Practical: Investigating the Rate of Transpiration (OCR AS Biology): Revision Note

Practical: Investigating the Rate of Transpiration

• 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)