Factors affecting the rate of transpirationSpec 4.2.3.2
You need to be able to explain the effect of changing temperature, humidity, air movement and light intensity on the rate of transpiration.
| Factor | Change | Effect on rate | Reason |
|---|---|---|---|
| Temperature | Increase | Increases | Water evaporates faster and diffuses out of the leaf faster |
| Humidity | Increase | Decreases | The air around the leaf already has a lot of water vapour, so the concentration gradient is smaller and diffusion is slower |
| Air movement | Increase | Increases | Moving air carries away water vapour from around the stomata, keeping a steep concentration gradient |
| Light intensity | Increase | Increases | Stomata open wider in the light, so more water vapour can escape |
The opposite change has the opposite effect: lower temperature, more humid air, still air and darkness all reduce the rate of transpiration.
Measuring the rate of transpiration
A potometer measures the uptake of water by a cut shoot. This is used as a measure of the rate of transpiration.
- Cut a shoot under water and fit it into the potometer, making sure the apparatus is full of water with no air bubbles.
- Dry the leaves and seal the joints so no air or water can leak.
- Introduce an air bubble into the capillary tube and record its starting position.
- Measure the distance the bubble moves in a set time.
- Change one factor, such as air movement from a fan, and repeat.
Rate of transpiration
An air bubble in a potometer moves 36 mm in 12 minutes. Calculate the rate of transpiration in mm per minute.
- Rate = distance ÷ time.
- Rate = 36 ÷ 12.
Answer: 3 mm/min
Volume of water taken up
The capillary tube has a cross-sectional area of 0.8 mm². The bubble moves 40 mm in 10 minutes. Calculate the volume of water taken up and the rate in mm³ per minute.
- Volume = cross-sectional area × distance = 0.8 × 40 = 32 mm³.
- Rate = volume ÷ time = 32 ÷ 10.
Answer: 32 mm³ of water; 3.2 mm³/min
Other maths skills
Repeat readings and find the arithmetic mean to reduce the effect of random errors. When you plot a graph, put the independent variable on the x-axis, choose scales that use most of the grid, label both axes with units and draw a line of best fit. The same ideas apply when you investigate the distribution of stomata by counting them in several fields of view (a sample) on a leaf.
Mean number of stomata
A student counts the stomata in three fields of view on the lower surface of a leaf: 24, 28 and 20. Calculate the mean.
- Add the counts: 24 + 28 + 20 = 72.
- Divide by the number of fields of view: 72 ÷ 3.
Answer: 24 stomata per field of view
Quick check
Which plant tissue transports water and mineral ions?
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Xylem.
What is translocation?
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The movement of dissolved sugars through phloem tissue from the leaves to the rest of the plant.
Why does an increase in humidity decrease the rate of transpiration?
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The concentration gradient of water vapour between the leaf and the air is smaller, so water vapour diffuses out more slowly.
What do guard cells control?
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The opening and closing of stomata, which controls gas exchange and water loss.