PHOTOTROPIC RESPONSES OF THE PULVINULES AND ASSOCIATED LAMINAR REORIENTATION IN THE TRIFOLIATE LEAF OF BEAN PHASEOLUS-VULGARIS (FABACEAE)

Movements of the terminal and lateral leaflets in the trifoliate leaf of pinto bean (Phaseolus vulgaris L.) cv. Bulgarian, in response to directional light, were studied by kinetic analysis. The pulvinule responds to unilateral light by contraction of the sector that intercepts the light and concomitant expansion of the opposite sector. The rate of the resulting laminar reorientation depends on the excitation of the photo-receptors in the pulvinule, which changes with angle of the incident light and as pulvinar topology changes in response to light. This dependence provides feed-back control of the eventual steady-state of the response. The pulvinule perceives directional light from all sides, but only as unilateral signals, and integrates these signals in its response. Phototropic responses of the pulvinule are modified by its asymmetry, which differs in the terminal and lateral leaflets. The asymmetry is expressed by axial torsion, superimposed on the positive phototropic curvature. The resulting laminar reorientations reflect this asymmetry. Laminar reorientation in directional light is a passive result of the pulvinar response and may combine changes in azimuth and axial rotation with changes in elevation.