Plant Transport Mechanisms: Xylem and Phloem

Exam Reminders and Important Dates

• Canvas Assignment 2: Due Friday, September 12 @ $5:00 \text{ p.m.}$
• Chapter 37 SmartBook: Due Wednesday, September 17 @ $11:59 \text{ p.m.}$
• Canvas Assignment 3: Due Friday, September 19 @ $5:00 \text{ p.m.}$
• EXAM 2: September 23

Transport in Plants: Chapter 36 Overview

• Plants use specialized mechanisms for transporting water, dissolved solutes, and organic molecules throughout their bodies.
• Xylem: Primarily responsible for moving water and dissolved solutes (minerals) unidirectionally from roots to shoots. This process is largely driven by transpiration.
• Phloem: Primarily responsible for moving organic molecules (sugars, hormones, mRNA, etc.) bidirectionally throughout the plant, from sources to sinks. This process is driven by turgor pressure.

Transport Mechanisms in Plants

Long-Distance Movement in Xylem

• The Challenge: How does a $10$ -story tall tree transport water from its roots to its uppermost leaves?
• Water enters the roots and moves into the xylem, which is the innermost vascular tissue.
• Water rises through the xylem due to a combination of factors, but primarily a "pulling" force.
• Most water exits the plant through stomata in the leaves via transpiration.

Forces Driving Xylem Transport

1. Root Pressure (Minor Push):
   • Some "pushing" force comes from the pressure of water entering the roots due to the continuous accumulation of ions.
   • This can cause water to move into the plant and up the xylem even without transpiration.
   • Guttation: The loss of water from leaves of some plants when root pressure is high and transpiration rates are low (e.g., humid mornings).
   • Significance: While present, root pressure alone is usually insufficient to explain long-distance xylem transport in tall plants.
2. Transpiration (Major Pull - Cohesion-Tension Theory):
   • This is the primary driving force for water movement in xylem.
   • Mechanism: Evaporation of thin films of water in the leaves (specifically from mesophyll cell walls into the air spaces) creates a negative pressure or tension in the xylem. This process is called transpiration.
   • This "negative pressure" or "pull" extends throughout the continuous water column from the leaves, down the stem, and to the roots.
   • Cohesion: Water molecules are highly attracted to each other due to hydrogen bonding. This property, known as cohesion, gives water high tensile strength. This allows the water column to remain unbroken as it is pulled upwards.
   • Adhesion: Water molecules also stick to the hydrophilic walls of the xylem vessels. This adhesion helps counter the force of gravity and further supports the water column.
   • Vapor Pressure Gradient: Transpiration is driven by a gradient in water vapor pressure, from $100\%$ relative humidity inside the leaf to much less than $100\%$ outside the stomata.
   • Xylem Structure: Tracheids and vessels have small diameters, which enhances the cohesive forces of water (tensile strength varies inversely with diameter), making them stronger than gravity. Adhesion to vessel walls further stabilizes the long water column.

Effects of Cavitation

• Cavitation (Embolism): Air bubbles can form in the xylem (e.g., due to freezing/thawing or excessive tension), breaking the continuous water column and reducing its tensile strength. This is analogous to a straw losing its suction when an air bubble forms.
• Minimizing Damage: Plants have anatomical adaptations to minimize cavitation damage:
  • Alternative Pathways: Connections among tracheids and vessels provide alternative routes for water flow around blocked areas.
  • Small Pores: Pores between xylem elements are smaller than typical air bubbles, which can help prevent bubbles from spreading.

Mineral Transport in Xylem

• Tracheids and vessels are also essential for the bulk transport of dissolved minerals.
• Minerals are relocated from the roots to metabolically active parts of the plant via the xylem.
• Abundant Minerals: Phosphorus, potassium, nitrogen, and sometimes iron can be abundant in xylem fluid.
• Immobile Minerals: Calcium, an essential nutrient, is largely immobile once deposited in a plant's tissues and cannot be easily moved from older to younger parts.

Water Movement at the Cellular Level

• Osmosis: Water primarily diffuses across plasma membranes via osmosis, moving from an area of higher water potential (lower solute concentration) to an area of lower water potential (higher solute concentration).
• Aquaporins: These are integral membrane proteins that form water-selective pores, significantly increasing the efficiency and speed of osmosis across cell membranes (facilitated diffusion of water). They do not change the direction of water flow, only its rate.
• Other Substances (Ions, Organic Compounds): Their transport across membranes depends on specific membrane-bound protein transporters:
  • Facilitated Diffusion: Movement down a concentration gradient with the help of a protein channel or carrier, no ATP required.
  • Active Transport: Movement against a concentration gradient, requiring energy (ATP).
    • ATP-dependent hydrogen pumps (Proton Pumps): These actively pump $H^+$ ions out of the cell, creating an electrochemical gradient (proton motive force). This gradient can then be used to co-transport other ions or substances.
    • Symport: A type of co-transport where two substances move in the same direction across the membrane, often with one moving down its electrochemical gradient (e.g., $H^+$ and sucrose).
    • Ion Channels: Specific protein channels facilitate the passive movement of ions (e.g., $K^+$) across the membrane, down their electrochemical gradient.
• Plasmodesmata: Cytoplasmic connections between adjacent plant cells that allow for direct symplastic transport of water and solutes (including some macromolecules) without crossing the plasma membrane multiple times.

Osmosis and Cellular Changes

The behavior of plant cells in different solutions demonstrates the principles of osmosis:

• Turgid Cell: If a plant cell is placed in pure water (a hypotonic solution with low osmotic concentration), water moves into the cell by osmosis. The cell expands and becomes firm or turgid. The cell wall prevents lysis, and the turgor pressure (pressure exerted by the protoplast against the cell wall) is crucial for plant support and growth.
• Plasmolysis (Shrinkage): If a plant cell is placed in a high concentration of sucrose (a hypertonic solution with high osmotic concentration), water leaves the cell by osmosis. The plasma membrane pulls away from the cell wall, and the cell shrinks, a process called plasmolysis.

Cell Walls and Turgor Pressure

• Cell walls are rigid structures that exert wall pressure, preventing the plant cell from rupturing when it takes in water.
• Solutes (e.g., sucrose) lower the relative concentration of water molecules. Since solutes are generally too large to cross the cell membrane, water moves from high to low concentration across the selectively permeable membrane.

Water and Mineral Absorption in Roots

• Root Hairs: Most water absorption occurs through root hairs, which are epidermal extensions that greatly increase the surface area for absorption.
• Mycorrhizal Fungi: The surface area for water and mineral absorption is further increased by symbiotic associations with mycorrhizal fungi, which are particularly helpful in phosphorus uptake.
• Pathways to Vascular Tissue: Once absorbed by root epidermal cells, water and minerals must move across several cell layers (cortex, endodermis) to reach the vascular tissues (xylem) in the stele.
• Ion Accumulation: Mineral ion concentration in soil water is typically much lower than inside plant root cells. Therefore, an expenditure of energy (ATP) via active transport (e.g., proton pumps) is required for ions to accumulate in root cells. This increased solute concentration in the stele contributes to water potential gradients.

Three Transport Routes Through Root Cells

Water and solutes can move through the root tissue via three main routes to reach the xylem:

1. Apoplast Route: Movement through the cell walls and the extracellular spaces between cells. This route avoids crossing the plasma membrane until the endodermis.
2. Symplast Route: Movement through the cytoplasm of cells, connected by plasmodesmata. Once inside one cell's cytoplasm, substances can move from cell to cell without repeatedly crossing membranes.
3. Transmembrane Route: Movement across cell membranes and through the cytoplasm of individual cells, often involving transport across the membranes of vacuoles within cells. This route offers the greatest control over substance movement.

The Endodermis and Casparian Strip

• The endodermis is a cylinder of cells that surrounds the vascular tissue (stele) in the root.
• Casparian Strips: These are waterproof bands of suberin (a waxy substance) embedded in the cell walls of endodermal cells.
• Function: The Casparian strips effectively block the apoplast route. Any water and dissolved minerals moving via the apoplast are forced to cross the plasma membrane and protoplast of an endodermal cell to reach the xylem. This allows the plant to selectively filter what enters the vascular tissue.
• Plasma membranes of endodermal cells contain various protein transport channels and proton pumps, which actively transport specific ions against their concentration gradients into the stele.

Stomata and Transpiration Rate

• Over $90\%$ of the water taken in by roots is released to the atmosphere via transpiration.
• Trade-off: Photosynthesis requires carbon dioxide ($\text{CO}_2$) from the atmosphere, which enters through stomata. However, open stomata also mean water loss.
• Regulation: Plants control water loss on a short-term basis by opening and closing stomata.

Guard Cells: Regulating Stomata

• Structure: Guard cells are specialized epidermal cells, unique for possessing chloroplasts. They have thicker inner cell walls (facing the pore) and thinner outer cell walls.
• Mechanism of Opening:
  1. Guard cells actively take up potassium ions ($K^+$), chloride ions ($\text{Cl}^-$), and malate. This process requires ATP, indicating active transport.
  2. The accumulation of these solutes lowers the water potential inside the guard cells.
  3. Water then enters the guard cells by osmosis.
  4. Increased turgor pressure causes the guard cells to swell. Due to their differential wall thickness, they bulge and bow outwards, pulling the stomatal pore open.
• Mechanism of Closing:
  1. Active pumping of sucrose out of guard cells (and other solutes) decreases their solute concentration.
  2. Water leaves the guard cells by osmosis.
  3. The guard cells become flaccid, straightening and closing the stomatal pore.

Factors Affecting Stomatal Opening and Closing

1. Temperature: Transpiration rates increase with increased temperature. Stomata generally close at high temperatures (>\text{34}^\circ\text{C}) to conserve water.
2. Wind Velocity: Increased wind velocity increases evaporation and thus transpiration rates.
3. Light: Blue wavelengths of light promote $K^+$ uptake by guard cells, leading to stomatal opening.
4. $\text{CO}<em>2$ Concentration: High concentrations of atmospheric $\text{CO}</em>2$ typically cause stomata to close.
5. Abscisic Acid (ABA): In response to drought stress (low water availability), the hormone ABA initiates a signaling pathway that closes stomata. ABA opens $K^+$, $\text{Cl}^-$, and malate channels, causing these ions to exit the guard cells, and water loss follows, leading to decreased turgor and closure.
6. Circadian Rhythms: Stomata typically open during the day for photosynthesis and close at night.
7. Alternative Photosynthetic Pathways: Plants with Crassulacean Acid Metabolism (CAM) open their stomata at night (cooler temperatures, higher humidity) to collect $\text{CO}_2$ and close them during the day, significantly reducing transpiration.

Plant Responses to Water Stress

Drought Stress Responses (Water Deficit)

Plants have various morphological adaptations to limit water loss in dry conditions:

• Dormancy: Entering a period of reduced metabolic activity to survive unfavorable conditions.
• Loss of Leaves: Shedding leaves (e.g., deciduous trees) reduces the total surface area for transpiration.
• Cuticle and Woolly Trichomes: Thick cuticles (waxy layers) and dense coverings of trichomes (hairs) on leaves reduce evaporation from leaf surfaces.
• Reduced Number of Stomata: Fewer stomata mean less opportunity for water vapor to escape.
• Recessed Stomata: Stomata located in pits or depressions on the leaf surface create a microenvironment of higher humidity, reducing the water potential gradient and thus transpiration. Deeply embedded stomata, abundant trichomes, and multiple epidermal layers are common in xerophytic plants.

Flooding Stress Responses (Low Oxygen)

Flooding can lead to reduced or abnormal growth due to limited oxygen availability for root respiration. Plants have adapted to low oxygen conditions:

• Aerenchyma: Formation of loose parenchymal tissue with large air spaces, particularly in aquatic plants (e.g., water lilies). This tissue facilitates gas exchange, collecting oxygen and transporting it to submerged parts of the plant. It also adds buoyancy.
• Larger Lenticels: Increased size of lenticels (pores on stems and roots) allows for more oxygen entrance.
• Adventitious Roots: Formation of new roots from non-root tissue, often above the water line, to access oxygen.

Growth in Saltwater (Salinity Stress)

Halophytes are plants that can grow in saline soils. They employ specific adaptations:

• Pneumatophores: Spongy, air-filled roots that emerge above the mud or water surface (e.g., mangroves). They have large lenticels for oxygen entrance into the submerged root system.
• Salt Secretion/Exclusion: Halophytes can secrete large quantities of salt through specialized glands on their leaves or block salt uptake at the roots.
• Organic Solute Accumulation: Producing high concentrations of organic molecules (compatible solutes) in root cells. This decreases the overall water potential of the roots, enhancing water uptake from saline soil and counteracting the osmotic effect of external salt.

Transport in Phloem

• Phloem Sap: The fluid transported in the phloem is called sap, which is rich in carbohydrates and nutrients.
• Substances Transported: Phloem transports a diverse array of organic molecules, including:
  • Various sugars (primarily sucrose)
  • Amino acids
  • Organic acids
  • Hormones
  • mRNA
  • Proteins
  • Ions
• Translocation: The movement of these organic molecules through the phloem. This process provides building blocks and energy for actively growing regions of the plant.
• Bi-directional Transport: Unlike xylem, phloem transport is bi-directional; substances can move both up and down the plant, from sources to sinks.
• Speed of Transport: Phloem substances can move remarkably fast, as much as $50$ to $100 \text{ cm/h}$, as demonstrated by radioactive tracer studies and aphid stylet sampling.
• Components: Phloem is composed of sieve tube cells (which form the main conduits) and companion cells (which metabolically support the sieve tube cells).

Pressure-Flow Theory: Mechanism of Phloem Transport

This theory explains how carbohydrates (primarily sucrose) are moved in the phloem from a source to a sink.

1. Source: A source is a location where sugars are produced or stored (e.g., mature leaves synthesizing sugars via photosynthesis, or storage organs mobilizing reserves).
2. Sink: A sink is a location where sugars are consumed or stored (e.g., growing root and stem tips, developing fruits, flowers, young leaves, storage organs accumulating reserves). Food storage tissues can act as either sources or sinks depending on the plant's needs.
3. Phloem Loading at Source:
   • Carbohydrates (sucrose) are actively transported from source cells into the sieve tubes of the phloem. This process is called phloem loading and requires energy (ATP).
   • The active transport of sucrose into the sieve tubes significantly increases their solute concentration.
   • This decreases the water potential within the sieve tubes.
   • As a result, water flows from the adjacent xylem into the sieve tubes by osmosis, increasing the turgor pressure within the sieve tubes at the source end.
4. Bulk Flow (Turgor Pressure Driven):
   • The high turgor pressure at the source creates a pressure gradient.
   • This pressure drives the bulk flow of the phloem sap (water and dissolved sugars) through the sieve tubes from the source towards the sink, similar to water flowing through a garden hose.
5. Phloem Unloading at Sink:
   • At the sink, sucrose is actively removed from the sieve tubes and delivered to the surrounding sink cells, where it is consumed for growth/metabolism or converted to storage polymers (e.g., starch).
   • The removal of sucrose increases the water potential within the sieve tubes at the sink end.
   • Water then follows the sucrose out of the sieve tubes by osmosis, often re-circulating into the xylem or being lost. This reduction in water reduces the turgor pressure at the sink.
6. Continuous Cycle: This continuous cycle of loading at the source, bulk flow, and unloading at the sink maintains the pressure gradient necessary for phloem transport.

Visual Summary (Integrated Transport)

• Xylem Transport: Achieved by moving water & dissolved solutes unidirectionally from roots to shoots, driven mainly by transpiration (water out through stomata) and some root pressure (water in from soil). Composed of tracheids and vessels. Movement occurs via three cellular routes: Apoplast, Symplast, Transmembrane.
• Phloem Transport: Achieved by moving organic molecules bidirectionally (up and down plant body), driven by turgor pressure. Composed of sieve tube cells and companion cells. Requires phloem loading (energy-dependent sugar transport from source into phloem) which drives sugars from source to sink. Transports sugars and hormones.