Measurements of electrical leaf surface conductance reveal recondensation of transpired water vapour on leaf surfaces

Electrical conductance (lambda) was measured continuously and in vivo on le af surfaces of Vicia faba and Aegopodium podagraria. lambda 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 treatm ent with NaNO3 aerosols, a sudden increase in lambda was observed at 73% r. h., which is close to the deliquescence point of the salt. Transpiration a nd electrical conductance of untreated leaves were measured simultaneously under conditions of constant r. h., while the photosynthetic photon flux de nsity and CO2 concentration of the air were varied to induce changes of sto matal aperture. At 35% r. h., changes of light and CO2 level revealed a str ong correlation between stomatal conductance (g(S)) and lambda for Vicia fa ba 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 cor relation between gS and lambda for stomata-bearing leaf surfaces indicates that an equilibrium exists between the ambient water vapour phase and the l iquid water phase on and within the cuticle. This is modified by transpired water vapour influencing the air humidity inside the boundary layer. Our r esults imply recondensation of transpired water vapour to salts on the leaf surface and its sorption to the cuticle.