To test the effect of humidity on transpiration, compare shoots at different humidities while keeping light, temperature and airflow as similar as possible. A potometer measures water uptake, which can be used as an estimate of transpiration rate under suitable conditions.
The 2027 SEC G3 Biology K325 syllabus covers environmental effects on transpiration. This guide shows how to interpret a humidity comparison and spot when another variable could also explain the result.
- Choose humidity as the variable to change.
- Keep airflow, temperature, light and leaf area comparable.
- Compare bubble movement per unit time.
- Explain the result using the water vapour gradient.
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Connect the reading to the process
Transpiration involves water evaporating from moist cell surfaces in the leaf and water vapour moving out, mainly through stomata. Higher humidity outside the leaf usually reduces the difference in water vapour concentration between the leaf air spaces and the surrounding air.
With a smaller gradient and comparable conditions, water vapour diffuses out more slowly. That usually reduces transpiration.
A potometer tracks water uptake by the shoot. Bubble movement can indicate changes in uptake when the apparatus is airtight and the capillary size is the same. Water uptake can differ from water loss because the shoot may use or store some water.
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Worked example | Two humidities, the same measuring time
In an illustrative comparison, similar shoots with comparable leaf areas are tested in identical capillaries. The shoots are allowed to adjust to each chamber. Airflow, light and temperature are held as similar as possible.
| Condition | Distance in 5 min | Movement rate |
|---|---|---|
| Lower humidity | 30 mm | 6 mm/min |
| Higher humidity | 15 mm | 3 mm/min |
Bubble movement per minute is lower in the more humid chamber. With the same capillary bore, this indicates a lower rate of water uptake in that setup. It supports a lower transpiration rate under higher humidity.
Connect the conclusion to the mechanism. More humid surrounding air reduces the water vapour concentration difference, so less water vapour diffuses out per unit time under otherwise similar conditions.
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Practise identifying a confounding variable
Suppose a pupil creates the humid condition by enclosing one shoot in a bag and leaves the other shoot in open air. The bag can also change airflow and temperature. A difference in uptake then has more than one possible explanation.
Use comparable chambers and vary humidity while monitoring the other conditions. Repeat the comparison with more than one shoot and compare rates over matching intervals.
If a question asks for one improvement, identify a specific variable the arrangement changes accidentally and say how to control it.
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Frequently asked questions
Does the bubble directly measure evaporation from the leaf?
It indicates water uptake through the potometer. Under suitable conditions that gives an estimate of transpiration.
Why match the leaf area?
Shoots with different leaf areas may have different total water loss even in the same air conditions.
Can I compare distance if the capillary widths differ?
Use the volume displaced per unit time or use matched capillaries. The same bubble distance can represent different volumes in tubes of different widths.
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