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
• TRANSPIRATION

  • Energy dependent process
  • The transformation of liquid to gas phase involves use of energy
  • 97-99.5% of water taken up is lost in transpiration
• 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

  • heat load is dissipated in the process due to the high heat of vaporization of water
  • If transpiration is extremely high 🡪 dehydration and desiccation 🡪 death
• Types of Transpiration (Based on the avenue of exit of water vapor)

  1. Cuticular transpiration
  2. Lenticular transpiration
  3. Stomatal transpiration
• Types of Transpiration (Based on the avenue of exit of water vapor)

  • Cuticular transpiration
  • Loss of water through cuticle
  • 5-10% of the water loss
• Types of Transpiration (Based on the avenue of exit of water vapor)

  • 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.
• Types of Transpiration (Based on the avenue of exit of water vapor)

  • Stomatal transpiration
  • Through the stomata
  • As much as 90% of the water lost from plants.
• What affects diffusion of water from leaf to atmosphere?

  • Relative humidity (RH) (%)
  • Vapor pressure deficit (VPD) ((pascal (Pa))
• 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 rarely exceeds 90%
  • water diffuses out from the plants to the atmosphere
• What affects diffusion of water from leaf to 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 (i.e. when RH of the atmosphere is 100%), there is no net movement of water
  • when the RH of 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
  2. Movement of water from root xylem to leaf xylem
  3. Movement of water from leaf xylem to the air
• Soil-Plant-Air Continuum of Water

  • 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
  • symplastic pathway - plasmodesmata
  • transmembrane or transcellular pathway - vacuolar membrane (tonoplast)and the plasma membranes
• Soil-Plant-Air Continuum of Water

  • Movement of water from root xylem to leaf xylem
  • transpiration-cohesion-adhesion theory
• Soil-Plant-Air Continuum of Water

  • Movement of water from leaf xylem to the air
  • influenced by RH and VPD
  • Towards lower water potential (Ψ; expressed in megaPascal, MPa)
• Factors that Affect Transpiration

  • Plant Factors
  • Environmental Factors
• 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.
• Factors that Affect Transpiration

  Environmental Factors

  1. Edaphic (soil) factors
  2. Atmospheric factors

  • Light
  • Relative humidity
  • Temperature
  • Wind velocity
  • Oxygen and carbon dioxide concentrations
• 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