Relationships between Motor Cell Ultrastructure and Leaf Movements in <i>Samanea saman</i>

Abstract

Abstract We examined the fine structure of motor cells in the secondary pulvinus of Samanea saman (Jacq.) Merrill, to aid in analyzing the cellular basis for K + and Cl − driven, turgor regulated circadian leaflet movements. Pulvini that were (a) open (horizontal) in the light, (b) closed (vertical) in the dark, or (c) at an intermediate angle after 96‐h incubation in H 2 O in darkness, were cut into cross sections, sub‐divided into quadrants, and prepared for electron microscopy by standard methods. The walls of many cells are ridged, appearing scalloped in cross section, the plasmalemma following the wall contours. Plasmodesmata localized in pit fields are most numerous in the inner cortex of the extensor (abaxial), and least numerous in the outer cortex of the flexor (adaxial) (3.9 and 0.7, respectively, per μm 2 wall area). Vacuole size, number of vacuoles per cell, and the condition of the electron dense precipitate within the vacuole also vary with cell location, multivacuolation being most pronounced in the outer cortex of the extensor. Chloroplasts are dimorphic: those in cells close to the vascular tissue are very large, circular in cross section, and contain huge starch deposits at all times, while those in the remainder of the cortex are smaller, usually oblong, and contain large starch deposits at the beginning of the dark period, but are devoid of starch at the beginning of the light period. However, both types of chloroplasts in excised pulvini incubated in H 2 O in darkness for 96 h still contain starch deposits, indicating that (1) light may promote starch degradation, or (2) starch degradation and resynthesis may be rhythmic.