Epinasty, Hyponasty, and Related Topics

Abstract

A nastic response is a growth curvature, where the direction of flexure is determined by the morphology of the organ and is not directly related to an external reference, although the response may be induced by an environmental stimulus (Ball 1969; see Sect. 4). A common, readily recognizable form of nastic curvature is epinasty, where in a lateral organ relative growth of the upper or adaxial side increases so that it becomes convex (De Vries 1872). In many dicotyledonous species, the growing leaf petiole has a well-developed epinastic capability (Crocker et al. 1932) (Fig. 1 A), while the entire leaf shows an epinastic response in some monocotyledons, for example narcissus, tulip, and hyacinth (Hitchcock et al. (1932). Epinasty is also used to describe curvature in an orthotropic organ such as a stem or pedicel, where the side that initiates the curvature by an acceleration in elongation growth becomes uppermost and the tip of the organ is inclined to form an angle with the vertical. The formation of the terminal hook in the etiolated seedling shoot and in the flower pedicel during its elongation phase are good examples (Kaldewey 1962, Kohji et al. 1979, 1981). The complimentary response, hyponasty, where the abaxial side grows most strongly, often occurs in leaf petioles as a sequel to epinasty, when it serves to straighten and restore the petiole’s original shape and orientation (Fig. 1C).