Phototropic stimulation does not induce unequal distribution of indole‐3‐acetic acid in maize coleoptiles

Distribution of endogenous diffusible auxin into agar blocks from phototropically stimulated maize coleoptile tips was studied using a bioassay and a physicochemical assay, to clarify whether phototropism in maize coleoptiles involves a lateral gradient in the amount of auxin. At 50 min after the onset of phototropic stimulation, when the phototropic response was still developing, direct assay of the blocks with the Avena curvature test showed that the auxin activity in the blocks from the shaded half‐tips was twice that of the lighted side, at both the first and second positive phototropic curvatures. However, physicochemical determination following purification showed that the amount of indole‐3‐acetic acid (IAA) was evenly distributed in the blocks from lighted and shaded coleoptile half‐tips at both the first and second positive phototropic curvatures. The even distribution of the IAA was also confirmed with the Avena curvature test following purification by HPLC. These results indicate that phototropism in maize coleoptiles is not caused by a lateral gradient of IAA itself and thus cannot be described by the Cholodny‐Went theory. Furthermore, the lower auxin activity in the blocks from the lighted half‐tips suggests the presence of inhibitor(s) interfering with the action of auxin and their significant diffusion from unilaterally illuminated coleoptile tips.