Effect of Some Substituted Benzoic Acids and Related Compounds on the Distribution of Callus Growth

It has been observed in this laboratory (15) that the normally polar distribution of callus growth on tobacco stem explants becomes non-po'ar in the presence of low levels of 2,3,5-triiodobenzoic acid (TIBA), while the amount of callus growth is not diminished. It was thought that this phenomenon might reflect a specific inhibition by TIBA of the polar transport of auxin. Subsequent studies (10, 11, 13) were carried out to test this hypothesis. TIBA was found (13) to inhibit transport of indole-3-acetic acid (IAA) in the classical Went (23) transport test and in tobacco stem segments as determined directly by chemical and bio-assay of IAA previously introduced into the plants and then extracted from the apical and basal portions of the stem segments. Other compounds found to cause a redistribution of callus growth and also to inhibit transport in the Went assay were 2bromo-3,5-dichlorobenzoic acid, -y-phenvlbutyric acid, 2,4-dichlorophenoxy acetic acid (2,4-D), and phenylacetic acid (10). The inhibitory effects on polar IAA transport by TIBA and 2,4-D have been independently confirmed by Hay (4) and by Zwar and Rijven (24). Another instance of apparent inhibition of IAA transport by TIBA has been reported by Kuse (6), who measured axillary bud growth and petiole extension in Ipomoea explants (consisting of a section of stem containing 1 node and its leaf) after 7 days' growth in nutrient solution. Bud growth was inhibited by the intact leaf, or by IAA applied to the cut end of a debladed petiole. TIBA applied midway along the petiole permitted bud growth on explants having either auxin source. The portion of the petiole distal to the TIBA was reported to have grown more than the proximal part. Kuse suggested that both effects resulted from accumulation of auxin above the point of TIBA treatment, as though transport had been prevented.