The positive hook: the role of gravity in the formation and opening of the apical hook.

Photographic observations on germinating seedlings of Lepidium sativum L., Cucumis sativus L., and Helianthus annuus L. showed that the hypocotyl hook is not present in the seed but forms during the early stages of growth. Evidence that gravity plays a major role in inducing curvature of the hypocotyl, and in maintaining the hook once it has been formed, was obtained from clinostat experiments, from the use of morphactin to remove geotropic sensitivity and from inversion of seedlings to change the direction of the geostimulus. In L. sativum and H. annuus gravity perception seemed to be the only mechanism responsible for hook formation. In C. sativus hook formation was additionally aided by the mode of emergence of the cotyledons from the seed coat but gravity played an indirect role in regulating such emergence. Further evidence that hook formation is linked to a georesponse was derived from a comparison of hypocotyl development in wild-type Arabidopsis thaliana seedlings with that of an ageotropic mutant, hook formation being found to occur only in the wild type. Hook formation and maintenance is discussed in terms of contrasting geosensitivity between the apical and basal ends of the hypocotyl and it is suggested that light-induced hook opening is a reversal to a condition of uniformly negative georesponse throughout the hypocotyl.