Investigating Temperature & Wind Speed (Cambridge O Level Biology)

• It is possible to investigate the effect of different environmental factors on the rate of transpiration using a piece of apparatus called a potometer
• Environmental factors that could be studied include:
  • Wind speed
  • Light intensity
  • Temperature
• There are 2 types of potometer
  • A mass potometer measures the change in mass of a plant as a measure of the volume of water that has evaporated from the leaves and stem
  • A bubble potometer measures the uptake of water to indicate the volume of water that is being lost by evaporation

Mass potometer and bubble potometer diagram

Mass and bubble potometers can be used to investigate the effect of environmental conditions on transpiration

Investigating the effect of light intensity on transpiration using a bubble potometer

Apparatus

• Water basin
• Scalpel or sharp scissors
• Potometer
• Vaseline
• Paper towel
• Timer
• Lamp
• Ruler

Method

• Cut a plant shoot underwater
  • This prevents air entering the xylem and forming blockages which may affect the uptake of water
• Assemble the potometer underwater and use Vaseline to seal the joins
  • The prevents air entering the potometer
• Set up the potometer on a workbench with the end of the capillary tube in a beaker of water
• Dry the leaves of the shoot
  • Water on the leaves can block the stomata and reduce water loss from the plant shoot
• Remove the capillary tube from the beaker of water to allow a single air bubble to form, then place the tube back into the water
• Set up a lamp 10 cm from the leaf
• Allow the plant to adapt to the new environment for 5 minutes
• Record the starting position of the air bubble using a ruler
• Leave for 30 minutes
• Record the end position of the air bubble on the ruler
• Reset the bubble in the potometer by opening the tap below the reservoir
• Repeat the experiment twice more at this light intensity
• Change the light intensity by moving the lamp, e.g. to 20 cm from the plant
• Carry out further repeats at this new light intensity, and at other light intensity levels

Investigating light intensity and transpiration diagram

A bubble potometer can be used to investigate the effect of light intensity on transpiration in a plant shoot

Results

• The further the bubble travels along the tube, the more water has been taken up by the plant
• We would expect that as light intensity increases, the bubble will travel further during the 30 minute time period
  • Transpiration rate increases with light intensity because more stomata tend to be open in bright light in order to maximise photosynthesis

Investigating other factors

• The same technique can be used to investigate the effect of other environmental factors:
  • Temperature can be investigated by setting up a potometer in rooms at different temperatures
  • Wind speed can be investigated using an electric fan to generate air movement