LectureTopic24-PlantTranport-II

Transport in Plants II

I. Bulk Flow in the Xylem

• Plants lose a large volume of water from transpiration, which is the evaporation of water from the plant’s surface.

• Water is replenished through the bulk flow of water and minerals, termed xylem sap, which flows from the steles of roots to the stems and leaves.

• The mechanism of sap rising: Is it pushed or pulled? It involves both mechanisms but primarily relies on tension created by transpiration.

II. Pushing Xylem Sap: Root Pressure

• Root Pressure Process:

  • At night, transpiration is minimal; root cells continue to pump mineral ions into xylem, lowering water potential inside the roots.

  • Water moves in from the root cortex, generating root pressure.

• Characteristics of Root Pressure:

  • Though it generates a positive pressure, it is relatively weak and is considered a minor contributor to xylem bulk flow.

  • Guttation can occur due to root pressure, where water droplets appear on the edges of leaves.

III. Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism

• Mechanism Description:

  • Water is pulled upward by negative pressure within the xylem.

  • Water vapor in leaf airspaces moves down its water potential gradient, exiting via stomata, lowering water potential (Ψ) in the mesophyll.

  • This negative pressure (tension) created by transpiration exerts a pulling force on water in the xylem, drawing water into the leaf.

IV. Cohesion and Adhesion in the Ascent of Xylem Sap

• Transpirational Pull:

  • The pull on xylem sap is transmitted from the leaves to root tips, facilitated by cohesion among water molecules and adhesion to the xylem cell walls.

  • Water molecules adhere to cellulose in xylem walls, helping overcome gravitational forces.

V. Stomata: Major Pathways for Water Loss

• Approximately 95% of water loss from plants occurs via stomata.

• Structure and Function of Stomata:

  • Each stoma is flanked by guard cells that control the stoma's diameter by changing shape.

  • Turgor pressure changes, mainly from the uptake and loss of K+ ions, result in the opening and closing of stomata.

VI. Stimuli for Stomatal Opening and Closing

• Typically, stomata open during the day and close at night to reduce water loss.

• Triggers for Stomatal Opening:

  • At dawn: Light, CO2 depletion, and an internal clock in guard cells trigger stomatal opening.

• Factors causing stomatal closure: Drought, high temperature, wind, and production of the hormone abscisic acid in response to water scarcity.

VII. Transpiration, Wilting, and Leaf Temperature

• Transpiration leads to significant water loss; insufficient replenishment causes water loss and wilting.

• However, transpiration also provides evaporative cooling, which lowers leaf temperature and prevents enzyme denaturation, highlighting a key tradeoff.

VIII. Adaptations That Reduce Evaporative Water Loss

• Xerophytes:

  • These desert-adapted plants may complete life cycles during the rainy season or have leaf modifications to minimize transpiration.

  • Some utilize crassulacean acid metabolism (CAM), allowing stomatal gas exchange at night to reduce water loss during the day.

IX. Transport of Sugars

• Products of photosynthesis are transported through phloem via translocation.

• Phloem Sap Characteristics:

  • An aqueous solution rich in sucrose, moving from a sugar source (mature leaves) to a sugar sink (tubers or growing tissues).

• A storage organ may act as both a sugar sink in summer and a sugar source in winter.

• Sugar can move via symplastic or apoplastic pathways, utilizing modified companion cells (transfer cells) to facilitate movement.

X. Bulk Flow by Positive Pressure: The Mechanism of Translocation in Angiosperms

• In angiosperms, researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure.

XI. Key Concepts (To Remember)

• Understand how root pressure is generated and the transpiration-cohesion-tension mechanism.

• Grasp how negative pressure is produced in leaves and its effect on water transport in the xylem.

• Describe stomatal structure and the factors controlling guard cell activity, leading to water transport implications.

• Know the stimuli for stomatal movement, mechanisms of sugar loading in phloem, and how positive pressure facilitates sugar transport.

• Recognize trade-offs involved in plant water transport.