Relationships between xanthoxin, phototropism, and elongation growth in the sunflower seedling Helianthus annuus L.

For phototropic curvature of a green sunflower seedling, only the hypocotyl has to be illuminated; the tip and cotyledons are not involved in stimulus perception. The etiolated seedling is phototropically insensitive, illumination of only the hypocotyl renders it sensitive. It is concluded that the photoreceptor is located within the responding organ. In curving seedlings, the endogenous indoleacetic acid (IAA) remains evenly distributed. However, the inhibitor, xanthoxin (Xa), accumulates on the illuminated side. The degree of phototropic response is generally related to the concentration of Xa. The amount of phototropic curvature is independent of the rate of elongation growth, the former can be changed without affecting the latter, and vice versa. The data conflict with the Cholodny-Went theory, whereas they support the hypothesis of Blaauw that the phototropic reaction is caused by the local accumulation of a growth-inhibiting substance on the irradiated side.