Abstract
Exogenous application of indoleacetic acid results in a significant suppression of the linear growth that is promoted by exogenous gibberellic acid in Avena stem segments in a fashion similar to that previously noted in Avena leaf base segments (van Overbeek and Dowding, 1961, Fourth International Conference Plant Growth Regulation). Treatment with the auxin transport inhibitors, methyl-2-chloro-9-hydroxyfluorene-(9)-carboxylate (CFM) or 2,3,5-triiodobenzoic acid (TIBA), alone promotes elongation growth of the stem segments over that of control growth. This effect is interpreted as being due to the interference in the transport of native indoleacetic acid by CFM and TIBA, thus removing the inhibitory effect of native indoleacetic acid on gibberellin-promoted growth in the internodal intercalary meristem. This results in a greater promotion of internodal growth by native gibberellins. In the presence of (2-chloroethyl) trimethylammonium chloride (CCC), the growth-promoting effects of CFM and TIBA are decreased, and the antiauxin, PCIB (4-chloro-phenoxyisobutyric acid), has no growth-promoting effects whatsoever. These results indicate that the CFM and TIBA-promoted growth require the continuous presence of gibberellins. They further support the view that native indoleacetic acid acts as a growth suppressor hormone in its regulation of gibberellin-promoted internodal extension in Avena shoots.