On phototropic Curvatures in Avena, caused by photochemical Inactivation of Auxin-a via its Lactone

1. The method of Skoog (1937) of determining the activity of auxin preparations on deseeded and decapitated coleoptiles of Avena is not only accurate and sensitive but gives also very constant results. 2. Van Overbeek’s statement (1936, a, b, c) that the growth substance curvatures are affected by illumination if auxin-a is applied has been confirmed. With /8-indole acetic acid no influence of light was stated. The decrease of the curvature with auxin-a by light must be ascribed to the photo-inactivation of the auxin-a-lactone. 3. In the plant auxin-a apparently can be transformed into its lactone. 4. Deseeded and decapitated coleoptiles show phototropic curvatures (base response) if auxin-a is applied as growth substance, and not with /?-indole acetic acid. 5. The base response in decapitated coleoptiles is to be ascribed only to the partial photo-inactivation of the auxin-a; no evidence of a “redistribution” (lateral transport) of auxin-a in the base of the coleoptile, as postulated by the Cholodny-Went theory, could be obtained. 6. Since this photo-inactivation in our experiments did not occur in vitro a sensibiliser must be present in the reacting cells. In the literature evidence is given that carotinoids may act as sensibiliser. The authors wish to tender their sincere thanks to Prof. Kogl, who supplied them with the hardly accessible auxin preparations. Botanical Laboratory, Utrecht, December, 1937.