2.2.2. Flowering Time

Flowering is an important agronomic trait of broccoli, as it influences maturity, head quality, hybrid seed production and geographical region adaptation. Flowering time is con-

sidered to be controlled by multiple QTLs. To detect QTLs/genes associated with this trait, segregation populations were generated by crossing broccoli with different germplasms, such as broccoli × cabbage, broccoli × Chinese kale and broccoli × broccoli. Different QTLs were detected in these studies, even using similar populations, such as broccoli × cabbage and broccoli × Chinese kale [21,33,44–47]. Most studies have implied that flowering is largely controlled by one or a few major QTLs [21,33,44–47]. As early as the late 1990s, using populations of broccoli (nonvernalization type) × cabbage (vernalization type), broccoli (late flowering type) × Chinese kale (early flowering type), several QTLs for flowering time were mapped [44–47]. Two subsequent studies used a similar population derived from broccoli × cabbage but obtained different results [48,49], possibly due to the differences of the specific germplasms used and the planting environmental conditions. Okazaki et al. detected six QTLs controlling flowering time (from February to July, 2001, Niigata, Niigata Prefecture, Japan), among which the major QTL in the interval BRMS215–F2-R4b, accounting for 36.8% of the phenotypic variance and *BoFLC2* in the interval is thought to be the candidate control of flowering time [48]. Similarly, using a broccoli × cabbage population, Shu et al. combined QTL-seq and a traditional linkage map to detect flowering time loci (from the spring of 2013 to the winter of 2014, Beijing China). A major QTL *Ef2.1* is located on C02 2.65–2.68 Mb, responsible for early flowering and explaining 51.5% of the phenotypic variation, and a homolog of *GROWTH-REGULATING FACTOR 6* (*BolGRF6*) is a possible candidate [49]. Using DH populations of broccoli × Chinese kale, Stansell et al. (2019) identified two QTLs *DM\_C03@6.4* and *DM\_C09@50.0* for days to maturity, two QTLs *DF\_C03@6.4* and *DF\_C09@50.0* for days to flowering, and the major QTLs *DM\_C09@50.0* and *DF\_C09@50.0* on C09 at approximately 50 Mb, explaining approximately 50% of the phenotypic variation [33].

Broccoli is usually sensitive and not feasible in high-temperature areas/seasons, which are thought to impede vernalization, resulting in defects in floral meristem development. Using tropical accessions in Taiwan, China, Lin et al. 2018 identified nonvernalizationresponsive QTLs that contribute to subtropical adaptation (high ambient temperature and short day length) [21]. The candidate gene *BoFLC3* identified in the major QTL *qDCI-3* may function as an alternative pathway for the control of flowering in temperate and tropical environments [21].
