or vapor
hydrologic cycle - the continuous circulation of water from land and sea to the atmosphere and back again
stomata - an opening on the leaf surface for the exchange of CO2 and water vapor
symplastic movement - the movement of water along with the dissolved ions from the cytoplasm of one cell into the next cell via plasmodesmata that physically join different plant cells
translocation - mass transport of photosynthates from source to sink in the phloem of vascular plants
transmembrane movement - the movement of water along with the dissolved ions from outside of the cell to inside across the plasma membrane
transpiration - loss of water vapor to the atmosphere through stomata
turgor pressure - the outward pressure generated by the inflow of water via osmosis across the semipermeable plasma membrane causing the plasma membrane to swell and push against the cell wall
water potential - the potential energy of a water solution per unit volume in relation to pure water at atmospheric pressure and ambient temperature
The structure of plant roots, stems, and leaves facilitate the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants. To understand how these processes work, we must first understand the energetics of water potential.
Water Potential
Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a 116-meter-tall tree. Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks (Figure 2.3.1). Plants achieve this because of water potential.
Water potential is a measure of the potential energy in water—specifically, water movement between two systems. Water potential can be defined as the difference in potential energy between any given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter Ψ (psi) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψpure H2O) is designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are expressed relative to Ψpure H2O.
The water potential measurement combines the effects of solute concentration (S) and pressure (P):
Ψwater = ΨS + ΨP
where Ψs = solute potential, and Ψp = pressure potential.
The addition of more solutes will decrease the water potential, and the removal of solutes will increase the water potential. The addition of pressure will increase the water potential, and the removal of the pressure (creation of a vacuum) will decrease the water potential.
Water always moves from a region of high water potential to an area of low water potential, until it equilibrates the water potential of the system. At equilibrium, there is no difference in water potential on either side of the system (the difference in water potential is zero). For water to move through the plant from the soil to the air (a process called transpiration), Ψsoil must be > Ψroot > Ψstem > Ψleaf > Ψatmosphere.
Let’s consider solute and pressure potential in the context of plant cells:
Solute potential (Ψs), also called osmotic potential, is negative in a plant cell and zero in distilled water, because solutes reduce water potential to a negative Ψs. The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content. Because of this difference in water potential, water will move from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential. Plant cells can metabolically manipulate Ψs by adding or removing solute molecules (figure 2.3.2).
Pressure potential (Ψp), also called turgor potential, may be positive or negative. Positive pressure (compression) increases Ψp, and negative pressure (vacuum) decreases Ψp. Positive pressure inside cells is contained by the rigid cell wall, producing turgor pressure. Pressure potentials can reach as high as 1.5 MPa in a well-watered plant. A Ψp of 1.5 MPa equates to 210 pounds per square inch (psi); for comparison, most automobile tires are kept at a pressure of 30-34 psi. A plant can manipulate Ψp via its ability to manipulate Ψs, as well as by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, Ψs will decline, water will move into the cell by osmosis, and Ψp will increase. Ψp is also under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing Ψp and Ψtotal of the leaf and increasing the water potential difference between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf (figure 2.3.2).
This video provides an overview of water potential, including solute and pressure potential.
And this video describes how plants manipulate water potential to absorb water and how water and minerals move through the root tissues.
Access for free at https://openstax.org/books/biology-2e/pages/30-5-transport-of-water-and-solutes-in-plants
Movement of Water and Minerals in the Roots
Negative water potential continues to drive movement once water (and minerals) are inside the root; Ψ of the soil is much higher than Ψ or the root, and Ψ of the cortex (ground tissue) is much higher than Ψ of the stele (location of the root vascular tissue). Once the water has been absorbed by a root hair, it moves through the ground tissue through one of three possible routes before entering the plant’s xylem:
- the symplast “Sym” means “same” or “shared,” so symplast is shared cytoplasm. In this pathway, water and minerals move from the cytoplasm of one cell into the next, via plasmodesmata that physically join different plant cells until eventually reaching the xylem (Figure 2.3.4).
- the transmembrane pathway In this pathway, water moves through water channels present in the plant cell plasma membranes, from one cell to the next, until eventually reaching the xylem (Figure 2.3.4).
- the apoplast “A” means “outside of,” so the apoplast is outside of the cell. In this pathway, water and dissolved minerals never move through a cell’s plasma membrane; instead, they travel through the porous cell walls that surround plant cells (Figure 2.3.4.).
Movement of Water Against Gravity
How is water transported up a plant against gravity, when there is no “pump” to move water through a plant’s vascular tissue? There are three hypotheses that explain the movement of water up a plant against gravity. These hypotheses are not mutually exclusive, and each contributes to the movement of water in a plant, but only one can explain the height of tall trees:
- Root pressure pushes water up.
- Capillary action draws water up within the xylem.
- Cohesion-tension pulls water up the xylem.
We’ll consider each of these in turn.
Root pressure relies on positive pressure that forms in the roots as water moves into the roots from the soil. Water moves into the roots from the soil by osmosis, due to the low solute potential in the roots (lower Ψs in roots than in soil). This intake of water in the roots increases Ψp in the root xylem, driving water up. 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:
- Surface tension occurs because hydrogen bonding between water molecules is stronger at the air-water interface than among molecules within the water.
- Adhesion is the molecular attraction between “unlike” molecules. In the case of the xylem, adhesion occurs between water molecules and the molecules of the xylem cell walls.
- Cohesion is the molecular attraction between “like” molecules. In water, cohesion occurs due to hydrogen bonding between water molecules.
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 cohesion-tension hypothesis (Figure 2.3.7.) is the most widely accepted model for the 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 the xylem. The cohesion-tension model works like this:
- Transpiration (evaporation) occurs because stomata are open to allow the gas exchange for photosynthesis. As transpiration occurs, it deepens the meniscus of water in the leaf, creating negative pressure (also called tension or suction).
- The tension created by transpiration “pulls” water in the plant xylem, drawing the water upward in much the same way that you draw water upward when you suck on a straw.
- Cohesion (water sticking to each other) causes more water molecules to fill the gap in the xylem as the top-most water is pulled toward the stomata.
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's 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 the 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.
Control of Transpiration
Transpiration is a passive process, meaning that ATP is not required for water movement. The energy that drives transpiration is the water potential difference 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 plants (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.
Xerophytes and epiphytes often have a thick covering of trichomes or of stomata that are sunken below the leaf’s surface. 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.
Dig Deeper
Chapter 23
Bear, Robert; Rintoul, David; Snyder, Bruce; Smith-Caldas, Martha; Herren, Christopher; and Horne, Eva, "Principles of Biology" (2016). Open Access Textbooks. 1.
https://newprairiepress.org/textbooks/1
Attributions
Biology 2e By Mary Ann Clark, Matthew Douglas, Jung Choi. OpenStax is licensed under Creative Commons Attribution License v4.0
Introduction to Organismal Biology at https://sites.gatech.edu/organismalbio/ is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
Botany (Ha, Morrow, and Algiers) is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Melissa Ha, Maria Morrow, & Kammy Algiers.