Transpiration

Transpiration Rate

The rate of transpiration depends on a number of factors:

• Temperature – warm weather increases the kinetic energy of the water molecules so they move out of the leaf faster.

• Humidity – damp air reduces the concentration gradient so the water molecules leave the leaf more slowly.

• Wind – the wind blows away the water molecules so that a large diffusion gradient is maintained.

• Light – light causes the stomata to open and so more water is lost.

If a question asks Give one factor that increases transpiration rate, make sure that you write ‘an increase in temperature’ not just ‘temperature’. Many candidates lose marks in this way.

Adaptations to reduce water loss

When plants are short of water, they do not want to waste it through transpiration. The trouble is they need to let carbon dioxide in, so water will always be able to get out. Water loss is kept as low as possible in several ways:

• Photosynthesis only occurs during the day, so the stomata close at night to reduce water loss. The guard cells lose water by osmosis and become flaccid. This closes the stoma.

• The stomata are on the underside of the leaf. This reduces water loss because they are away from direct sunlight and protected from the wind.

• The top surface of the leaf, facing the Sun, is often covered with a protective waxy layer.

Although transpiration is kept as low as possible, it does help plants by cooling them down and supplying leaves with minerals. It also provides water for support and photosynthesis.