Measurements of electrical leaf surface conductance reveal re‐condensation of transpired water vapour on leaf surfaces

Electrical conductance (λ) was measured continuously and in vivo on leaf surfaces of Vicia faba and Aegopodium podagraria. λ increased with rise and decreased with fall in humidity, exhibiting a hysteresis during an applied humidity cycle [90–20–‐90% relative humidity (r.h.)]. After treatment with NaNO3 aerosols, a sudden increase in λ was observed at 73% r.h., which is close to the deliquescence point of the salt. Transpiration and electrical conductance of untreated leaves were measured simultaneously under conditions of constant r.h., while the photosynthetic photon flux density and CO2 concentration of the air were varied to induce changes of stomatal aperture. At 35% r.h., changes of light and CO2 level revealed a strong correlation between stomatal conductance (gS) and λ for Vicia faba leaves. This was also found at 90, 75, 60, 45 and 25% r.h. on the lower but not on the astomatous, upper surface of Aegopodium podagraria. The correlation between gS and λ for stomata‐bearing leaf surfaces indicates that an equilibrium exists between the ambient water vapour phase and the liquid water phase on and within the cuticle. This is modified by transpired water vapour influencing the air humidity inside the boundary layer. Our results imply re‐condensation of transpired water vapour to salts on the leaf surface and its sorption to the cuticle.