FUSION AND FISSION OF PLASMA-MEMBRANE MATERIAL ACCOMMODATES FOR OSMOTICALLY INDUCED CHANGES IN THE SURFACE-AREA OF GUARD-CELL PROTOPLASTS

Stomatal movement requires large and repetitive changes in cell volume and consequently changes in surface area. The patch-clamp technique w as used to monitor changes in plasma-membrane surface area of individu al guard-cell protoplasts (GCPs) by measuring membrane capacitance (C- m), a parameter proportional to the surface area. The membrane capacit ance increased under hypoosmotic conditions and decreased after hypert onic treatment. As the specific capacitance remained constant, this de monstrates that osmotically induced changes in surface area are associ ated with incorporation and removal of membrane material. Osmotically induced fusion and fission of plasma-membrane material was not affecte d by removal of extracellular Ca2+ Dialysing protoplasts with very low (<2 nM) or high (1 mu M) Ca2+ had no effect on changes in C-m under h ypo- and hyperosmotic conditions. However, the rate of change in surfa ce area was dependent on the size of the difference in osmotic potenti al applied. The larger the osmotic difference and thus changes in memb rane tension caused by water influx or efflux, the faster the change i n C-m. The results therefore demonstrate that osmotically induced fusi on and fission of plasma-membrane material in GCPs are Ca2+-independen t and modulated by membrane tension.