Onion is grown in more than 160 countries (FAO, 2014) and is ranked consistently as the third highest valued vegetable crop in the United States (U.S. Department of Agriculture, 2016). Long-day storage cultivars are used extensively on every continent (Serra, 2002) and are the predominant onion market class in the northern United States (Davie, 2017; Lucier, 2011). These cultivars maintain a high level of quality while in storage for extended periods and are well adapted to the shorter growing seasons of northern climates.
Vernalization, the process during which exposure to cold temperatures over an extended period expedites or induces floral initiation (Chouard, 1960), is an important feature for breeding and producing seed from biennial root vegetables. Onion is biennial and is widely understood to require vernalization to induce flowering. Premature flowering during crop growth, also known as bolting, greatly reduces the quality of storage roots and bulbs, and can render an entire crop unsaleable if present on a large scale. In well-adapted cultivars, the minimum length of vernalization required for flowering, or the critical vernalization period, is long enough that a brief period of vernalizing temperatures does not result in bolting. Thus, vernalization serves a protective role against bolting during bulb production. However, seed producers and plant breeders also wish to understand and manipulate vernalization to achieve consistent, uniform flowering each year.
The path to floral initiation in onion is complicated. Extensive work on the molecular basis of vernalization and flowering in Arabidopsis thaliana, sugar beet (Beta vulgaris), and temperate grasses such as oats (Avena sativa), wheat (Triticum aestivum), rye (Secale cereale), and barley (Hordeum vulgare) have demonstrated that the mechanisms for vernalization and flowering are not conserved across orders (Bouché et al., 2017). Discovery of the FLOWERING LOCUS T (FT)-like gene AcFT2, with an upregulation that correlates highly with vernalization-induced flowering, provided evidence that onion is dissimilar from other well-characterized monocot flowering pathways (Bouché et al., 2017; Lee et al., 2013).
Much of the published research on onion vernalization has focused on identifying conditions under which vernalization occurs. Temperatures ranging from 2 to 17 °C can result in vernalization, with an optimum between 7 and 12 °C (Wiebe, 1990). Shishido and Saito (1977) and Brewster (1983) have proposed the minimum size in which bulbs and seedlings are responsive to chilling. These studies have played an important role in defining the conditions for vernalization in onion, particularly as seedlings or sets, but do not describe the length of vernalization required to achieve uniform flowering in long-day bulbs.
Vernalization can be accomplished during both the seedling and bulb stages. Efforts to vernalize onion seedlings began as early as 1945 with both the intent of controlling bolting and accelerating flowering for seed production (Heath, 1945). Seedling vernalization involves exposing young plants to vernalizing temperatures before bulbing to induce flowering. Early attempts at vernalizing seedlings were met with mixed results until the discovery that seedlings must pass a juvenile phase before they are receptive to chilling (Holdsworth and Heath, 1950). The juvenile phase is cultivar dependent and can range from the 4- to 14-leaf stage, after which the plants are vernalized for ≈60 d (Khokhar, 2014; Rabinowitch, 1985; Shishido and Saito, 1976). Although seedling vernalization is a useful tactic for producing seed rapidly, and it has applications in marker-assisted selection and backcrossing, there are limitations for its use in phenotypic selection of bulb traits because this procedure does not allow for evaluation of the mature bulb. In addition, such a procedure may favor bolting-susceptible genotypes, which may not be preferred in breeding programs.
A meta-analysis using vernalization experiments published between 1963 and 1983 was used to create a vernalization model for onion (Streck, 2003). This model confirmed the previously published optimum vernalization temperature of 10 °C and reported that vernalization is complete after 60 d of chilling. Although this analysis made use of data from Japanese, American, and Brazilian onion cultivars, only the Brazilian cultivars were evaluated as bulbs; the Japanese and American cultivars were vernalized as seedlings. Vernalization requirements for seedlings cannot necessarily be translated to bulbs, in part because of the presence of bulb endodormancy in many onion cultivars. Bulb endodormancy is observed at harvest and can last from weeks to months, depending upon the genotype (Carter et al., 1999). Because onion seedlings are vernalized while they are actively growing, the effects of dormancy are not factored into seedling vernalization models. Despite some information on optimal vernalization conditions and endodormancy, the temporal aspects of vernalization and flowering in long-day onion bulbs have not been investigated.
Through this study, we sought to quantify the temporal aspects of vernalization in long-day storage onions and to gain an understanding of the relationship between vernalization and flowering. A series of time course experiments were conducted over 4 years with F1 hybrid and doubled haploid onion bulbs stored at 10 °C for varying lengths of time. We characterized vernalization and dormancy in long-day storage onion bulbs for cultivars that were representative of the material grown in Wisconsin.
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