Effect of photoperiods before, during and after vernalization on flower initiation and development and its varietal difference in Japanese bunching onion (Allium fistulosumL.)

SummaryTo control the bolting of Japanese bunching onion (Allium fistulosum L.) photoperiodically, the effect of photoperiods before, during and after vernalization on flower initiation and development and the varietal differences were investigated using the two mid-season flowering cvs Kincho and Asagi-kujo, and a late-season flowering cv. Cho-etsu. A long-day photoperiod (LD, 16 h) given before vernalization inhibited flower initiation. Especially, the bolting rate of ‘Asagi-kujo’ decreased by about a half, compared with the short-day photoperiod (SD, 8 h). The interaction between the effect of night temperature (3°C, 7°C, 11°C or 15°C) and the effect of the photoperiod (SD and LD) during vernalization was also investigated. In ‘Kincho’, LD did not affect flower initiation at 3°C, but inhibited flower initiation at 7°C, 11°C and 15°C. In ‘Asagi-kujo’, flower initiation was significantly inhibited by LD under all temperature conditions. This inhibitory effect was stronger at 11°C and 15°C than at 3°C and 7°C. In ‘Cho- etsu’, LD significantly inhibited flower initiation at 3°C and 7°C, and flower initiation rarely occurred at 11°C and 15°C. In this study, generally, LD during vernalization inhibited flower initiation in all cultivars. Thus Japanese bunching onion required a short-day photoperiod in flower initiation, which was stronger in ‘Asagi-kujo’ and ‘Cho-etsu’ than in ‘Kincho’. From these results, we conclude that low temperature and a short-day photoperiod complementarily induce flower initiation in Japanese bunching onion. Varietal differences exist in the requirement of low temperature and a short-day photoperiod: the primary requirement in ‘Kincho’ is low temperature and that in ‘Asagi-kujo’ is a short-day. After flower initiation, the early stage of flower development is day-neutral, and after the floret formation stage, a long-day photoperiod promotes flower development and elongation of the seedstalk.