By the end of this lesson
- Predict effects of four named factors.
- Explain effects using gradients and stomata.
- Interpret transpiration data with limiting caveats.
BIG QUESTION
Why can the same plant lose water rapidly on one day and slowly on another?
Environmental conditions alter evaporation, diffusion gradients and stomatal opening.
Temperature and humidity alter gradients
Higher temperature gives water molecules more kinetic energy and increases evaporation; warm air can also hold more water vapour.
High humidity reduces the water-vapour concentration gradient between leaf and air, so diffusion slows.
Wind removes the boundary layer
Still air near the leaf becomes humid and forms a boundary layerA layer of still, humid air next to a leaf surface that reduces the water vapour gradient and so slows transpiration.. Wind replaces it with drier air, steepening the gradient and usually increasing transpiration.
At very high water loss, stomata may close, so simple trends need biological context.
Light controls stomata
Increasing light intensity commonly causes guard cells to open stomata for carbon dioxide uptake. The larger pore also increases water loss.
In darkness, many stomata close. Succulent and drought-adapted plants can show different patterns.
Graph skills: SLAPUK and choosing a scaleBubble distances are small decimals: choose a decimal scale.
Environmental effects
Predictions assume other factors are controlled and the plant is not severely water-stressed.
| Factor increases | Typical effect | Reason |
|---|---|---|
| Temperature | Rate increases. | Faster evaporation and diffusion. |
| Humidity | Rate decreases. | Smaller vapour gradient. |
| Wind speed | Rate increases. | Humid boundary layer is removed. |
| Light intensity | Rate increases then may plateau. | Stomata open; another factor can become limiting. |
Language that earns precision
- boundary layer
- Layer of relatively still, humid air beside a leaf.
- humidity
- Amount of water vapour in air.
- limiting factor
- Factor in shortest supply that restricts a process rate.
Common misconceptions
Claim: Wind closes stomata and always lowers transpiration.
Correction: Moderate wind usually removes humid air and increases loss; severe stress may trigger closure.
Claim: Higher humidity increases transpiration because more water is present.
Correction: It reduces the gradient for water-vapour diffusion.
Claim: Light directly evaporates water.
Correction: Its principal specified effect is often stomatal opening, while radiant heating may also affect temperature.
Match the answer to the command word
Name the gradient or stomatal response, not just the direction.
Read axes and units; describe the trend before explaining it.
Recall, apply, then justify
Explain why wind usually increases transpiration rate.
Hint
Think about humid air next to the leaf.
Suggested marking points
- Water vapour accumulates in a boundary layer.
- Wind removes/replaces this humid air.
- The steeper gradient increases diffusion through stomata.
Explain how increasing humidity affects transpiration.
Hint
Compare leaf air spaces with external air.
Suggested marking points
- External air contains more water vapour.
- The gradient from leaf to atmosphere becomes smaller.
- Water-vapour diffusion/transpiration rate decreases.
A plant shows lower water uptake at noon than at 10:00 despite higher temperature. Explain this result.
Planning hint
A predicted physical effect can be overridden biologically.
What a strong design includes
- Higher temperature alone tends to increase evaporation.
- By noon the plant may have lost substantial water.
- Guard cells may lose turgor or receive closure signals.
- Stomata close, reducing the diffusion pathway.
- Reduced aperture can outweigh the temperature effect.
- Water uptake is also only a proxy for transpiration.