Heteroblasty and the Evolution of Flowering Phenologies

Flowering phenologies have rarely been examined from a developmental perspective. The production of a flower or inflorescence, however, is an integral part of the ontogeny and architecture of an organism. As a result, basic features of plant developmental morphology have the potential to define options for variation in phenology and to determine, in part, the consequences of this variation for other life-history traits. Two interrelated models of morphological evolution are applied to the analysis of variation in phenology. These models identify the developmental processes of dissociation and addition/deletion as potentially powerful mechanisms of change in the timing of flower production. As a consequence of heteroblasty and architectural features of plant shoots, however, changes in flowering time that occur via dissociation and addition/deletion result inevitably in altered vegetative morphology. If these vegetative characters influence fitness, then response to selection for earlier or later flowering is potentially constrained. A review of the available literature shows that dissociation and addition/deletion do occur in association with changes in flowering phenology. Additional studies that integrate developmental and evolutionary approaches are required to determine the extent to which whole-plant or shoot-level developmental and morphological properties may enhance or constrain phenological evolution.