2018 ASHS Annual Conference

Vernalization, the process in which exposure to cold temperatures over an extended period expedites or induces floral initiation, is an important feature for cultivating and producing seed from biennial root vegetables. Onion (Allium cepa L.) is biennial and is widely understood to require vernalization to induce flowering. Additionally, long day onion bulbs undergo a period of endodormancy which begins prior to harvest and lasts for several weeks, depending upon the genotype. Understanding the appropriate duration of vernalization and techniques for overcoming bulb dormancy are important for onion breeding and seed production. Endodormant bulbs of two long-day cultivars, ‘Cortland’ and ‘Sherman’ were treated with hydrogen peroxide solutions at various concentrations over a three-year period and were monitored for root and leaf (sprout) development. We found a two to four-hour exogenous treatment of 20% (weight by volume) hydrogen peroxide to be highly effective at initiating uniform root growth in endodormant bulbs. When compared to a purified water control, the 20% treatment resulted in a 61.3% average reduction in the time to rooting in 2016. We also observed improved uniformity in rooting time between ‘Cortland’ and ‘Sherman’ in all three years of this work. A series of time course experiments were conducted over four years with F₁ hybrid and doubled haploid onion bulbs stored at 10°C for varying lengths of time. For all genotypes, a strong negative relationship was observed for sprouting, scape emergence, flowering, and time from sprouting to scape emergence as storage lengths at 10°C increase. Additionally, the bulb-to-bulb variation in the number of days to conversion showed significant decreases between 10 and 14 weeks of cold storage. We found the optimum chilling time at 10°C for the genotypes studied to be 14 weeks. However, on a percentage basis, we observed scape emergence and flowering across all cold storage treatment groups, including 90% of the bulbs in treatments which were not exposed to vernalizing temperatures. We propose a combination of precise vernalization with hydrogen peroxide for breaking bulb dormancy as a novel method for breeders, researchers, and seed producers seeking rapid, uniform flowering and seed production in onion in the span of a single calendar year.