Measuring Water Uptake

Under normal circumstances, the rate of transpiration can be measured by the uptake of water. This is because if water leaves the plant, then water is taken up by the xylem.

A potometer measures the rate of water uptake of a leafy shoot. It can be used to observe the rate of transpiration. This is what the apparatus should look like:

A detailed diagram showing an experiment set-up to measure water uptake in plants. A green plant with a "Cut shoot" is placed into a water-filled tube that connects to a "Reservoir". This reservoir has a "Volume scale" to measure water levels. Adjacently, a "Capillary tube" containing an "Air bubble" leads to a "Beaker of water". Labels are provided for each component for clarity.

Method

1. Cut a shoot from a plant.

  • It is important that the plant shoot is cut underwater so that the xylem remains filled with water and there are no air bubbles.

2. Place the shoot in the tube.

  • But make sure there is no moisture on the leaves as this can affect the rate of transpiration.

3. Set up the apparatus shown in the diagram above.

  • It’s crucial to make sure that the setup is airtight and without any bubbles, as this is the most challenging part of the process. To prevent air bubbles, you can set up the apparatus under water and seal any gaps with Vaseline.

4. Remove the capillary tube from the beaker of water, so that one air bubble forms, then put the tube back in the water.

5. Mark or record the starting point of the air bubble so that you can use a ruler to measure the distance the bubble travels.

6. Leave the apparatus for a given period of time and then record the final position of the air bubble.

  • During transpiration, water evaporates from the leaf. As a result, new water will be taken up by the plant to replace the evaporated water. This will move the air bubble along the tube, towards the opening that contains the plant.

To calculate the rate of transpiration, divide the distance the bubble travelled by the set period of time. An example of the units your result can be is centimetres per minute (cm/min).

Evaluating the potometer

It is important to keep in mind that measuring the rate of transpiration using a potometer is an indirect measurement. It measures the water uptake rather than the water evaporating from the leaves. But water taken up by the plant can also be used for photosynthesis and other processes. So the actual rate of transpiration may be lower.

You can reset the apparatus to investigate other variables, such as temperature, light intensity, air movement or relative humidity.

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