Hormonal Relations in the Phototropic Response: III. The Movement of C-labeled and Endogenous Indoleacetic Acid in Phototropically Stimulated Zea Coleoptiles.

The effect of phototropic stimulation of Zea coleoptile tips on the distribution of both endogenous indoleacetic acid (IAA) and applied C(14)-labeled IAA was determined. The tips rested on bisected agar blocks. More IAA was found in the blocks under the shaded side of the coleoptile tips than those under the irradiated side. However, no significant difference was observed between the total amounts of IAA, endogenous or labeled, in the irradiated and shaded sides of the experimental system. In addition, less endogenous auxin was found in the shaded tissues than in their irradiated counterparts. It is suggested that phototropism following unilateral irradiation with first positive radiant densities might be a consequence of lateral inequalities in the ability of the irradiated and shaded tissues to transport auxin basipetally.