Calculating Irregular Leaf Area With the LI-6400XT Portable Photosynthesis System

The LI-6400XT Portable Photosynthesis System calculates photosynthetic rates, transpiration, and other variables based on the leaf area inside the chamber. The standard leaf chamber has an aperture of 2 cm x 3 cm, so if the sample leaf covers the entire aperture, then the leaf area is 6 cm2.

But what if the leaf doesn’t cover the entire chamber opening? In this case, the user will need to measure or estimate the leaf area so that the calculations are correct.

Here is a simple method to measure the area of the leaf inside the chamber during a measurement. It does require removing the leaf from the plant (destructive sampling), but it provides a more accurate value than a cursory approximation.

  1. Perform the photosynthesis measurement. Don’t forget to match the IRGAs before you start—matching is especially important for plants with small leaf areas.
  2. Before removing the leaf from the chamber, trace the outline of the bottom gasket on the leaf surface with a pen.
  3. Open the chamber and remove the leaf.
  4. Remove the leaf from the plant.
  5. Find a spare gasket.
  6. Line up the outside edge of the gasket over the pen marks.
  7. Trace the inside of the gasket on the leaf surface with the pen.
  8. Cut out the area traced by the inner pen mark. This was the portion of the leaf that was inside the chamber during the measurements.
  9. Measure the area of this section. You can easily use the LI-3100C Area Meter or the LI-3000C Portable Area Meter with the LI-3050C Transparent Belt Conveyor Accessory to measure this leaf area. You can also scan the section, or take a photograph of it resting on a white table or piece of paper, next to a ruler or measuring tape. Then use image processing software, such as ImageJ, to analyze the pixels and calculate area. An alternative method, though less precise, is to trace the leaf onto graphing paper and estimate the area.
  10. Recompute the data using the correct leaf area. This is further described in Chapter 13 of the LI-6400XT manual.