Floral induction in woody angiosperms

The long juvenile period of trees is a severe impediment to study of reproductive biology and genetic inheritance, and is a serious constraint for traditional breeding programs. Very little effort has been directed toward the development of practical methods for inducing early flowering in most woody angiosperms, particularly members of the genus Populus. This review is intended to stimulate interest in, and provide direction for, future research in this area. An emphasis is placed on techniques that can be applied easily and inexpensively. Inductive treatments discussed include: phytohormones (gibberellins, cytokinins, abscisic acid, ethylene and auxin); growth retardants (paclobutrazol, uniconazole, daminozide, chlormequat and cimecthacarb); physical constraints (girdling, root restriction and shoot training); cultural conditions (photoperiod, mineral nutrition, moisture stress and temperature); and grafting. Recent reports on stimulation of flowering by genetic transformation with floral homeotic genes are also described. Several research avenues which appear promising are proposed for near-term study.