Lesson 9 of 12 / Nutrition and transport in flowering plants
Testing transpiration factors
What does a moving potometer bubble actually measure?
I can interpret potometer results and explain effects of air movement, temperature, humidity and light.
I can investigate and explain four environmental effects on transpiration and wilting.Syllabus
Syllabus K224 / K225, Topic 8(k). Investigate and explain four environmental effects on transpiration and wilting.
About 7 min
Make a guess. You are not marked.
A plant is put in front of a fan. What happens to its rate of water loss?
Show the answer
It goes up, because moving air removes humid air.
Wind carries away the moist air near the leaf, keeping a steep water-vapour gradient for diffusion.
The key idea
A potometer measures water uptake as an estimate of transpiration. Environmental factors change evaporation, the vapour gradient or stomatal opening.
| Condition | Distance in 4 min (mm) | Uptake (mm/min) |
|---|---|---|
| Still air: reference | 12 | 3 |
| More air movement | 24 | 6 |
| Higher humidity | 8 | 2 |
| Higher temperature | 20 | 5 |
| Lower light | 4 | 1 |
Reference condition: 12 mm in 4 minutes gives 3 mm per minute.
Dashed bubble: start. Solid bubble: after 4 minutes. The arrow points towards the shoot as it takes up water. This is a schematic capillary and shoot; support, reservoir and seals are omitted.
Controls: same leaf area, intact water column, sealed apparatus, adjustment time and unchanged other conditions. Uptake estimates transpiration; some water is retained or used. The arrows are not a timed simulation.
Explanation
A potometer measures how fast a leafy shoot takes up water. As the shoot takes up water, an air bubble moves along a narrow tube. The distance the bubble moves per minute shows the rate. Most of the water taken up replaces water lost by transpiration, so this estimates the rate of transpiration.
Cut the shoot under water, so that air does not get into the xylem, and make sure all the joints are sealed. Change only one condition at a time, and repeat each reading.
Moving air carries away the damp air around the leaves. This keeps a steep water vapour concentration gradient, so transpiration is faster. Dry air does the same. Higher temperature also usually speeds up evaporation and diffusion.
Another method is to weigh a whole plant in a pot. Put the pot and soil in a plastic bag tied round the stem, so that water can only be lost from the leaves, not from the soil. Weigh the plant every hour. The loss in mass per hour shows the rate of transpiration. Compare, for example, still air with a fan, keeping everything else the same.
Brighter light often increases transpiration, because the stomata open wider. But if a plant is very short of water, its stomata close, so the rules do not always apply. Look at the actual data before explaining them.
Step by step
- 1
State what is measured
Use bubble distance or water volume per time, with correct units.
- 2
Control the comparison
Keep shoot/leaf area, other environmental factors and apparatus conditions comparable.
- 3
Explain the observed change
Link the changed factor to evaporation, a vapour gradient or stomatal opening.
Worked example
Still air and moving air
Illustrative readings show a bubble moves 12 mm in 4 minutes in still air and 24 mm in 4 minutes with a fan. Compare rates and explain.
One way to explain it
The rates are 3 and 6 mm per minute, so measured uptake is twice as fast with the fan. Moving air removes humid air near the leaf, steepening the vapour gradient and increasing transpiration under these controlled conditions.
Why this answer works
- Divide each distance by its time.
- Make a numerical comparison.
- Explain the gradient while recognising water uptake as the measurement.
Watch out for this
A student says: "The bubble directly measures every molecule of water lost from the leaf." What is wrong with this?
Show the answer
It measures uptake through the apparatus. Uptake is an estimate of transpiration because some water is retained or used in plant processes.
Check your understanding
Why does higher humidity usually reduce transpiration when other conditions are held constant?
Show the answer
The water-vapour concentration difference between leaf air spaces and the outside air is smaller.
A smaller gradient reduces outward diffusion of water vapour.
Exam-style questions
Check your understanding
In a potometer the air bubble moves 36 mm in 4 minutes. What is the rate of water uptake?
Show the answer
9 mm per minute
Rate = distance / time = 36 / 4 = 9 mm per minute.
Quick recall
What does a moving potometer bubble actually measure?