Measuring rate of water uptake by a plant shoot using a potometer
Class practical
In this activity the rate of water uptake, due to transpiration, by a shoot from a woody plant, is measured by timing how long a bubble takes to move a set distance, in the capillary tube of an h-shaped potometer.
Lesson organisation
Decisions will depend on the availability of suitable potometers. This could be a demonstration or an experiment for groups of up to three individuals – observer, timekeeper and scribe.
Apparatus and Chemicals
For each working group – equipment to investigate factors affecting rate of transpiration:
Lamp
Light meter
Fan
Anemometer
Vaseline or nail varnish
Graph paper
For each working group – basic equipment:
Plant material (Note 1)
Large sink – to assemble apparatus under water (Note 2)
Potometer – designs vary
Scalpel or scissors, 1
Clamp stand with boss and clamp, 1
Marker pen, 1
Stop clock, 1
Beaker
Water
Plastic ruler
Paper towels
Thermometer (-10 °C to 110 °C), 1
Food colouring, optional (Note 2)
Health & Safety and Technical notes
Some people find sap from plants irritating to the skin.
Take care when cutting the plant shoot.
Take care when assembling and handling the glass potometer: it is easy to break the long glass tubes and cut or stab yourself with the broken ends. Be prepared with first aid for cuts from broken glass, and brief students how to deal with breakages.
1 Use shoots from a shrub or tree with leaves that have thin waxy cuticles, such as. beech, lilac, Spirea. Leaves with thicker waxy cuticles (very glossy leaves) do not work as well.
2 You must cut the shoots under water and you must assemble the potometer under water. This requires a large sink. If air gets into the xylem vessels of the plant, it can form air locks which will prevent the plant taking up water and so prevent steady transpiration.
3 The potometers should be left for the leaves to dry. Alternatively dry the leaves gently with a paper towel. The potometer will not work properly until any excess water on the leaves has evaporated or been removed.
4 Assemble the potometers before the lesson, since there is a real art in setting them up. This will give the leaves time to dry, and give technicians a chance to check they are working before the students begin to take measurements.
5 Adding food colouring to the water makes it easier to see the air bubble in the capillary tube.
Ethical issues
There are no ethical issues with this procedure.
Procedure
SAFETY: Take care when handling this apparatus as glass tubing is quite fragile.
Make sure students know what to do if anything gets broken.
Have first aid kit ready for cuts.
Preparation
a Set up the apparatus as in the diagram.
b Leave undisturbed so that the shoot equilibrates to the conditions.
c Starting about 2 cm from the free end of the capillary tubing, mark the tubing at 1 cm intervals using the ruler and marker pen. Make as many marks as possible (at least 6).
Investigation
d Introduce a bubble into the capillary tubing by lifting the whole potometer upwards. To do this, loosen the screw on the boss and slide the boss up the clamp stand so that the capillary tube comes out of the water in the beaker. Retighten the screw on the boss.
e Gently blot the end of the capillary tube with a piece of paper towel; an air bubble should appear in the capillary tube.
f Loosen the screw on the boss and lower the potometer, so that the capillary tube just goes back into the water in the beaker. Retighten the screw on the boss.
g There are two ways of taking measurements:
- Start the stop clock when the bubble of air touches the first marked line. Stop the clock when the bubble has travelled a fixed distance (2 or 3 cm) and touches the appropriate marked line.
OR:
- Start the stop clock when the bubble of air touches the first marked line. Allow the bubble to travel upwards for a fixed period of time. Depending on the rate of movement, this could be 10 or 30 seconds or longer. Mark how far the bubble has moved with a different coloured pen. Measure the distance between the first mark and this second coloured mark.
h Calculate the rate of movement of the air bubble (and hence rate of transpiration) using the formula given here. SI units are cm/s.
Distance moved by the air bubble/Time taken for the air bubble to move that distance
i Change a factor that might affect how quickly the plant loses water by transpiration - see teaching notes.
j Repeat from d to i.
Teaching notes
Note that many plants close their stomata when the light intensity is insufficient for photosynthesis. Stems harvested in the autumn or winter may not transpire very much. You can use a high-intensity light to promote photosynthesis and get transpiration going again. Fluorescent strip lights or halogen lamps are better than ordinary bench lamps.
You could give students guidance on investigating a particular factor affecting the rate of transpiration, or they can choose from a list and develop their own ideas. Use five or six values for each variable tested.
- Light intensity. Place the plant shoot and potometer at different positions round the laboratory to alter the light intensity. Or you could use a lamp set at different distances from the plant. Check the intensity of light reaching the plant by placing a light meter by the leaves of the shoot.
- Wind speed. Using a fan to create different wind speeds, either by setting the fan to different speeds or by placing the plant shoot and potometer at different distances from the fan. Check the wind speed at the plant with an anemometer placed just in front of the plant at leaf height.
- Temperature. It may be possible to find places which are at different temperatures; but it may not be easy to ensure that all other variables are controlled.
- Surface area. There are at least two ways of estimating the effect of surface area. An added extra dimension would be to try to establish if the upper or lower surface is more important in transpiration. Stomata are usually more abundant on the lower surface of leaves.
Estimating effect of surface area
Method 1: Measure the water uptake by the shoot. Remove a number of leaves from the shoot and measure the rate of water uptake again. Keep removing leaves until all the leaves are off the plant shoot. Label the leaves as you remove them, then estimate their area by placing on squared paper and tracing their outlines. Add together the areas of the leaves to find the total area and multiply by two to get the total surface area, as each leaf has an upper and a lower side. Add the leaf areas together in the reverse order that they were removed to get figures for the change in the surface area of leaves attached to the plant shoot.
Method 2: Measure the water uptake by the shoot. Use vaseline - or nail varnish, but this could damage the leaves - to cover one or both sides of a leaf. Test again. Make a note of the order of vaselining/ varnishing the leaves. Calculate the surface area of the leaves as above. This could get messy!
Health and safety checked, September 2008
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