D. L. R. De Silva

The Stomatal Physiology of Calcicoles in Relation to Calcium Delivered in the Xylem Sap

Physiological mechanisms in calcicoles in simulated highly calcareous habitats have been investigated using Campanula glomerata, Centaurea scabiosa and Leontodon hispidus. Diffusion resistance of the leaves was unaffected by high concentrations (15 mol m-3 ) of rhizospheric calcium in all three species, and in C. scabiosa and L. hispidus there was no inhibition of leaf extension even at 20 mol m-3. Free calcium concentrations in samples of xylem sap taken from the roots were found to be very close to those in the rhizosphere. However, stomata on isolated epidermis of C. scabiosa and L. hispidus closed in response to elevated free calcium in the same manner as those of Commelina communis, a calcium-neutral plant. It is concluded that the calcicoles must possess an efficient mechanism to remove high concentrations of free calcium delivered into the leafs apoplast by the transpiration stream. If the xylem sap reached the apoplast around the stomata containing even 5-10% of its free calcium, stomatal function would be disturbed. If these species are representative of calcicoles in general, the leafs mechanism for preventing excess calcium from reaching the stomatal guard cells may be indispensable. The capacity to remove or sequester most of the calcium delivered in the xylem may be a key factor in determining whether a plant is a calcicole or not.

Abscisic Acid, Calcium Ions and Stomatal Function

One of the most clearly defined roles for abscisic acid (ABA) is the induction of stomatal closure when water supply is suboptimal (Mansfield & Davies, 1981). Ca2+ is also known to induce stomatal closure but its role as a regulator of guard cell turgor has received little attention. As Elliot and coworkers (1983) reported that calmodulin (CaM) antagonists influence growth regulator action it seemed logical to investigate whether Ca2+ and ABA interacted in influencing stomatal aperture.