Biology Topic
Factors Affecting Transpiration
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Study focus - 3 things
- Light, temperature, humidity, and wind affect the rate of transpiration
- Hot, bright, windy, dry conditions usually increase transpiration
- Simple experiments can compare water loss under different conditions
Section 1Light
In bright light, stomata usually open so carbon dioxide can enter for photosynthesis. Open stomata allow more water vapour to leave. This usually increases the rate of transpiration.
Section 2Temperature and humidity
Higher temperature gives water molecules more energy, so evaporation happens faster. Low humidity means the air is dry, so water vapour diffuses out of the leaf more quickly. High humidity slows transpiration because the air already contains plenty of water vapour.
Section 3Wind
Wind blows away the moist air around the leaf. This keeps a steep concentration gradient between the inside of the leaf and the outside air. As a result, windy conditions usually increase transpiration.
Section 4Simple experiments
A potometer can estimate transpiration by measuring water uptake. A simpler classroom method is to cover a leafy shoot or potted plant with a clear plastic bag and observe water droplets forming. Fair tests should change only one factor at a time.
Examples
Two similar plants are tested. One is placed by a fan, and one is kept in still air. Which should lose water faster?
The plant by the fan should lose water faster because wind removes moist air around the leaves, increasing transpiration.
A plant in a humid greenhouse loses water slowly. Why?
Humid air already has lots of water vapour, so the concentration gradient from leaf to air is smaller and transpiration slows.
Now let's water it
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