# Cell wall water shields stomata against falling leaf airspace humidity

## TL;DR

The study shows that cell wall water helps protect stomata from low humidity in the leaf airspace, keeping them open even under dry conditions.

## Contribution

The novel contribution is demonstrating that cell wall water shields stomata against falling leaf airspace humidity, maintaining stomatal conductance.

## Key findings

- Stomata in Vicia faba L. remained open with high conductance even when inner leaf humidity was below saturation.
- Cell wall water potential remained stable until the tissue dried, at which point stomata closed.
- Findings align with OnGuard model predictions about humidity changes under water stress.

## Abstract

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 (g
s), 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 g
s 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.

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 (g
s), 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 g
s 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.