Lesson 7 of 9 / Nutrition and transport in flowering plants
Testing transpiration factors
What does a moving potometer bubble actually measure?
In this lesson: Interpret potometer results and explain effects of air movement, temperature, humidity and light.
About 7 min
The key ideaA 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
In a bubble potometer, water uptake by a leafy shoot moves an air bubble along a narrow tube. Bubble distance per unit time compares uptake rates when the tube cross-section is unchanged. With known cross-sectional area, distance multiplied by area gives volume. Uptake estimates transpiration because most absorbed water replaces water lost, but some water is used or retained by the plant.
Cut and assemble the shoot under water to reduce air entering the xylem, and ensure airtight, watertight connections. Keep leaf area and the apparatus comparable, allow adjustment to each condition and repeat readings. Change only one environmental factor at a time.
More air movement removes humid air around a leaf, maintaining a steeper water-vapour concentration gradient. Lower humidity also increases that gradient. Higher temperature generally increases evaporation and diffusion, provided water supply and stomatal opening are not restricting the process.
Greater light intensity often increases transpiration because stomata open for gas exchange, but other conditions must be controlled. Severe water stress can cause stomata to close, so these predictions are conditional rather than unlimited rules. Compare actual data before assigning a mechanism.
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.
Is this true? "The bubble directly measures every molecule of water lost from the leaf."
It measures uptake through the apparatus. Uptake is an estimate of transpiration because some water is retained or used in plant processes.