Estimating transpiration rate - potometer

• a potometer is a special piece of apparatus used to estimate transpiration rates - actually measures water uptake by a plant, but it is assumed that water uptake by a plant is directly related to water loss by the leaves - can use it to estimate how different factors affect the transpiration rate
• Cut a shoot underwater


  1. Cut at a slant

  2. To prevent air from entering the xylem

  3. To increase the SA available for water uptake
• Assemble potometer


  1. Assemble under the water

  2. Insert the shoot with the apparatus still underwater

  3. So no air can enter
• Remove the apparatus from the water


  Keep the end of the capillary tube submerged in a beaker of water
• Check the apparatus


  Ensure it is watertight and airtight
• Dry the leaves


  1. Allow time for the shoot to acclimatise

  2. Shut the tap
• Remove the end of the capillary tube from the beaker of water

  1. Until one air bubble has formed

  2. Put the end of the tube back into the water
• Start a stopwatch


  1. Record the distance moved by the air bubble per unit time e.g. per hour

  2. The rate of air bubble movement is an estimate of the transpiration rate
• • Only change one variable (e.g. temp) at a time
  • All other conditions (e.g. light int/ humidity) must be kept constant
• reservoir of water used to return bubble to start for repeats
• capillary tube with a scale
• bubble moves right
• as the plant takes up water, the air bubble moves along the scale