A POSTULATED ROLE FOR CALCIUM-OXALATE IN THE REGULATION OF CALCIUM-IONS IN THE VICINITY OF STOMATAL GUARD-CELLS

Calcium ions are known to play an important part in signal transductio n in stomatal guard cells. In Commelina communis L., stomatal opening in isolated epidermis is strongly inhibited if the calcium concentrati on in the incubation medium is 0.1 mol m(-3) or greater. It can be ass umed that in the intact leaf, the apoplastic concentration of free cal cium in the vicinity of the guard cells must be kept below this level if interference with stomatal functioning is to be avoided. When C. co mmunis was grown with 15 mol m(-3) calcium in the rhizosphere, the con centration of free calcium in the xylem sap in the shoot was found to be 3.76 mol m(-3). A mechanism is clearly needed for reducing this con centration as the sap traverses the apoplast between the xylem and the stomatal guard cells. Evidence is presented here that the deposition of calcium oxalate in cells of the leaf achieves the necessary regulat ion. The protective role of the six specialized subsidiary cells in th is species appears to be specially important. It is suggested that the regulation of apoplastic free calcium may take an important contribut ion to the effective stomatal control of gas exchange.