cpac 8

• Potometer


  A device used to measure the uptake of water, and therefore the rate of transpiration, in a leafy shoot
• Potometers


  • Can be mass (based on measuring the change in mass of a plant over time) or bubble (based on measuring the movement of a bubble along a length of capillary tubing over time)
  • It is important that the potometer is airtight when using potometry to measure rate of transpiration
• Rate of transpiration


  Can be affected by abiotic factors such as light intensity, humidity, wind speed and temperature
• Equipment


  • Large leafy shoot
  • Capillary tubing
  • Ruler
  • Clamp and stand
  • Beaker
  • Stop clock
  • Fan/dark cupboard/lamp/plastic bag/incubator
  • Petroleum jelly
  • Small cable ties
  • Scissors
• Setting up the potometer

  1. Fill the capillary tube and rubber connector with water and insert the leafy shoot into the rubber connector (do this step underwater)

  2. Clamp the capillary tube into the stand and place the bottom of the capillary tube into the beaker of water

  3. Smear petroleum jelly around the join to maintain airtight conditions

  4. Leave for 5 minutes to allow a bubble to be drawn up into the capillary tube
• Measuring the rate of transpiration

  1. Time the length of time for the bubble to move a certain distance along the capillary tube or measure the movement of the bubble along the capillary tube in a certain length of time

  2. Convert this measurement into a rate of transpiration via dividing volume of water taken up by time
• Repeating the experiment


  Change the abiotic variable, e.g. by placing the plant 10 cm further from the light source
• Hazards


  • Biohazard (contamination)
  • Lamps (temporary damage to eyes)
  • Bags/stools (tripping)
• Safety precautions


  • Use disinfectant; wash hands with soap after handling
  • Do not look directly at lamp
  • Keep under desks and away from workspace
• In emergency


  • Seek assistance
  • Wait for afterimage to disappear; seek appropriate assistance if needed
  • Seek appropriate medical assistance; clean spillages
• Graphing


  Plot a graph of rate of transpiration against abiotic factor
• Temperature affects rate of transpiration

  It increases the rate of diffusion and evaporation from the stomata
• Light intensity affects rate of transpiration


  It affects the rate of photosynthesis, which in turn affects the number of stomata which are open
• Humidity affects rate of transpiration


  It affects the rate of diffusion and evaporation by decreasing the concentration gradient between the plant and the atmosphere
• Wind speed affects rate of transpiration

  It increases the concentration gradient by mechanically removing water from the outside of the stomata