Hook formation in sunflower seedlings is directed by both positive gravitropism and a form of circumnutation

The apical hook that forms at the apex of most dicotyledonous seedlings during germination has in the past been variously described either as the consequence of positive gravitropism or as an autonomous process. Recently, however, emphasis has shifted more on the role of positive gravitropism as the inducer of this structure. To address the question on the nature and origin of the hook, experiments were performed on sunflower seedlings, not only under gravitational conditions but also by means of a clinostat, i.e. in the presence of multidirectionally compensated gravitational stimulation. The results of this work show that at least two processes are involved in the formation of a hook: positive gravitropism and an autonomous movement. Consequently, both Darwin, with his conviction that an innate process such as circumnutation was involved, and the supporters of positive gravitropism, were partly correct. Moreover, it was found that, in contrast to the hook formed under normal unidirectional gravitational conditions, the one formed on the clinostat showed a preferential plane for its development: the plane that passes parallel to the closed cotyledons. This characteristic also suggests that a form of circumnutation may be part of the basic mechanism for hook formation.