The regulation of apoplastic calcium in relation to intracellular signalling in stomatal guard cells

It is now well established that calcium has a substantive role in intracellular signalling in stomatal guard cells. Much less attention has, however, been paid to the importance of maintaining an apoplastic calcium concentration which permits this mechanism to operate. Data are presented for the calcicole Leontodon hispidus, in which the xylem sap may contain over 16 mM free calcium, showing that the concentration of calcium in the guard cell apoplast may be as low as 0.10 mM. This appears to be achieved by the deposition of considerable amounts of calcium oxalate in the mesophyll, particularly in the palisade tissue. Of the calcium delivered directly into the epidermis by the transpiration stream, a large proportion is diverted into the trichomes.

It seems unlikely that stomatal regulation over long distances (e.g. from root to shoot) is accomplished via apoplastic calcium, but over shorter distances (the span of a few cells in the epidermis) signalling via calcium in the apoplast is entirely feasible and it is worthy of further investigation.