Ethylene, gibberellins, auxin and the apical control of branch angle in a conifer, Cupressus arizonica.

Decapitation, gibberellin A3, high light, their combination, and certain levels of indole-3-acetic acid increase ethylene evolution and also induce branch hyponasty (upturning) in seedlings of Cupressus arizonica Greene, the increase in ethylene preceding obvious hyponasty. Exogenous ethylene also causes branch hyponasty and branches of seedlings maintained in an atmosphere scavenged of ethylene by mercuric perchlorate grow downwards. It is concluded that ethylene may play a role in the apical control of branch angle in some conifers. The positive effect of ethylene in increasing branch hyponasty may be direct, or reflect changes in levels of endogenous auxin and/or gibberellin.