Measuring water uptake - Potometers

Investigating transpiration

The uptake of water can be measured using a potometer. Under normal circumstances, the rate of water uptake gives a measure of the rate of transpiration.

A simple potometer is a piece of capillary tubing to which a plant has been connected. The water uptake is measured by recording the time taken for a bubble in the tube to move a set distance.

Aim

To find the rate of water uptake of a plant.

Diagram of how to conduct an experiment of how to find the rate of water uptake in a plant

Step one

The potometer is filled with water.

Variables

  • Independent variable - time.
  • Dependent variable - the distance moved by the bubble along the capillary tube.
  • Control variables - temperature, air flow or draughts, adequate supply of water.

Risks

Care must be taken when inserting the plant shoot into the rubber tubing attached to the capillary tube and with the scissors used to cut the plant shoot.

Example results

Time (min)Distance moved by bubble (mm)
00
516
1032
1547
2064
2580
3096

Analysis of results

Transpiration is the loss of water vapour from the stomata of the leaves and so cannot be measured directly. Instead we assume that the water taken up by the plant is equal to the water lost and, the rate of the movement of the air bubble gives an indication of the amount of water being lost from the plant.

\text{rate of movement} = \frac{\text{total distance}}{\text{total time}} = \frac{96~\text{mm}}{30~\text{min}} = 3.2 mm/min

Evaluation

As this method is only an indication of the rate of transpiration, it is most useful when comparing transpiration in different environments.

It could be used to measure the rate of transpiration:

  • with or without a fan to demonstrate the effect of air movement
  • at different temperatures
  • at high or low humidity
  • at different light intensities
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