Apparatus & Techniques: Using a Potometer
- Air movement, humidity, temperature and light intensity all have an effect on the rate at which transpiration occurs
- The table below explains how these four factors affect the rate of transpiration when they are all high; the opposite effect would be observed if they were low
Transpiration Rate Factors Table
- A potometer can be used to investigate the effect of environmental factors on the rate of transpiration
Potometer Experiment
Apparatus
- Plant shoot
- Cutting board
- Scalpel/scissors
- Paper towels
- Potometer
- Volume scale
- Beaker
- Capillary tube
- Stopwatch
- Vaseline
Method
- Cut a shoot underwater
- This is done to prevent air from entering the xylem
- Place the shoot in the tube
- Set up the apparatus as shown in the diagram
- Make sure it is airtight, using vaseline to seal any gaps
- If air enters the apparatus the readings will be affected
- Dry the leaves of the shoot
- Any moisture present on the leaves will affect the rate of transpiration
- Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water
- Set up the environmental factor you are investigating
- Allow the plant to adapt to the new environment for 5 minutes
- Record the starting location of the air bubble
- Leave for a set period of time
- Record the end location of the air bubble
- Change the light intensity or wind speed or level of humidity or temperature (only one - whichever factor is being investigated)
- Reset the bubble by opening the tap below the reservoir
- Repeat the experiment
- The further the bubble travels in the same time period, the faster transpiration is occurring and vice versa
Investigating transpiration rates using a potometer
Environmental factors can be investigated in the following ways:
- Airflow: Set up a fan or hairdryer
- Humidity: Spray water in a plastic bag and wrap around the plant
- Light intensity: Change the distance of a light source from the plant
- Temperature: Temperature of room (cold room or warm room)
Examiner Tip
Remember when designing an investigation to ensure a fair test you must keep all factors the same other than the one you are investigating.
