What it shows
A potometer measures how fast a cut shoot takes up water, which is close to how fast it transpires. Changing wind or light shows which factors speed water loss from leaves.
The science
Water lost by transpiration is replaced by water drawn up the xylem, so the distance a bubble or a coloured column moves along the tube per minute shows how fast the shoot is taking up water. Wind carries away the humid air around the leaves and makes water evaporate faster. Light opens the stomata, and warmth raises the evaporation rate, so uptake increases. High humidity slows it. Water uptake is close to, but not exactly, the same as water lost, because some water is used in photosynthesis and some stays in the plant.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A leafy twig 20 cm long, such as balsam, coleus or mint
- A 5 mL syringe without a needle
- About 30 cm of clear aquarium air tube that fits the syringe tip
- A basin of water and a few drops of food colour
- A ruler or a printed 1 mm scale
- A small table fan
- Petroleum jelly and a clamp or stand
- A stopwatch or phone timer
Safety
For this project: An adult must cut the twig with secateurs and handle the fan, keeping it away from water and wires. Do not use mains-powered equipment near spills.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fill the basin, the tube and the syringe with coloured water, working under water so no air bubbles are trapped.
Cut the twig under water at a slant and fit its stem snugly into the open end of the tube, seal the join with petroleum jelly.
Mount the tube along a ruler on a stand with the syringe at the lower end, and introduce one small air bubble into the tube.
Let the plant settle in still, shaded air for ten minutes, then mark the bubble position as the start.
Measure how far the bubble moves in five minutes, repeat three times and calculate the mean rate in mm per minute.
Repeat in the same room with the fan on low blowing on the leaves, and again in bright light from a window.
Between runs push the syringe plunger gently to move the bubble back to the start of the scale.
Plot a bar graph for still shade, fan and bright light, and write which factor increased water uptake the most.
What you should see
The bubble moves slowest in still, humid shade, faster in bright light, and fastest with the fan on. The mean rates from three runs support the order.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does a potometer actually measure?
The rate of water uptake by a shoot, which is close to the rate of transpiration.
2Why cut the stem under water?
To stop air from entering the xylem, which would break the water column.
3Why does wind increase transpiration?
It removes humid air around the leaf, keeping a steep gradient so more water vapour diffuses out.
4Why repeat each run three times?
To reduce random error and use an average.
Take it further
Compare the rate for the same shoot before and after you strip off half of the leaves, and calculate rate per unit leaf area.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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