Manipulation of Apical Dominance in Sorghum with Growth Regulators 1

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 mM gibberellic acid (GA3) 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 GA3 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 GA3 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, GA3 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.