Annualization of the Long Day Onion Breeding Cycle through Threshold Vernalization and Dormancy Disruption

Abstract

Background: Genetic gain is a function of several parameters including generation time. Onion is a biennial crop that typically requires two full calendar years per generation or breeding cycle. Annualization of onion breeding could double the rate of genetic gain but is technically difficult because of bulb vernalization and dormancy requirements.Methods: Based on recent results that demonstrate vernalization thresholds for long-day onion, we conducted an experiment to assess whether shorter vernalization times could result in faster, uniform flowering with dormancy-broken onion bulbs. Bulbs from eight varieties were subjected to a 15% hydrogen peroxide treatment for 4 h in order to break dormancy. This treatment was followed by either 6 or 12 weeks of vernalization after the first physical signs of dormancy release. The vernalized plants were transferred to 16-h daylengths in a 20 °C greenhouse for flowering. Results:The treatments imposed in this study resulted in the completion of a full generation in less than 12 months for all eight varieties examined. As the length of vernalization at 10 °C increased, we observed greater uniformity in floral initiation and a higher proportion of scape emergence. Plants that were vernalized for 6 weeks flowered 24% earlier than those in the 12-week treatment group. However, we observed greater uniformity in scape emergence both within and among varieties when vernalized for 12 weeks. Conclusions:We have developed a framework for an annual cycle breeding system that offers the ability to phenotype bulb traits. We have also identified aspects of the system that will benefit from further refinements. Overall, this annualization system may be useful for advancing generations in onion breeding and reducing the time required to produce new varieties.

Recent Advances in Onion Genetic Improvement

“…Bulbs are then placed in nursery pots and planted in a 20 • C greenhouse under 16-h daylength to promote root formation, and then transferred after three weeks to a 10 • C temperature-controlled growth chamber with fluorescent lighting under 16 h of daylength with a photon flux density of 10.3 µmol m −2 s −1 . After the appropriate vernalization period (from 12-14 weeks depending on genotype), bulbs can be transferred back to the 20 • C greenhouse for flowering and seed production [3].…”

“…This method will require additional fine-tuning before it can be adopted on a large scale. A short period of chilling may not be adequate for promoting flowering in some long-day onion germplasm; however, for many of the commercial germplasm sources investigated, shorter vernalization times coupled with dormancy breaking treatments can significantly shorten the time for onion breeders to move from seed to seed in their breeding nurseries [3]. Bulb annualization will likely not double the rate of genetic gain in long-day onion, but it may reduce the time from initial crossing to varietal release by several years.…”

“…Flowering can be initiated in the first year; however, bulb selections cannot be made [2]. Research has been done in the long-day onion to shorten the generation time to one year [3]. To achieve a generation time of one year,…”