Effects of turgor potentials of epidermal cells neighbouring guard cells on stomatal opening in detached leaf epidermis and intact leaflets of Vicia faba L. (faba bean)

Leaflets of Vicia faba L. (faba bean) were used to determine whether the mechanical forces resulting from the turgor potentials (Φ p ) of the larger epidermal cells neighbouring guard cells play a significant role in regulating stomatal aperture. When Φ p , of epidermis and Φ p of bulk leaflet tissue were compared at midday, Φ p of epidermis were only 15–25% those of bulk leaflet tissue at all but the most negative leaflet water potentials (Φ). When plants were bagged to increase Φ by reducing vapour pressure differences between leaflets and air, Φ p of bulk leaflet tissue increased to predawn values, but Φ p , of epidermis increased to only = 20% of predawn values and stomata opened to their widest apertures. Stomatal apertures were positively correlated with Φ p of bulk leaflet tissue but they were not correlated with Φ p of epidermis. Reductions in epidermal Φ p , began predawn, before stomata were open, and reached minimum values at midday, when stomata were open. We conclude that, in Vicia faba , (1) reduction of Φ p of epidermal cells begins predawn, reducing the counterforce to stomatal opening that would exist if full epidermal turgor were maintained throughout the day, and (2) changes in Φ p , of leaf epidermal cells do not play a significant role in regulating stomatal aperture.