Cuticular conductance of adaxial and abaxial leaf surfaces and its relation to minimum leaf surface conductance

Cuticular conductance to water (g_cw) is difficult to quantify for stomatous surfaces due to the complexity of separating cuticular and stomatal transpiration, and additional complications arise for determining adaxial and abaxial g_cw. This has led to the neglect of g_cw as a separate parameter in most common gas exchange measurements. Here, we describe a simple technique to simultaneously estimate adaxial and abaxial values of g_cw, tested in two amphistomatous plant species. What we term the ‘Red-Light method’ is used to estimate g_cw from gas exchange measurements and a known CO₂ concentration inside the leaf during photosynthetic induction under red light. We provide an easy-to-use web application to assist with the calculation of g_cw. While adaxial and abaxial g_cw varies significantly between leaves of the same species we found that the ratio of adaxial/abaxial g_cw (γ_n) is stable within a plant species. This has implications for use of generic values of g_cw when analysing gas exchange data. The Red-Light method can be used to estimate total cuticular conductance (g_cw-T) accurately with the most common setup of gas exchange instruments, i.e. a chamber mixing the adaxial and abaxial gases, allowing for a wide application of this technique.