Transpiration

Definition

• Loss of water vapor from aerial parts of plants, mainly through stomata.
• Called a “necessary evil”: helps cooling but causes water loss.
• Rate: ~400–500 g water/day per medium plant.
• Related process: Guttation – loss of liquid water through hydathodes.
• Transpiration accounts for ~99% of water uptake; <1% used in metabolism.
• Maximum transpiration occurs in midday due to high temperature and light.
• Minimum transpiration occurs at night (except in CAM plants).
• Transpiration is directly proportional to leaf area.

2. Types of Transpiration

Additional facts:

• Transpiration coefficient: water transpired per unit dry matter produced.
• Evapotranspiration = Transpiration + Soil evaporation.

3. Structure and Function of Stomata

• Each stoma has 2 guard cells controlling the pore.
• Dicots: kidney-shaped; lower epidermis.
• Monocots: dumbbell-shaped; both surfaces.
• Accessory/subsidiary cells: support guard cells in some species.

Mechanism:

• High turgor → stomata open; low turgor → close.
• Ion movement: K⁺ in → water in → open; K⁺ out → water out → close.
• Hormone: ABA → closes stomata under stress.
• Other regulators: Blue light → stomatal opening; circadian rhythm also influences.

• Stomata respond to blue light → phototropins trigger opening.
• Circadian rhythm: stomata open even without light during certain periods.
• Stomatal density: High in sun leaves, low in shade leaves.
• Guard cell turgor: regulated by K⁺, Cl⁻, malate ions.
• Monocots often have parallel venation and dumbbell-shaped stomata.

4. Mechanism of Ascent of Sap

• Cohesion–Tension Theory (Dixon & Joly, 1894):
  • Transpiration pull at leaves creates negative pressure → water moves upward via xylem.
  • Cohesion (water–water) + Adhesion (water–xylem walls) + Capillarity.
• Max ascent ~100 m (tallest trees).
• Other less accepted theories: Root pressure, Capillarity.

• Factors Affecting Transpiration

External (Environmental) Factors

• Light: opens stomata → increases transpiration.
• Temperature: increases evaporation; 10°C rise can double transpiration.
• Humidity: high humidity → reduces transpiration.
• Wind: removes saturated layer → increases rate.
• Soil water: low moisture → stomata close, reduces transpiration.

Internal (Plant) Factors

• Leaf area, size, orientation.
• Stomatal density & distribution.
• Cuticle thickness (thicker → less transpiration).
• Leaf hairiness → reduces transpiration.
• Sunken stomata → xerophytes adaptation.

Measurement of Transpiration

• Potometer: Measures water uptake.
• Cobalt chloride paper: Blue → pink on water vapor contact.
• Weighing method: Measures loss in fresh weight.
• Lysimeter: Field-scale transpiration.

Significance of Transpiration

• Cooling leaves → prevents overheating.
• Transpiration pull → ascent of sap, mineral transport.
• Maintains water balance and turgor.
• Facilitates gas exchange.
• Excessive transpiration → wilting, leaf drop.

Anti-Transpirants; Chemicals that reduce water loss.

Types:

• Reduce water loss; useful in drought conditions.
• Stomatal closing: ABA, PMA.
• Film forming: wax, silicone oils.
• Reflectant: Kaolin, lime spray.
• Growth retardants: CCC → reduce transpiring leaf area.

Wilting; Loss of turgidity due to water loss.

Types:

• Temporary wilting: reversible, midday.
• Incipient wilting: early signs of permanent wilting.
• Permanent wilting: irreversible, plant death.

• Wilting point: soil moisture level below which plants cannot regain turgor.

Water Use Efficiency (WUE)

• WUE = Dry matter produced / Water transpired.
• Higher in C₄ plants (low photorespiration).
• Improved by: drought-tolerant varieties, mulching, anti-transpirants.

• WUE higher in C₄ plants than C₃ plants.
• Transpiration ratio indicates water needed per unit biomass.
• Low transpiration ratio → better water economy.

Transpiration Ratio

• Transpiration ratio = Water transpired / Dry matter produced.
• Indicates water loss per unit biomass.
• Low ratio → more efficient water use.

Additional Competitive Exam Points

• Maximum transpiration pull: ~2 MPa.
• Evapotranspiration: total water loss from plant + soil.
• Transpiration coefficient: varies among crops (e.g., wheat 500–900, rice 500–1000).

• Transpiration accounts for ~10 times more water loss than photosynthesis.
• Maximum pull in xylem: -2 MPa.
• Leaf rolling, thick cuticle, wax deposition → anti-transpiratory adaptations.
• Stomatal ratio (upper/lower surface) affects transpiration; dicots usually hypostomatic.
• Boundary layer resistance → slows down transpiration; reduced by wind.