Vernalization, Dormancy, and the Annualization of Onion (Allium cepa) for Breeding

The biennial life cycle of onion (Allium cepa) provides a significant challenge for plant breeders. The lengthy vernalization period, coupled with bulb dormancy in some varieties, mandates a two year generation time from seed to seed; roughly six months of which is spent holding mature bulbs in cold storage. There is remarkably little publicly available information on the critical vernalization period for long-day storage onions, and even less on the nature of onion bulb endodormancy. The goal of this research is two-fold: 1) to characterize the requirements for vernalization, dormancy, and floral induction in cultivated onion, and 2) to develop a reliable protocol for breeders that will allow for onion seed to be produced on an annual cycle in a winter greenhouse while still offering a bulb evaluation. A time course experiment conducted over two years with F1 hybrid onion bulbs (Cortland & Sherman, Bejo Seeds) held at 10°C was used to determine the minimum vernalization time required for floral competency. Bulbs were removed from cold storage at two week intervals, planted in a greenhouse at 15°C, and monitored for sprouting, scape emergence, and flowering. The date of each developmental change was recorded for individual bulbs. A great deal of bulb to bulb variability was observed in the critical vernalization period for floral competency. Additionally, a 9 week endodormancy period was observed, which may have an impact on when vernalization is initiated. The optimum cold treatment to satisfy the vernalization requirement and overcome dormancy for Cortland and Sherman bulbs was found to be between 14 and 16 weeks at 10°C. The average times to scape emergence in the greenhouse for these vernalization lengths were 85 and 74 days post-chilling, respectively. In the 2013-2014 season, field grown onion bulbs flowered and produced viable seed in a greenhouse within approximately 13 months. This seed was planted for the 2014 field season and is the first instance of approximating an annual cycle for onions in our breeding program. The results from this experiment have established a baseline for the critical vernalization period in long-day storage onions, and will prove to be valuable in creating an annualization protocol for breeders. Furthermore, this work has laid the foundation for future experiments with RNA sequencing of the doubled haploid line CUDH 2107 to identify candidate genes responsible for vernalization, dormancy, and floral development.