Cell wall water shields stomata against falling leaf airspace humidity.

Plants lose water by transpiration through stomatal pores. However, it remains a matter of debate whether relative humidity (RH) in the substomatal cavity may fall below saturation and guard cells experience strong differences water potential driven by RH in the cavity. We developed a gas exchange chamber to control RH and CO2 at the inner epidermal surface. Vicia faba L. stomata remained open with high stomatal conductance (gs), even when RH inside was reduced substantially below saturation. Concurrent measurements showed no resolvable decline in bulk cell wall water potential, even with 50 %RH inside, provided the wall space was hydrated. Only when the tissue was allowed to dry did the wall water potential fall below -2 MPa, the stomata close, and gs collapse to values near zero. These findings concurred with OnGuard model predictions showing large decreases in RH in the leaf under water stress. The observations highlight a steady-state flux from liquid in the cell wall to vapour in the substomatal cavity and across the stomatal pore; they implicate cell wall water in shielding the stomata against leaf airspace humidity; and they pose a challenge to consider the kinetics of evaporative flux behind stomatal transpiration.