2.2 Plant Physiology: Transpiration

1. Concept and Importance of Transpiration

• Definition: Release of water vapor from aerial parts of plants, primarily through stomata.
• Process: Water absorption → Movement to leaves → Evaporation → Diffusion through stomata.
• Significance:
  • Creates Transpirational Pull (drives ascent of sap).
  • Transports minerals.
  • Provides cooling effect.
  • Maintains turgor pressure.

2. Ganong’s Potometer

• Purpose: Measures the rate of water uptake by a leafy shoot (assumed to be transpiration rate).
• How it works: Measures the movement of an air bubble in a capillary tube as water is drawn by the plant.
• Limitations: Measures uptake, not directly transpiration; requires airtight seal; cutting shoot can cause damage; sensitive to environmental factors.

3. Factors Affecting Transpiration

• Environmental Factors:
  • Light Intensity: Increases (promotes stomatal opening).
  • Temperature: Increases (faster evaporation).
  • Humidity: Decreases (reduces water potential gradient).
  • Wind Speed: Increases (removes humid air).
  • Soil Water Availability: Decreases if scarce.
• Plant Factors: Stomatal density, leaf area, cuticle thickness, root-to-shoot ratio.

4. Experiments on Transpiration

• Loss in Weight: Potted plant or leafy shoot loses weight/water level decreases over time.
• Cobalt Chloride Paper: Turns pink in presence of water vapor; used to show unequal transpiration from leaf surfaces (more from lower surface of dorsiventral leaves).

5. Mechanism of Stomatal Transpiration: Potassium Ion Exchange Theory

• Stomatal Opening:
  1. Light stimulates guard cells to produce ATP.
  2. H⁺ pumped out, creating electrochemical gradient.
  3. K⁺ influx into guard cells (and Cl⁻).
  4. Water potential decreases, water enters by osmosis.
  5. Guard cells become turgid, bow outwards, opening stomata.

5. Mechanism of Stomatal Transpiration: Potassium Ion Exchange Theory (Cont.)

• Stomatal Closing:
  1. Absence of light or water stress (ABA production).
  2. K⁺ efflux from guard cells.
  3. Water potential increases, water leaves by osmosis.
  4. Guard cells become flaccid, straighten, closing stomata.

6. Adaptations to Reduce Transpiration

• Thick Cuticle: Reduces epidermal evaporation.
• Sunken Stomata: Stomata in pits create humid microenvironment.
• Hairy Leaves (Trichomes): Trap moist air.
• Rolled Leaves: Reduce exposed surface area.
• Reduced Leaf Area: Small/needle-like leaves, spines.
• Succulence: Fleshy stems/leaves store water.
• CAM Photosynthesis: Stomata open at night.
• Deep Root Systems: Access deeper water.

7. Guttation and Bleeding

• Guttation: Exudation of xylem sap drops from leaf margins/tips through hydathodes.
  • Occurs when root pressure is high and transpiration is low (e.g., at night).
  • Contains dissolved minerals, unlike dew.
• Bleeding (Weeping): Exudation of xylem sap from a cut or injured part of a plant.
  • Also driven by root pressure.