Cycles of floral and vegetative development in Metrosideros excelsa (Myrtaceae)

The signals involved in floral induction and the time frame from induction to flowering are not known in the tree Metrosideros excelsa. To understand this course of events and the role played by environment, bud development and shoot growth in 12-15-yr-old adult trees were monitored over 2 yr. Microscopy of the developing buds was conducted concurrently. Typically, apical portions of elongating vegetative shoots abscised in spring. The resulting pairs of distal axillary buds overwintered, forming inflorescences or vegetative shoots the following spring. By midwinter, these buds were committed to either the floral or vegetative state, as evidenced by the development of three-flowered cymules in the axils of the scales in the floral buds or leaves in the apical region of vegetative buds. By early spring, all the floral organs had differentiated in the flowers of each cymule. Floral development occurred only in buds borne on vegetative shoots; the terminal buds in maturing inflorescences either aborted or remained vegetative. Larger buds tended to be borne in the better-lit parts of the tree canopy and to result from the relatively early timing of bud break and shoot abscission the previous spring-summer. In general, larger buds were more likely to be floral and to bear more cymules in each inflorescence. However, some of the largest buds borne on particularly vigorous shoots were vegetative, possibly because of the early development of leaves in the resting bud before the onset of inductive shortening days in autumn. Patterns of flowering varied between trees, with the extreme being alternate reproductive behavior in which almost the entire canopy of buds was vegetative and floral in successive years.