Agriculture and Food 7–12 · Years 9–10

Measuring transpiration with a bubble potometer on a balance

Plant Production 1

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Water evaporating from leaves pulls water up through the stem, so the rate a shoot takes up water rises with wind, warmth and light and falls in still, humid air.

Safety card

Low riskAn adult supervises

Hazards

cuts from the blade
glass capillary breakage
water near the fan and lamp

Controls

technician assembles the potometer, as SAPS suggests
keep electrical leads away from water
handle the capillary by the bung

Note

No hazardous chemicals. General laboratory rules in NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.

What you need

leafy shoot with a stem about 4 mm across that fits the bung
potometer: capillary tube whose internal bore is stated by the supplier (capillary tubing is sold by bore), fitted through a bung into a water-filled reservoir, with a tap or syringe to reset the bubble
bucket or sink of water for cutting and assembling under water; sharp blade
petroleum jelly to seal joints
ruler graduated in millimetres, stop clock
balance reading to 0.01 g
desk fan, lamp, clear plastic bag, thermometer
1 cm grid paper to measure leaf area

How to do it

The technician or teacher cuts the stem under water at a slant and fits the shoot into the bung under water so no air enters, then seals the joint with petroleum jelly and dries the leaves.
Introduce an air bubble into the capillary, wait 5 minutes for the shoot to settle, then record the distance the bubble moves in 5 minutes. Repeat three times.
Record the mass of the whole potometer on the balance at the start and the end of the whole run, with the fan switched off and the reading settled each time, because air moving over the balance pan disturbs a 0.01 g reading.
Change one condition at a time: a fan on low, the lamp at 30 cm, then a plastic bag over the shoot. Let the shoot settle for 5 minutes after each change before measuring.
Take the capillary's internal bore from the supplier's specification or the potometer's own label and use the model to convert bubble distance to volume. The bore is not measured here: a millimetre ruler, the only length-measuring instrument on this list, cannot resolve an internal radius of 0.25 to 1.0 mm.
At the end, trace every leaf onto grid paper, count squares to find total leaf area, and express uptake per square centimetre of leaf.

What you should see

Uptake is faster with the fan and with the lamp than in still room air, and slowest with the bag over the shoot, where the air becomes humid. A bubble moving 30 mm in 5 minutes in a capillary of 0.5 mm internal radius is 23.6 microlitres, or 4.7 microlitres per minute. Volume goes as the square of the radius, so a bore 20% away from its stated value puts every volume 44% out, which is why the entry reports the bubble distances as well as the volumes and why the volumes are not compared between potometers of different bore. Water moving at that rate weighs only about 0.024 g per 5 minutes, close to the balance's 0.01 g resolution, so the balance readings are compared over the whole run. The volume taken up and the mass lost are compared; they need not be equal, because the potometer measures uptake, and some water taken up stays in the shoot. The learner knows it worked when repeat bubble readings in one condition agree, the bubble rates put the bagged shoot slowest and the fan and lamp faster than still air, and the balance shows a mass loss over the whole run.

What changes

What you change: condition (still air, fan, lamp, bagged shoot)
What you measure: bubble distance per 5 minutes (mm), water uptake (microlitres per minute), mass lost over the whole run (g)
What you keep the same: same shoot
5 minute settling time
measuring interval
room temperature recorded

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Roots pump water up the plant the way a heart pumps blood.
Transpiration is wasted water that does the plant no good.
A potometer measures exactly the water lost from the leaves.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Agricultural Technology Years 7–10 Syllabus (2019)AG5-6 AG5-11 AG5-12
Agriculture Technology 7–10 Syllabus (2024), new syllabus, implementation from 2027AGT5-APG-01
Australian Curriculum v9AC9TDE10K04AC9S9I02

Sources

The pages the author read to write this activity.

All Concept Studio activities