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Plant Detectives Manual: a research-led approach for teaching plant science

Activity 8: Measuring stomatal conductance to water using a porometer

8.1) Introduction and objectives

Stomata confer a physical barrier to water vapour diffusion. The conductance to water vapour is a major determinant to water loss and availability of CO2 for photosynthesis. Although both CO2 and water vapour can be quantified by using an infrared gas analyser (like the LiCor, Activity 7), another instrument, the porometer (Fig. 9), can provide reliable estimates of stomatal conductance. The cycling porometer works by measuring the time it takes for a leaf to release enough water vapour to change the relative humidity in a small chamber by a fixed amount. By comparing this amount of time with a calibration plate, the stomatal conductance can be calculated (Monteith et al. 1988).


Unlike with the LiCor, environmental conditions, such as light, temperature and relative humidity, cannot be controlled in the porometer chamber. The porometer, however, is easy to calibrate and use, and it provides readings much faster than the LiCor, making it the tool of choice when sampling many plants.


The objectives of Activity 8 are to:

  1. become familiar with the use of the porometer
  2. quantify and compare stomatal conductance between wild type and mutant plants.



Figure 9. The porometer

Figure extracted from brochure ‘AP4 Porometer Data Sheet’.


8.2) Materials

  1. porometer type AP4, Delta-T
  2. porometer calibration plate and plastic ziplock bag
  3. Whatman paper, pre-cut to fit into calibration plate
  4. plastic bag
  5. water
  6. dry desiccant

8.3) Procedure

Porometer datasheets and manual: Delta-T Devices.

8.3.1) Porometer calibration

The AP4 is supplied with a moulded polypropylene calibration plate with six groups of holes; the rate of diffusion of water vapour through these holes has been carefully verified. Water vapour is provided by backing the plate with dampened paper. The sensor head is clipped onto the calibration plate, and readings are stored from each of the six standard calibration positions.



Calibration plate, damp filter paper

  1. Place damp filter on the back of the calibration plate to cover all holes. Seal with sticky tape. Make sure to remove air pockets before sealing

Plastic bag

  1. Place the calibration plate inside the ziplock bag and keep it at the same temperature as the location at which the measurements will be run. TIP: it is best to prepare the calibration plate on the day before running the experiment.


  1. Select Calibration from main menu.


  1. Select RH and press + or – to disable the pump; open the head and wave it about to determine the ambient RH. Adjust the Set RH value to the closest to that of ambient RH. Press Set to change other values (like the cup type, or units) or Go or Exit to return to the Insert Plate screen.


  1. Place the head in the corresponding position of the calibration plate that is indicated in the Position column and press Go to start. Repeat this step at the corresponding position when prompted by the software.


  1. After the six positions are completed, press Fit Curve. Accept if error is <5%. NOTE: you can come back to individual positions if there is a major discrepancy between the theoretical and estimated values.

8.3.2) Porometer measurement

Clamp a leaf with the measuring head and press Go (NOTE: the red button in the measuring head). Attempt to measure three leaves per plant, five plants per genotype.

8.4) Expected outcomes

  1. Using the data from the porometer compare conductance measurements for your wild type and mutant plants. See Appendix B for information on how to assess whether there are statistically significant differences in conductance between your wild type and mutant plants.


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