Geotropic Response of Wheat Coleoptiles in Absence of Amyloplast Starch

Young coleoptiles of wheat (Triticum durum var. Henry), depleted of amyloplast starch by incubation at 30°C with gibberellin plus kinetin, retained their geotropic responsiveness. Depleted coleoptiles curved upward more slowly than controls, but this was commensurate with their slower growth. The ratio of curvature to growth was about 50° per mm of elongation in both cases. Newly excised coleoptiles, though containing much more starch than incubated controls, curved only about 25° per mm. The tissue treated in gibberellin plus kinetin appeared to contain no starch when examined (a) freshly squashed, (b) as fixed material sectioned thin and stained by the PAS procedure, and (c) as electron micrographs. Shrunken, starch-free amyloplasts could be identified in certain regions, but these did not show evidence of asymmetric distribution under the influence of gravity. The possibilities that other organelles function as statoliths are considered, and it is concluded not only that georeception is independent of starch grains, but further that it may not be due to statoliths at all in an ordinary sense.