About this experiment
Use a virtual potometer to measure water uptake from the travel of an air-bubble marker. Change humidity, wind, light, leaf area and capillary radius, then compare displacement with the actual water volume.
Learning goals
- Convert bubble travel into water volume using capillary cross-section.
- Relate transpiration to vapour-pressure deficit and stomatal opening.
- Distinguish a larger instrument displacement from a larger biological uptake.
How to use it
- 1Run a trial and record bubble displacement, elapsed time and uptake.
- 2Double the capillary radius without changing leaf conditions and compare travel.
- 3Close the stomata or set humidity to saturation and repeat.
- 4Shade the leaf and compare uptake over equal observation times.
The model
Assumptions
- A specified conductance scales with leaf area, pore opening, light and wind. Vapour-pressure deficit uses saturation pressure at the selected leaf and air temperature.
- Uptake is treated as equal to transpiration. Growth, tissue storage, leaks, xylem damage and differences between leaf and air temperature are omitted.
- The capillary has a uniform circular bore. One mL is 1000 cubic mm, so bubble displacement is volume divided by cross-section.
- The run stops when its 200 mm scale ends, or at 60 minutes. Environmental changes start a new constant-condition trial.
- Dragging the bubble selects the model time corresponding to its position; it does not force water through a living leaf.
Try this in class
- Q1Why does a narrower capillary make uptake easier to detect?
- Q2Why does wind have little effect when humidity is saturated?
Frequently asked questions
Does moving the bubble change the transpiration rate?
No. It selects a measurement time. The environmental controls determine the model rate.
Does a potometer directly measure water lost from the leaf?
It measures water uptake. This teaching model assumes that uptake matches transpiration over the trial.