Factors affecting transpiration
The rate of water loss by transpiration changes with environmental conditions. Important factors include air movement, humidity, light intensity and temperature.
| Environmental condition | Effect on transpiration rate | Explanation |
|---|---|---|
| Higher wind speed | Increases | Moving air removes water vapour from around the leaf. This maintains a larger water-potential gradient between the leaf and the surrounding air, increasing diffusion of water vapour. |
| Higher humidity | Decreases | More water vapour in the surrounding air reduces the water-potential gradient from the leaf, so water vapour diffuses away more slowly. |
| Higher light intensity | Increases | The rate of photosynthesis increases and stomata open to permit gas exchange, providing a route through which more water vapour can leave. |
| Higher temperature | Increases | Water molecules have more kinetic energy, so evaporation from mesophyll surfaces and diffusion from the leaf occur more rapidly. |
1
Gradient effects
Air movement and humidity alter the gradient between the leaf and the surrounding atmosphere, changing the rate at which water vapour diffuses away.
2
Conditions within the leaf
Light intensity affects stomatal opening, while temperature changes the rate of evaporation and diffusion.
Exam Tip: Explain the complete causal link. For example: increased wind speed → water vapour is removed from around the leaf → a steeper water-potential gradient is maintained → water vapour diffuses out faster.