Background

For simplicity, most common commercial gas exchange devices mix gases from the upper and lower cuvettes to estimate bulk gas exchange parameters from the mix. However, in some cases, it is desirable to analyse the characteristics of the leaf gas exchange on each leaf surface independently, such as when analysing adaxial and abaxial cuticular permeability in planta (Márquez et al., 2022), the stomatal contribution to the gas exchange of each surface (Wall et al., 2022), the estimation of water vapour (Wong et al., 2022) and CO₂ (Márquez et al., 2023) gradients within the leaf, etc.

It has been common to use elaborate in-house built gas exchange systems to measure adaxial and abaxial gas exchange independently [see Jarvis and Slatyer (1966), Jones and Slatyer (1972) and Wong et al. (1985)]; however, it requires proficiency in electronics calibration, sensor tuning, environmental conditions control, and expertise in designing the tubing and manifolds. The LI-6800 incorporates automatic calibration protocols, well-tuned sensors, and its design permits modifying the chamber without major interference with the other parts of the system, allowing us to easily conjoin two instruments to analyse the adaxial and abaxial surfaces of a leaf.

The chamber modification demands re-measuring the boundary layer conductance to water for each cuvette to be accounted for in the computation of the gas exchange parameters. In this protocol, we walk the reader through the process of mounting the two LI-6800s together and measuring the boundary layer conductance following a method adapted from Parkinson (1985) to be used in LI-6800 devices. We provide an add-on script for the LI-6800 to include the new boundary layer conductance estimations for each cuvette in the calculations. The add-on script also includes an update to the latest theory for estimating gas exchange parameters presented by Márquez et al. (2021), which is a more precise physical approach to the electrical resistance analogy for gas exchange. The new boundary layer conductance and equations are included in the instrument's computation, display, variables, and spreadsheet results using the add-on code.