When the apical bud of a plant seedling is actively growing, the outgrowth of lateral buds is suppressed. This phenomenon is called "apical dominance". Apical dominance is released by excision of the apical bud or by application of auxin transport inhibitors, such as 1-naphthylphthalamic acid (NPA) and 2,3,4-triiodobenzoic acid (TIBA) to the apical bud or the internode of seedlings. However, direct application of auxin to the lateral buds of decapitated seedlings did not maintain apical dominace. We demonstrated that application of natural auxin-inhibiting substances, raphanusanin B and 6-methoxy-2-benzoxazolinone (MBOA) not only to the apical bud or the internode but also directly to the lateral buds of pea (Pisum sativum L. cv. Alaska) seedlings released apical dominance in either intact or IAA-treated, decapitated seedlings. These results suggest that antagonist(s) of the auxin-inhibiting substances, raphanusanin B or MBOA are synthesized during auxin translocation from the apical buds. We aimed to isolate and identify lateral bud growth inhibitor (s), which transport from the apical bud towards the lateral buds, from pea seedlings. The lateral bud growth inhibitor was isolated from etiolated pea seedlings and identified as indole-3-aldehyde. The indole-3-aldehyde content was significantly higher in the diffusates from explants with apical bud and IAA-treated decapitated explants, in which apical dominance is maintained, than in those from decapitated ones releasing apical dominance. When the indole-3-aldehyde was applied to the cut surface of etiolated decapitated plants or directly to the lateral buds, it inhibited outgrowth of the lateral buds. These results suggest that indole-3-aldehyde plays an important role as a lateral bud growth inhibitor in apical dominance of pea seedlings. On the other hand, flower development of the lateral buds was accelerated in Japanese pear Pyrus pyrifolia (Burm.) Nak. when vertical shoots were bent at a 45℃ angle. This acceleration was associated with notable changes in plant hormone levels. We propose that the bending of pear shoots alters hormone levels, thereby increasing the ability of the lateral buds to compete for assimilates. In the present study, we explore bioactive substance(s) involved in flower development of the lateral buds, and isolated and identified 3,5-di-O-caffeoylquinic acid from the lateral buds of bending-treatment pear shoots.
