Light-grown sunflower seedlings contain 10−7−3 x 10−7 M indoleacetic acid (IAA). The even distribution of this endogenous IAA in straight hypocotyls does not change during phototropic curvature. The diffusate from hypocotyls contains substance(s) influencing the elongation rate of Avena coleoptile segments but hardly any IAA. Phototropic curvature of the hypocotyl requires the presence of illuminated cotyledons. Illumination of cotyledons inhibits hypocotyl growth. It is concluded that the phototropic response of the sunflower hypocotyl is regulated by factors promoting and inhibiting cell elongation other than IAA.
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Bruinsma et al. (1975) studied this question.