Effect of Photoperiod under Low Temperature on the Growth and Bolting of Japanese Bunching Onion (Allium fistulosum L.)

The effect of 8, 16, and 24 hr photoperiods for 30, 45, and 60 days at 5 °C on the growth and bolting of Japanese bunching onion (Allium fistulosum L. cv. Kincho and Choetsu) was investigated.In both cultivars, 24 hr photoperiod treatment increased plant height and leaf number. Sixteen hr photoperiod increased plant height and leaf sheath diameter only 'Choetsu', but had no effect on the growth of 'Kincho'.In the 30 - day treatment, 16% of treated plants bolted under 24 hr, 54% under 16 hr, and 77% under 8 hr in 'Kincho'. When the treatments were longer than 30 days, the percentage of bolting plants increased in the 16 - hr and 24 - hr photoperiods. Thus, in the 60- day treatment, no differences in percentage of bolting were observed among 3 photoperiods. The inhibitory effect of long-day also decreased in 'Choetsu' as the treatment was prolonged; long-day photoperiod delayed bolting and increased the number of leaves until seedstalk formation in both cultivars.It was considered that flower initiation of Japanese bunching onion was accelerated by a short-day photoperiod, but the acceleration of flower initiation by short day was reduced when it was sufficiently exposed to low temperature. Therefore, we conclude that Japanese bunching onion requires short-day facultatively in the flower initiation process and the inhibition on the flower initiation by a long photoperiod can be overcome by low temperature.