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Discuss four plant factors that influence the rate of transpiration.

GeneralClass 12AllAnswered 27 Mar 2026
Answer

Internal (Plant) Factors Affecting Transpiration:

1. Frequency and Distribution of Stomata

  • Definition: Number of stomata per unit leaf area (stomatal density)
  • Typical Range: 100-1000 stomata/mm²
  • Relationship: Generally, higher stomatal density → higher transpiration
  • Exceptions:
    • Xerophytes have numerous stomata but keep them closed during day (water conservation)
    • Sunken stomata reduce transpiration despite high numbers
  • Variation:
    • Dorsiventral leaves: More stomata on lower (abaxial) surface
    • Isobilateral leaves: Equal distribution on both surfaces
    • Floating leaves: Stomata only on upper surface

2. Number of Open Stomata

  • Dynamic Factor: Changes throughout the day
  • Influence: Directly proportional to transpiration rate
  • Regulatory Factors:
    • Light: Stomata open in light (K⁺ ion accumulation in guard cells)
    • CO₂ concentration: Low CO₂ → stomata open
    • Water status: Water deficit triggers closure (ABA hormone)
    • Temperature: Moderate increase → opening; extreme heat → closure
  • Circadian Rhythm: Many plants show diurnal stomatal movement
  • CAM Plants Exception: Stomata open at night, closed during day

3. Water Status of Plant (Soil Water Availability)

  • Critical Factor: Determines water availability for transpiration
  • Mechanism:
    • Adequate water: Normal transpiration, stomata open
    • Water deficit:
      • Reduced turgor in guard cells → stomatal closure
      • Abscisic acid (ABA) synthesis increased
      • Hydraulic signal from roots reduces leaf water potential
    • Feedback Loop: Transpiration → water deficit → stomatal closure → reduced transpiration
    • Measurement: Leaf water potential (Ψ<sub>w</sub>) indicates plant water status
    • Permanent Wilting Point: Soil water content below which plants cannot recover turgor

4. Canopy Structure

  • Definition: Arrangement of branches and leaves in the crown
  • Components Affecting Transpiration:
    • Leaf arrangement (phyllotaxy): Overlapping vs. non-overlapping
    • Leaf angle: Vertical leaves reduce solar radiation absorption
    • Leaf size and shape: Large, broad leaves → higher transpiration
    • Leaf surface characteristics:
      • Cuticle thickness: Thick cuticle reduces cuticular transpiration
      • Trichomes (hairs): Create boundary layer, reduce transpiration
      • Sunken stomata: Protected from air currents
    • Architectural Effects:
      • Compact canopy: Reduced air movement, higher humidity → lower transpiration
      • Open canopy: Better air circulation → higher transpiration
    • Adaptive Significance: Xerophytic plants have reduced leaf surface area (e.g., spines in cacti)

Additional Plant Factors:

  1. Root:Shoot Ratio: Extensive root system supports higher transpiration
  2. Age of Leaf: Mature leaves transpire more than young leaves
  3. Leaf Orientation: Horizontal vs. vertical positioning

Interplay: These factors work synergistically; plants balance water loss with CO₂ uptake for photosynthesis.

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