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

Abstract

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.

Phytochrome-Mediated Phototropism in De-Etiolated Seedlings

“…6) is consistent with results obtained in other dicotyledonous seedlings irradiated with B light or WL (16,17,28,31) and with the hypothesis that it is the R:FR gradient established across the hypocotyl that elicits the bending response. This observation is very important with respect to the ecological significance of stem phototropism.…”

Turning heads: The biology of solar tracking in sunflower

“…Notably, hypocotyls of de-etiolated seedlings bend toward direct illumination by blue light, but the same phototropic response is not observed with red light [33]. Bending of etiolated seedling hypocotyls is similarly dependent on light quality [9]. Notably, the precision of heliotropic leaf and floral movements is maintained with fidelity under blue light illumination but not under red light illumination in many species [e.g., 22,27,34], and exposure of one mature leaf to weak blue light results in the bending of the stem in juvenile sunflower plants [35].…”

“…A role for non-foliar structures in phototropism or heliotropism is not unprecedented. Reception for the light stimulus driving floral heliotropism in snow buttercups (Ranunculus adoneus) occurs in the peduncles subtending moving flowers [8], and phototropic movements by de-etiolated sunflower seedling hypocotyls occur even when the shoot tip and cotyledons are covered with foil or removed [9].…”

“…The difference in auxin content between illuminated and shaded sides of sunflower seedlings has not been replicated despite repeated efforts and use of direct measures of auxin molecule content [9,30,31]. More recent evidence from sunflower and several other species supports an alternative model in which phototropic stem curvature is caused by the generation of auxin inhibitors in illuminated tissue [40], and a series of elegant studies has specifically implicated the auxin inhibitor 8-epixanthatin in sunflower.…”

“…1). Although phototropic bending can be elicited in the hypocotyls of young sunflower seedlings [9,10], heliotropic movement of the shoot apex does not begin until later developmental stages [10,11; B. Blackman, S. Harmer, unpublished data], indicating fundamental differences exist that distinguish these two processes. Notably, and contrary to conventional wisdom, solar tracking of sunflower inflorescences slows to a halt by anthesis, and then the mature blooms maintain an easterly orientation until senescence [10,12].…”