Where does all the calcium go? Evidence of an important regulatory role for trichomes in two calcicoles

In previous studies of the calcicoles Centaurea scabiosa and Leontodon hispidus, the stomata on isolated epidermis closed partially when the concentration of calcium in the medium was above 1 mol m(-3). This is a much smaller concentration than that believed to be delivered into the leaves in xylem sap when the plants are growing in a calcium-rich medium, and hence the mechanism for 'protecting' stomata from excessive exposure to free calcium is thought to be of great physiological significance. It is shown here that, in the leaves of both species, a substantial amount of the calcium they contain is located within mesophyll cells, and virtually all. of that which does enter the epidermis is contained within trichomes, probably as calcium oxalate. The amounts of calcium in the vicinity of the stomata thus remain small despite high concentrations elsewhere, ensuring that the essential role of Ca2+ in intracellular signalling in guard cells can continue to be performed without disturbance.