transpiration

• transpiration - loss of water vapour from aerial parts of plant, mostly through stomata in leaves
• some water may evaporate through upper leaf surface, but loss is limited by the waxy cuticle

  • most water vapour leaves through stomata, opening a gaseous exchange for photosynthesis

  → since photosynthesis occurs when there is sufficient light, the majority of water vapour is lost during the day
• The importance of transpiration

  essential for plant survival, as water is lost from the leaf, it must be replaced from below

  • transport useful mineral ions
  • maintains cell turgidity
  • supplies water for growth, cell elongation + photosynthesis
  • supplies water that, as it evaporates, can keep the plant cool on a hot day
• Environmental factor - Light intesity

  in light, stomata open open to allow gaseous exchange for photosynthesis - higher light intensity increases transpiration rate
• Environmental factor - Temperature

  higher temperature increases rate of transpiration in three ways

  • it will increase rate of evaporation from cell surfaces so water vapour potential in leaf rises
  • increased rate of diffusion, water molecules have more kinetic energy
  • decrease relative water vapour potential gradient in air, so more rapid diffusion out of leaf
• Environmental factor - relative humidity

  higher relative humidity in air will decrease the rate of water loss due to lower water vapour potential gradient between leaf and air spaces
• Environmental factor - Air movement

  air moving outside leaf will carry away water vapour that has just diffused out of the leaf, maintaining high water vapour
• Environmental factor - Water availability

  if little water in soil, plant cannot replace water that is lost if insufficient in soil, then stomata closes and leaves wilt