Seasonal changes in flowering order and form in the inflorescence of a delphinium-like spray-type chrysanthemum (Chrysanthemum morifolium Ramat.)

Some chrysanthemum strains show delphinium-like flowering, with reverse flowering order in the inflorescence; these strains flower from the bottom of the inflorescence towards the terminal flower bud. In order to establish a stable cultivation method for these strains, we investigated seasonal changes in flowering order and form in the inflorescence of these strains under short-day conditions throughout the year. When the short-day treatment started in late spring to early sunmer, the florets within an inflorescence flowered normally (i.e., basipetally, starting from the terminal flower). When the short-day treatment started in summer to winter, they flowered in reverse order (i.e., acropetally, starting from the bottom flower). When the short-day treatment started in late summer to winter, apical buds formed a rosette and did not flower for a while. After rosette formation, stem elongation began again and flowering was either normal or reversed, and was extraordinarily delayed (until the next spring). The appearance of reverse flowering order and rosette formation was unstable and varied among strains. When the short-day treatment started in early spring, flowering once more occurred in reversed order. When the short-day treatment started in mid-spring, they flowered normally again. Delphinium-like flowering was observed twice a year, in autumn and spring. Delphinium-like flowering appears to be closely related to rosette formation and to be caused by an intermediate physiological condition between normal flowering and rosette formation.