Chapter 4.5 Transport

Transport in Plants

Overview

The overall structure of plant roots, stems, and leaves is intricately designed to efficiently transport water, nutrients, and photosynthates (products of photosynthesis) throughout the plant's systems, ensuring proper growth and metabolism.

Main Tissues Responsible for Transport:

• Phloem: A living tissue responsible for the translocation of nutrients and sugars produced during photosynthesis. It transports these substances from sources (where they are made or stored) to sinks (where they are used or stored).

• Xylem: A vascular tissue that conducts water and dissolved minerals upward from the roots to the rest of the plant, supporting vital processes such as photosynthesis and nutrient uptake.

Water and Nutrient Absorption

• Roots: The primary site for absorption of water and minerals from the soil. Specialized structures like root hairs increase surface area, enhancing absorption efficiency.

• Transport via Xylem: Once absorbed, water and nutrients move upward through the xylem, facilitated by various physiological mechanisms.

Stomatal Function

Stomata are small openings on the leaf surface that allow gas exchange (CO2 intake and O2 release) but also lose water vapor. Plants have developed various adaptations to minimize water loss through stomatal regulation and structural modifications.

Example of Adaptation:

• Dragon's Blood Tree (Dracaena cinnabari): This species exemplifies adaptation to arid environments. Its unique structure enables the redirection of scarce water resources directly to the roots, exhibiting an evolutionary response to its habitat.

qWater Transport (4.5.1)

Water Potential (4.5.1.1)

• Definition: Water potential indicates the potential energy in water, effectively driving its movement from regions of high water potential (less negative) to low water potential (more negative), influencing the overall hydration status of plant cells.

• Influencing Factors: Key factors affecting water potential include solute concentration, physical pressure, gravitational force, and matric potential (interaction between water and solid surfaces).

• Equation: Ψ = Ψs + Ψp + Ψg + Ψm, where each component represents the contribution of solutes, pressure, gravity, and matric effects respectively.

Transpiration (4.5.1.2)

• Definition: Transpiration is the process of water vapor loss from the plant, primarily through stomata. This not only aids in cooling the plant but also creates a negative pressure that facilitates water movement from roots to leaves.

• Environmental Influences: Factors such as light intensity, temperature, humidity, and wind significantly affect the rate of transpiration:

  • Light: Increases stomatal opening, hence increasing transpiration rates.

  • Temperature: Higher temperatures accelerate evaporation, increasing transpiration.

  • Humidity: Low humidity levels can lead to higher transpiration rates due to steeper vapor pressure gradients.

  • Wind: Increases transpiration by reducing the humidity layer around the leaves, enhancing evaporation.

Adaptations to Reduce Transpiration (4.5.1.2.1)

• Structural Modifications: Plants have evolved several adaptations to minimize transpiration:

  • Waxy Cuticle: A waterproof layer that reduces water loss.

  • Thick Trichomes: Specialized hairs that can reflect light and reduce leaf temperature, lowering transpiration rates.

  • Sunken Stomata: Positioned in pits to reduce exposure to air, thereby decreasing water loss.

  • Multi-layered Epidermis: Provides an additional barrier against water loss.

• Plants in arid conditions often exhibit these adaptations prominently to retain water.

Cohesion-Tension Theory (4.5.1.3)

• Explanation: This theory describes how water is transported up through the xylem in plants, relying on the creation of negative pressure through transpiration at the leaf surface, coupled with cohesive forces between water molecules and adhesive forces between water and the xylem walls.

• Mechanics of Tension: As water evaporates from stomata, the tension created pulls more water up from the roots through the xylem, enabling efficient nutrient transportation.

Water Absorption (4.5.1.4)

• Root Hairs: These extensions increase the surface area for water and nutrient uptake. They are crucial in enhancing the efficiency of absorption.

• Root Pressure: Generated by osmotic forces within the root cells, it enables a push of water upwards into the xylem channels.

• Pathways for Water Movement: There are three principal pathways:

  • Apoplast Pathway: Water movement occurs through the spaces around cells, bypassing the cell membranes.

  • Symplast Pathway: Water moves through the cytoplasm of cells via plasmodesmata, allowing for quick distribution.

  • Transmembrane Pathway: Water crosses cell membranes, allowing for selective uptake of nutrients.

Translocation: Assimilate Transport (4.5.2)

Sources and Sinks (4.5.2.1)

• Sources: Generally consist of mature leaves and storage organs like bulbs or tubers, where excess products of photosynthesis are stored.

• Sinks: Young shoots, developing fruits, and roots function as sinks, drawing the necessary nutrients and carbohydrates for growth and development.

• Transport Mechanism: The movement of water and nutrients predominantly occurs through phloem from the sources to the sinks.

Phloem Loading (4.5.2.2)

• Mechanism: Sucrose enters the phloem tissue through both apoplastic pathways (involving active transport) and symplastic pathways (direct cytoplasm flow). This loading is essential for efficient translocation of sugars.

Pressure-Flow Hypothesis (4.5.2.3)

• Concept: The accumulation of sugars in the phloem creates a high solute concentration, leading to water movement from the xylem into the phloem due to osmosis. This increase in hydrostatic pressure pushes sap toward growing sinks.

• Unloading Mechanism: At sinks, sucrose is transported out of the phloem into surrounding tissues, which lowers osmotic potential, allowing water to re-enter the xylem for re-circulation.

Chapter Summary (4.5.3)

Both water (transported via xylem) and sugars (transported via phloem) are vital for plant functionality. The process of water movement is significantly influenced by water potential as described through the cohesion-tension theory. Transpiration and root pressure act as primary drivers for water movement, while translocation efficiently distributes sugars from sources to sinks, promoting plant growth and development.