The phototropic pulvinus of bean Phaseolus vulgaris L. functional features

The laminar pulvinus of bean (Phaseolus vulgaris L.) is a highly specialized organ for reorienting the lamina, and exhibits positive phototropic curvature. Structural and ultrastructural features of the pulvinus were studied to determine their possible role in its phototropic response. The vascular tissue forms a flexible, relatively inextensible central core enclosed by a starch sheath and surrounded by a multi-layered motor tissue. Phototropic curvature is a result of opposite anisotropic changes in volume of the cells in the exposed and opposite sectors of the motor tissue. Radial inflexibility of the epidermis and axial plasticity constrain these changes to the pulvinar axis. Anisotropic changes in volume of motor cells reduce the osmotic work involved. Motor cells exhibit features that are associated with high synthetic activity: thick cytoplasm with numerous ribosomes, polysomes, RER and SER, well-developed mitochondria and a large nucleus. Numerous, well-developed chloroplasts, with little starch, are increasingly abundant toward the periphery. The intercellular system is limited and partially filled with matrix. Stomata are absent and the motor tissue lacks vascularization. These features support the suggestion that the primary role of the chloroplasts in the photonastic response is photophosphorylation and photosynthetic electron transport (Koller et al., 1995 ([13])).