The effects of intermittent immersion of Avena seedlings in solutions of IAA on the response of the coleoptiles to unilateral illumination in the region of that producing the second positive curvature were studied by means of automatic time-lapse photography which enabled the growth-rate and curvature to be recorded simultaneously. Phototropic induction occurred even after the coleoptiles had absorbed sufficient IAA from a 10-4 M. solution to raise their rate of elongation to about twice the normal value. Phototropic curvature, which had been temporarily inhibited by a curvature in the opposite direction induced by the IAA, became evident as soon as this curvature had ceased to operate. In coleoptiles, supplied with IAA after the commencement of a phototropic curvature, the response was temporarily suppressed. It was resumed as soon as the effects of the exogenous IAA had disappeared. The ability of the coleoptiles to produce a slight phototropic response persisted even when their growth-rate had been greatly reduced by previous removal of the endosperm. Increasing the growth-rate by supplying the starved seedlings with IAA or sucrose, separately or together, failed to increase the response. Decapitation did not prevent phototropic induction, but delayed the onset of the response. Application of IAA by intermittent immersion in a 0.1 mg./l. solution, after the decapitated coleoptiles had been exposed to unilateral illumination, increased the rate of growth but reduced the response. The results suggest that in these experiments phototropic induction was not mediated by any direct action of light on the displacement, inactivation, or rate of synthesis of an endogenous auxin. They are in agreement with the hypothesis that the stimulus caused an asymmetrical distribution of a co-factor of auxin.
Copyright (C) 1962, Society for Experimental Biology
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