Introduction

Among biological systems for studying transport and signalling functions, stomatal guard cells are one of a very few highly successful cell models for higher plants. The dominance of the guard cell in this respect owes much to its unique situation and physiological function, features that have been used to considerable experimental advantage. Unlike the vast majority of cells in higher plants, guard cells lack functional plasmodesmata at maturity, and so are physically isolated from the neighbouring epidermal and mesophyll cells (Wille & Lucas, 1984; Willmer & Fricker, 1996). As a direct consequence, electrophysiological investigation of intact guard cells is possible without the complications of electrical coupling between cells. Thus, despite their comparatively small size, quantitative voltage clamp studies of guard cells have been carried out from several species, notably Nicotiana, Arabidopsis and Vicia. Furthermore, because this cell type can be isolated relatively easily, at least in the limited numbers required for such studies, guard cells have afforded virtually the only opportunity for comparisons of ion channel and other transport characteristics recorded in enzymatically isolated protoplasts and in situ (Lemtiri-Chlieh, 1996).

No less important, access into the network of regulatory processes that control guard cell membrane ion pumps and channels has been possible through the unique physiology of stomata. Guard cells are situated at the end of the transpiration stream within the plant. They surround small pores in the epidermis through which gas exchange for photosynthesis within the leaf mesophyll takes place. It is also through these pores that the bulk of water loss from the plant occurs.