STUDIES ON THE MECHANISM OF GROWTH INHIBITING EFFECT OF LIGHT

Abstract

A substance which inhibits indoleacetic acid (IAA)-and naphthaleneacetic acid (NAA)-induced elongation of Avena coleoptile section and IAA-induced Avena coleoptile curvature was found in an ether soluble neutral fraction of water extract of sunflower leaves and in agar blocks containing the diffusate from young sunflower leaves. This substance also inhibits the growth of isolated sunflower epicotyl. The Rf value (0.9) of the substance on a paper chromatogram developed with ammoniacal iso-propanol indicates that it is identical with the inhibitor reported by AUDUS et al. (1956), but not with inhibitor-β. The inhibitor can be transported from leaf to stem, and the transport seems to be accelerated by illuminating the leaf. The auxin diffused from sunflower leaf into agar block may be identical with IAA. A substance, which has the same properties as the inhibitor from sunflower leaf, was obtained in crystalline form from the leaf of Jerusalem artichoke. The mechanism of growth inhibition caused by this crystalline substance seems to involve inactivation of a sulfhydryl group. The reason why the stem growth of sunflower seedlings is reduced by strong light is discussed: the amount of the inhibitor transported from leaf to stem is increased under strong light, and in the stem, growth inhibition is caused by a direct effect of this inhibitor on growth and by its inhibiting effect on the transport of IAA from leaf to stem.