B1. Transpiration

• Transpiration 


  Process where water moves in Liquid Form in plants, and released in Vapor Form through aerial parts, but mostly in leaves, to the atmosphere. Energy dependent process. 97-99.5% of water taken up is lost in Transpiration. H2O(liquid) -539-540. cal g-¹> H2O (Gas)
• Importance of Transpiration  

  1. Keeps cells hydrated
  2. Maintains favorable turgor pressure for the transport of nutrients absorbed by the roots from the soil
  3. Cools the plant
• Types of Transpiration (CLS)

  1. Cuticular
  2. Lenticular
  3. Stomatal
• Cuticular Transpiration

  • Loss of water through cuticle
  • 5-10% of the water loss
• Lenticular Transpiration

  • Lenticels - pores in the outer layer of a woody plant stem
  • In deciduous species (trees which sheds off leaves) and in some fruits, water loss through lenticels may be quite substantial.
• Stomatal Transpiration

  • Through stomata
  • Up to 90% water lost from the plants
• What affects diffusion of water from leaf to atmosphere?

  Relative Humidity (RH) (%)

  • actual water vapor in the air; water vapor pressure in leaf
  • In leaves 100% RH; in atmosphere, RH exceeds 90%
  • water diffuses out from the plants to the atmosphere

  Vapor Pressure Deficit (VPD) (Pascal(Pa))

  • Actual water vapor pressure - water vapor pressure at saturation at the same temperature
  • When VPD is 0 Pa (when RH of the atmosphere of 100%), there is no net movement of water
  • when the RH in the atmosphere is low, the VPD is High, and the rate of Transpiration is faster
• Soil-Plant-Air Continuum of Water

  1. Movement of water from the soil to the root xylem

  • Extracellular or apoplastic route - water moves through non-living parts, e.g capillary spaces of the cell walls and intercellular spaces
  • Intracellular route

  1. Symplastic pathway
  2. Transmembrane or Transcellular pathway - vacuolar membrane (tonoplast)
• Soil-Plant-Air Continuum of Water

  • 2. Movement of water from root xylem to leaf xylem
  • Transpiration-Cohesion-Adhesion Theory (TCA)
  • 3. Movement of water from leaf xylem to the air
  • Influenced by RH and VPD
  • Towards lower water potential
• Transpiration of Water in Xylem

  Refer to the Image below:

  
• The Transpiration Cohesion-Adhesion Theory

  1. Water vapor leaves the air spaces of the plant via the stomates
  2. This water is replaced by evaporation of the thin layer of water that clings to the mesophyll cells
  3. Tension (pulling) on the water in the xylem gently pulls the water toward the direction of water loss
  4. the Cohesion of water is strong enough to transmit this pulling force all the way down to the roots
  5. Adhesion of water to the cell wall also aids in resisting gravity
• Factors that Affect Transpiration

  • Plant Factors

  1. Efficiency of Evaporative Surface
  2. Efficiency of water absorption
  3. Other surface/stomatal modifications
  4. Phytohormones
  5. Canopy Structure

  • Environmental Factors

  1. Edaphic (soil) factors
  2. Atmospheric factors

  • Light
  • Relative Humidity (RH)
  • Temperature
  • Wind velocity
  • Oxygen and Carbon Dioxide Concentrations
• How Plant and Environmental Factors affect Transpiration

  1. Leaf Number: more leaves, more Transpiration
  2. Number, size, position of stomata: more and large, more Transpiration, under leaf, less Transpiration
  3. Cuticle: waxy cuticle, less evaporation from leaf surface
  4. Light: more gas exchange as stomata are open
  5. Temperature: high temperature, more evaporation, more diffusion
  6. Humidity: high humidity, less Transpiration
  7. Wind: more Wind, more Transpiration
  8. Water availability: less water in soil, less Transpiration (winter, plant lose leaves)