The Site of Reception of Phototropic Stimulation and the Occurrence of Growth Inhibitors in Radish Seedlings

Abstract

Summary To investigate whether the cotyledons and/or apex of seedlings of radish (Raphanus sativus var. hortensis f. gigantissimus Makino) are necessary for the perception of phototropic stimulus, they were removed immediately before unilateral illumination that induced the curvature. Their excision did not significantly reduce the phototropic curvature of the hypocotyls. The influence of removal of cotyledons and apex on the distribution of the growth inhibitors, cis- and trans-raphanusanins and raphanusamide, between lighted and shaded halves of hypocotyls was investigated using a physicochemical assay. The lateral gradient of the growth inhibitors between the lighted and shaded halves was little influenced by the removal. The gradient was caused by light-induced biosynthesis at the lighted side. These results show that in radish hypocotyls the phototropic stimulus is absorbed directly by the hypocotyl and that the inhibitors that induce the curvature in this plant increase at the lighted side of the hypocotyls.