Ian R. Macdonald

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.

The role of the apex in normal and tropic growth of sunflower hypocotyls.

Regional growth in vertical and horizontal etiolated sunflower hypocotyls from which the apical hook tissue had been either partly or wholly excised, was measured 24 h later, the regions having been demarcated with resin beads. Removal of the cotyledons (an excision which included the distal end of the shoot apex) had little effect on growth during this period but excision of the apical hook significantly reduced growth. In vertically orientated seedlings, removal of half of the hook severely reduced growth in all other growing regions and removal of the entire hook totally inhibited growth. This inhibition of growth was not a consequence of the removal of the region of growth but a consequence of the removal of a region on which growth was dependent. In horizontal seedlings, the situation was more complex inasmuch as a horizontal orientation itself induced growth in previously non-growing regions. This new growth was localised in its extent and was not as severely affected by progressive excision of the hook as was growth in vertical seedlings. The results are discussed in terms of overall growth co-ordination in the hypocotyl.