Transpiration and Stomata

• Transpiration Rate


  • Affected by 4 main things:
  • Light intensity
  • Temperature
  • Air flow
  • Humidity
• Light intensity


  The brighter the light, the greater the transpiration rate
• As it gets darker

  Stomata begin to close
• Photosynthesis can't happen in the dark, so stomata don't need to be open to let CO2 in
• When stomata are closed


  Very little water can escape
• Temperature


  The warmer it is, the faster transpiration happens
• When it's warm, the water particles have more energy to evaporate and diffuse out of the stomata
• Air flow


  The better the air flow around a leaf (e.g. stronger wind), the greater the transpiration rate
• If air flow around a leaf is poor, the water vapour just surrounds the leaf and doesn't move away
• This means there's a high concentration of water particles outside the leaf as well as inside it, so diffusion doesn't happen as quickly
• If there's good air flow, the water vapour is swept away, maintaining a low concentration of water in the air outside the leaf
• Diffusion then happens quickly from an area of higher concentration to an area of lower concentration
• Humidity


  The drier the air around a leaf, the faster transpiration happens
• If the air is humid, there's a lot of water in it already, so there's not much of a difference between the inside and the outside of the leaf
• Diffusion happens fastest if there's a really high concentration in one place, and a really low concentration in the other
• Estimating transpiration rate


  1. Measure water uptake by the plant

  2. Set up apparatus with air bubble in capillary tube

  3. Record distance moved by bubble per unit time

  4. Keep conditions constant
• Guard cells


  • Kidney shape opens and closes stomata
  • Fill with water when plant has lots of water, making stomata open
  • Lose water when plant is short of water, making stomata close
  • Thin outer walls and thickened inner walls make the opening and closing work
  • Sensitive to light and close at night to save water
  • More stomata on underside of leaves than top
• Guard cells are adapted for gas exchange and controlling water loss within a leaf