Transpiration is the loss of water vapour from a plant, mainly from the leaf, which is the main organ for transpiration. Water evaporates from the moist surfaces of the mesophyll cells into the air spaces, then diffuses out through the open stomata. Most transpiration therefore happens through the stomata; a small amount escapes directly through the cuticle.
Why transpiration is useful
- It creates a "pull" (the transpiration stream) that draws water and mineral salts up the xylem from the root to the leaf.
- The evaporating water cools the leaf on a hot day.
- It keeps cells supplied with water needed for photosynthesis and turgidity.
Key idea
Transpiration = evaporation of water from mesophyll + diffusion of water vapour out of the stomata. It drives the transpiration stream in the xylem.
Factors affecting the rate
Anything that speeds up evaporation or diffusion raises the transpiration rate:
- Light — brighter light opens the stomata, so the rate rises.
- Temperature — higher temperature increases evaporation, raising the rate.
- Humidity — high humidity lowers the rate because the concentration gradient of water vapour is smaller.
- Wind — moving air removes water vapour from the leaf surface, keeping the gradient steep and raising the rate.
Example
On a hot, dry, windy afternoon transpiration is fast, so the plant may wilt if the roots cannot absorb water quickly enough. On a cool, still, humid morning the rate is much lower.
Controlling water loss
The plant controls transpiration mainly through the guard cells. If the plant is short of water the guard cells lose turgor and close the stomata, cutting water loss (but also slowing photosynthesis). This is a compromise between saving water and taking in carbon dioxide.
Remember
High humidity lowers transpiration; wind, heat and light raise it. Closing stomata reduces water loss but also reduces gaseous exchange.