Effect of photoperiod on flower bud development of ornamental sunflowers (Helianthus annuus L.)

The effect of photoperiod on newer bud development in four ornamental sunfl owers in Japan was investigated in an effort to shorten the growing period and to improve commercial cultivation techniques. The number of days from sowing to flower bud development for all cultivars was shortened in the short-day treatment; the same treatment shortened the period between flower bud differentiation to anthesis (FDF) for 'Big Smile' and 'Sunrich Orange', whereas, a long-day treatment hastened FDF for 'Taiy o'. A combination of short/long- day, or day - neutral treatments advanced FDF in 'Valentine'. There were no significant differences in plant height a mong the cultivars after the onset of flower bud differentiation. The long- day treatment yielded taller plants compared to the other treatments. Ther e was no difference in the flower diameter between the photoperiodic treatm ents for all cultivars except 'Big Smile'. These results indicate that some cultivars require different day-lengths, depending on their growth stage, i.e., from sowing to flower bud initiation and from flower bud differentiat ion and development to anthesis. Bence, we recommend that the proper photop eriodic treatment for each growth stage be established for each cultiver.