An Improved System to Measure Leaf Gas Exchange on Adaxial and Abaxial Surfaces.

Measurement of leaf carbon gain and water loss (gas exchange) in planta is a standard procedure in plant science research for attempting to understand physiological traits related to water use and photosynthesis. Leaves carry out gas exchange through the upper (adaxial) and lower (abaxial) surfaces at different magnitudes, depending on the stomatal density, stomatal aperture, cuticular permeability, etc., of each surface, which we account for in gas exchange parameters such as stomatal conductance. Most commercial devices measure leaf gas exchange by combining the adaxial and abaxial fluxes and calculating bulk gas exchange parameters, missing details of the plant's physiological response on each side. Additionally, the widely used equations to estimate gas exchange parameters neglect the contribution of small fluxes such as cuticular conductance, adding extra uncertainties to measurements performed in water-stress or low-light conditions. Accounting for the gas exchange fluxes from each side of the leaf allows us to better describe plants' physiological traits under different environmental conditions and account for genetic variability. Here, apparatus and materials are presented for adapting two LI-6800 Portable Photosynthesis Systems to work as one gas exchange system to measure adaxial and abaxial gas exchange simultaneously. The modification includes a template script with the equations to account for small fluxes. Instructions are provided for incorporating the add-on script into the device's computational sequence, display, variables, and spreadsheet results. We explain the method to obtain an equation to estimate boundary layer conductance to water for the new setup and how to embed this equation in the devices' calculations using the provided add-on script. The apparatus, methods, and protocols presented here provide a simple adaptation combining two LI-6800s to obtain an improved system to measure leaf gas exchange on adaxial and abaxial surfaces. Graphical overview Figure 1.Diagram of the connection of two LI-6800s.Figure adapted from Márquez et al. (2021).