Involvement of Gibberellins in the Regulation of Tillering in Welsh Onion (Allium fistulosum L.)

We investigated the effects of applied gibberellin A(3) (GA(3)) and uniconazole P (UCP), a GA biosynthesis inhibitor, on tittering in Welsh onion, also known as Japanese bunching onion 'Hangzhou', a cultivar with very high tittering capacity. The number of tillers was increased by GA(3) treatment and reduced by UCP treatment. The tillering-inhibitory effect of UCP was counteracted by GA(3) treatment. GAs were considered to be involved in not axillary bud outgrowth but its initiation for the following results: UCP treatment raised the leaf position with the first tiller, and the tillering-promoting effect of GA(3) treatment became apparent about 8 weeks after the treatment, which mostly corresponded to the period during which an axillary bud initiated near the shoot apex develops to become visible. To clarify the relationship between the diversity of tittering capacity in Welsh onion cultivars and GAs, we investigated the levels of endogenous GAs and the responsiveness to GA(3) treatment in several cultivars possessing different tittering capacities. There was a negative correlation between the tittering capacity and the level of GA(4), a major bioactive GA in Welsh onion. On the other hand, there was a positive correlation between the tittering capacity and the responsiveness to GA(3) treatment; GA(3) treatment markedly promoted tillering in 'Hangzhou', but not in 'Raitei-Shimonita' and 'Hanemidori-Ipponbuto', cultivars with very low tittering capacity. These results suggest that the sensitivity to GAs is a factor causing the diversity of tittering capacity in Welsh onion.