Bolting Property of Spring-harvested Kujo Bunching Onions

Abstract

Bunching onion (Allium fistulosum L.) is a green vernalization-type plant. Cold treatment of plants that reach the mature phase induces flower bud formation, and subsequent high-temperature and long-day conditions lead to bolting. Because bolting reduces the commercial value of bunching onions, plants should be grown large enough to harvest before bolting. We sowed seeds of two spring-harvest cultivars of Kujo bunching onions (‘Kujofuto’ and ‘Haru-Anjo’) and a year-round-harvest cultivar (‘Super-Anjo’) between early August and mid-October and studied the effects of the growth stage during cold exposure in the field. Cold exposure of plants that did not reach the mature phase because of later sowing reduced the rate of bolting and delayed bolting progress of ‘Kujofuto’, but only delayed bolting progress of ‘Super-Anjo. We evaluated the timing of 50% bolting, when commercial value loss becomes prominent, and found that ‘Super-Anjo’ is an earlier bolting cultivar than ‘Kujofuto’. ‘Haru-Anjo’ is a later bolting cultivar; because of its slower bolting progress, most plants can be harvested before bolting, even when bolting has begun, and so there is a reduced risk of losing commercial value by bolting. All three cultivars have different genotypes regarding the speed of flower-bud growth, and each property prominently appears when not reaching the mature phase. Because a later-bolting genotype in ‘Haru-Anjo’ also appears even when not reaching the mature phase, we can select them to further reduce the bolting risk.