Bulb division and morphological variations in onions (Allium cepa L.) in response to exogenous gibberellin treatments

• Inner bulb division is less in short-day than in intermediate- and long-day onions. • Exogenous gibberellic acid-3 (GA 3 ) treatment increases internal bulb division. • Exogenous GA 3 treatment promotes outer bulb division in intermediate-day cultivar 'Momiji No 3′. • GA 3 treatment results in spindle-shaped bulbs, likely by affecting cell elongation. • Assessment of endogenous gibberellin levels may reveal the underlying mechanisms. Bulb division is a key factor impacting the external quality and marketability of onion ( Allium cepa L.) crops. However, their underlying physiological mechanisms remain largely unknown. In this study, we aimed to elucidate the factors that contribute to bulb division using two experiments. In the first experiment, 11 onion cultivars commonly grown in Japan were cultivated under autumn-sowing (over-winter) conditions to compare the differences in bulb division rates between the cultivars. The results revealed that inner bulb division was significantly lower in short-day cultivars than in intermediate- and long-day cultivars. In the second experiment, exogenous gibberellic acid-3 (GA₃) was applied to onion seedlings to assess its effect on bulb division. GA₃ treatment markedly increased the incidence of internal bulb division in ‘Sonic’, a cultivar with a naturally low division rate, whereas it also promoted external bulb division in ‘Momiji No 3’, a cultivar with a naturally high internal division rate. Furthermore, GA₃-treated bulbs exhibited a spindle-shaped morphology, suggesting that gibberellin (GA) affects both bulb division and shape through its regulatory role in cell elongation. These findings indicate that GA plays a crucial role in bulb division and shape development in onions. Future research must explore the differences in GA sensitivity among cultivars, endogenous GA biosynthesis during growth, and the developmental dynamics of lateral bud formation to further elucidate the genetic and physiological mechanisms underlying bulb division.