Practical: Investigating the Rate of Transpiration (OCR A Level Biology)

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

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)