Plant cells regulate their cytoplasmic calcium concentration ([Ca]cyt) to lower than 1.0 µM, despite a steep gradient (probably around 1000 x; Meir, Juniper and Evans, 1991) favouring Ca2+ influx into the cell (reviewed by Evans, Briars and Williams, 1991). This low concentration is maintained by the action of active calcium transport systems, located at the plasma membrane (e.g., Butcher and Evans, 1987a, b; Rasi-Caldogno, Pugliarello, DeMichelis, 1987; Gräf and Weiler, 1989), endoplasmic reticulum (e.g., Bush and Sze, 1986; Brauer, Schubert and Tsu, 1990; Bush, Biswas and Jones, 1989) and tonoplast (e.g., Schumaker and Sze, 1985, Blumwald and Poole, 1986; and see also Evans, 1988 and Evans et al., 1991, Briskin, 1991 for reviews). Plant active calcium transporters fall into two categories; P-type ion translocating ATPases and calcium-proton antiporters; it has been suggested that the former are located at the plasma membrane and endoplasmic reticulum, while the latter is located at the tonoplast, although alternative locations have also been suggested (reviewed by Evans et al. 1991.).