EFFECTS OF TURGOR POTENTIALS OF EPIDERMAL-CELLS NEIGHBORING 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 t he mechanical forces resulting from the turgor potentials (Psi(p)) of the larger epidermal cells neighbouring guard cells play a significant role in regulating stomatal aperture, When Psi(p) of epidermis and Ps i(p) of bulk leaflet tissue were compared at midday, Psi(p) of epiderm is were only 15-25% those of bulk leaflet tissue at all but the most n egative leaflet water potentials (Psi), When plants were bagged to inc rease Psi by reducing vapour pressure differences between leaflets and air, Psi(p) of bulk leaflet tissue increased to predawn values, but P si(p) of epidermis increased to only approximate to 20% of predawn val ues and stomata opened to their widest apertures, Stomatal apertures w ere positively correlated with Psi(p) of bulk leaflet tissue but they were not correlated with Psi(p) of epidermis, Reductions in epidermal Psi(p) began predawn, before stomata were open, and reached minimum va lues at midday, when stomata were open, We conclude that, in Vicia fab a, (1) reduction of Psi(p) of epidermal cells begins predawn, reducing the counterforce to stomatal opening that would exist if full epiderm al turgor were maintained throughout the day, and (2) changes in Psi(p ) of leaf epidermal cells do not play a significant role in regulating stomatal aperture.