Plant water loss is tightly balanced with water uptake to maintain beneficial water status. The most important control on water transport is the change of stomatal aperture, which governs water diffusion from the leaf interior to the atmosphere, as well as the opposite flow of carbon dioxide (CO₂) into the photosynthesizing mesophyll. To balance transpiration and photosynthesis, guard cells may sense and integrate many environmental as well as physiological signals related to photosynthesis, the transpirational demand of the atmosphere, and the plant's current hydraulic status (Buckley, 2005; Roelfsema and Hedrich, 2005). Atmospheric humidity is one of the key environmental signals that stomata need to sense to adjust water loss. The sensing mechanisms involved in the stomatal humidity response are nonetheless still not identified.