Manipulation of Apical Dominance in Sorghum with Growth Regulators¹

Abstract

The hormonal control of apical dominance (inhibition of lateral bud formation and development) in sorghum [ Sorghum bicolor (L.) Moench] was studied by applying plant growth regulators to two cultivars—SM100 (weak apical dominance) and BT×378 (strong apical dominance). In the field, SM100 produced an average of 1 tiller per 2 plants before anthesis and BT×378 produced none. Following anthesis, apical dominance diminished in both cultivars. Spray applications of 0.2 m M gibberellic acid (GA 3 ) during the initial 7 weeks of seedling growth completely inhibited bud outgrowth before anthesis in SM100. Bud outgrowth increased rapidly in both cultivars after termination of GA 3 applications. This rapid increase in bud outgrowth was similar to the normal release from apical dominance ocurrlng in untreated plants following anthesis except that it was earlier, occurred at a more rapid rate, and a larger final number of buds was released. Thus, two aspects of the normal pattern of tiller bud development were mimicked by GA 3 application: i) intensification of the inhibition of bud outgrowth before anthesis and ii) acceleration of bud outgrowth occurring after anthesis. In plants grown hydroponically in the greenhouse, GA 3 again inhibited bud outgrowth in SM100 plants during the vegetative period. A role for gibberellin(s) in sorghum apical dominance is suggested because bud outgrowth was promoted in BT×378 plants by a gibberellin synthesis inhibitor, ancymidol. Evidence for the participation of auxins and cytokinins in sorghum apical dominance was also obtained. The auxin naphthaleneacetic acid inhibited bud outgrowth in SM100. In BT×378 the auxin transport inhibitor naphthylphthalamic acid and the cytokinin benzyladenine caused bud outgrowth which resembled that obtained by apical bud removal.