Rapid movements of plants organs require solute-water cotransporters or contractile proteins

Plant organs such as leaves or petals move as a result of changes in the sh ape and/or the volume of motor cells. In a similar manner, stomates open an d close when the turgor in the guard cells changes. The time scale of such movements ran-es from several milliseconds to hours. The most detailed stud ies of cellular movement have been done with stomata. Stomatal opening requ ires the uptake of K+ by guard cell, and the presence of K+ channels in the ir plasma membranes and tonoplasts that mediate this K+ flux (Fischer, 1968 ). If water follows K+ flux by osmosis, then the rate of flux would determi ne the rate of volume change (Schroeder et al., 1984). This explanation is frequently used as a paradigm for other nyctinastic and seismonastic moveme nts (Moran et al., 1990; Fleurat-Lessard et al., 1997; Moshelion and Moran, 2000). For example, in Mimosa pudica, the observed potassium exchanges and the nature of the anatomical structures of pulvinule and pulvinus have led to suggestions that movements are the result of modifications in the volum e of motor cells at the base of each leaflet and leaf by a "lever" effect. However, none of the movements in plants is really understood at the molecu lar level. Here, we discuss the constraints brought to the paradigm of move ment by considering the values of cell membrane transport parameters and th e characteristic time of the movement.