The Rate of Transpiration

• Transpiration rate if affected by 4 main things:

  • light intensity
  • temperature
  • air flow
  • humidity
• Light intensity:

  • the brighter the light, the greater the transpiration rate
  • stomata begin to close as it gets darker
  • photosynthesis can't happen in the dark, so they don't need to be open to let carbon dioxide in
  • when the 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 (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, meaning there is 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, so diffusion happens quickly
• 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