Abstract
To prevent bolting and to develop a new cropping system for harvest in June, the effects of tunnel covering and a 16-hr photoperiod (LD) on the growth, flower initiation and bolting of Japanese bunching onion (Allium fistulosum L.) cultivars 'Kincho', 'Asagi-kujo' and 'Cho-etsu' were investigated. The maximum air temperature was 20 - 25°C higher under the tunnel than in the open field. Growth during the treatments was accelerated by tunnel covering and LD, solely or combined. With 'Cho-etsu', tunnel covering alone effectively prevented bolting, because devernalization was induced by a high day temperature, but with 'Kincho' and'Asagi-kujo', it did not; but when it was combined with LD, the bolting rate was decreased to 7-24%. The inhibitory effect of LD alone on the bolting rate was not observed for 'Kincho' and 'Asagi-kujo'. However, with 'Cho-etsu', LD alone sufficiently prevented bolting; when combined, LD and tunnel covering prevented bolting completely. These results indicate that the 16-hr photoperiod induced devernalization of Japanese bunching onion. The possibility of harvesting some mid-season varieties, such as 'Kincho' and 'Asagi-kujo' in June was successfully established. As the bolting rate did not correspond to the developmental stage of flower buds during the treatments in 'Asagi-kujo', abortion of developing flower buds may be induced by high day temperature.
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