Breaking bud dormancy in Erythronium japonicum Decne. (Liliaceae) by natural and artificial chilling

This study was conducted to investigate flower bud differentiation and to determine the chilling requirements for bud dormancy breaking and sprouting in Erythronium japonicum Decne. (Liliaceae). Dormant bulbs grown in an open field were randomly selected from June 2012 to December 2013 for observation of flower bud differentiation and development. The 3–4 year-old bulbs and > 10 year-old bulbs were used for vegetative bud development and flower bud development, respectively. For natural chilling treatments, the plants were transferred from an experimental field to a growth module every month from September to February. Artificial chilling treatments were applied at 5°C for 0, 2, 4, 8, and 12 weeks in October and November. The cumulative chill unit (CCU) of low temperatures below 5°C was calculated. Flower bud initiation and differentiation in E. japonicum were observed in May, and from June to July, respectively, followed by flower bud elongation in August. Sprouting and bud break did not occur throughout the experiment when dormant bulbs were transferred from September (0 CCU) to November (450 CCU). However, all plants sprouted when the dormant bulbs were transferred after January (1,794 CCU). Prolonged cold storage also promoted sprouting of dormant E. japonicum. No cold storage or 4 weeks (672 CCU) of cold storage at 5°C in October did not induce sprouting and bud break. However, when the dormant bulbs were stored at 5°C for 12 weeks (2,061 CCU) in October, percent sprouting increased to 72.7%. Chilling treatment for 8 weeks (1,483 CCU) or 12 weeks (2,155 CCU) at 5°C in November resulted in 72.7 and 100% sprouting, respectively. In conclusion, at least 1,483–1,794 CCU may be recommended for dormancy breaking in the forcing culture of E. japonicum.