Regulation of flowering in Aster (Aster oricoides L.) by supplemental lighting

Photoperiodic control of aster growth and development with emphasis on the shift from vegetative to reproductive stage was investigated. After one month of establishment, aster became competent to respond to the inductive photoperiodic signal as shown by the occurence of bolting, a flowering syndrome generally exhibited by plants with rosette growth habit. A 15 hr daylength repeated for 19 cycles was inductive to bolting. Increasing daylength from 12 to 16 hr decreased the number of days to bolting and was found associated with a rise in the level of gibberellin-like substances as measured by dwarf rice bioassay, likewise, the number of days to appearance of visible flower buds and first flower opening were reduced. Advanced flowering was a consequence of early bolting suggesting that concomitant with stem elongation, aster meridian cells were evoked into a florally determined state. At the microscopic level, cell elongation at the sub-apical region became evident 4 days after the commencement of long day treatment. Vegetative apex of plants exposed to long days followed by short days (less than 13 hours) was then transformed to intermediate, transition, prefloral, and finally to inflorescence apex with flower primordia initiated about 1 1/2 months from the onset of cell elongation, indicating that bolting and flowering are two independent developmental events although the former appeared to be a necessary prelude to the latter. Moreover, flower initiation in plants exposed to short days after bolting occurred a month earlier than those exposed to continuous long days. Taken together, these indicate that aster has a dual-daylength requirement. It is a long-short day plant, specifically quantitative long day for bolting and short day for flower initiation. In terms of horticultural application, 15 hr daylength was about 32.5% and 2.5 times higher, respectively, in yield and earnings compared to gibberellic acid treatment which is the newly adopted practice of local growers in regulating flowering in aster.