How a Potometer Works

A potometer consists of a cut leafy shoot attached to a tube of water with a capillary tube and a scale. As the shoot loses water by transpiration, it draws water from the tube. An air bubble introduced into the capillary tube moves along the scale as water is taken up, allowing you to measure the rate of water uptake (distance moved per unit time or volume per unit time).

The key assumption is that almost all the water taken up by the shoot is lost by transpiration through the leaves, so water uptake rate ≈ transpiration rate.

Testing Different Conditions

Common factors to investigate:

  • Wind: use a fan to increase air movement around the leaves; this removes humid air from around stomata, increasing the diffusion gradient and thus transpiration rate
  • Humidity: enclose the shoot in a plastic bag to increase humidity; high humidity reduces the concentration gradient of water vapour, decreasing transpiration rate
  • Light: in light, stomata open wider for gas exchange during photosynthesis, increasing transpiration; in darkness, stomata close
  • Temperature: higher temperature increases the kinetic energy of water molecules, increasing the rate of evaporation from mesophyll surfaces

Setting Up Without Errors

Critical steps for reliable results:

  • Cut the stem under water to prevent air bubbles entering the xylem vessels (which would block water flow)
  • Ensure all joints are sealed with petroleum jelly (Vaseline) to prevent air leaks
  • Allow the shoot to acclimatise for several minutes before starting measurements
  • Use the same shoot for all conditions (or identical shoots of the same species, same number of leaves, same stem diameter)
  • Reset the bubble by opening a tap to push it back to the start position