Development of late-bolting F1 hybrids of Chinese cabbage (Brassica rapa L.) allowing early spring cultivation without heating

We developed new F1 hybrids of Chinese cabbage (Brassica rapa L. ssp. pekinensis) that allow cultivation earlier in spring without heating by introducing extremely late-bolting alleles at two homologs of the flowering repressor Flowering Locus C (BrFLC2 and BrFLC3) from non-heading ‘Leafy Green Parental Line No. 2’. These new F1 hybrids were produced by the following four steps. First, the extremely late bolting selected lines were developed. These selected lines headed in spring after overwintering cultivation, whereas the conventional F1 cultivars flowered. Secondly, an investigation of the three plantings showed that our F1 hybrids formed heads when seeds were sown from mid-February to early March, whereas the conventional F1 cultivar did not form heads because of premature bolting. Thirdly, we identified some F1 hybrids with extremely late bolting during early spring cultivation in an investigation of many F1 hybrids. Finally, based on an investigation across four cold regions for 2 years, we compared the commercialization rate, defined as the proportion of plants greater than 2000 g in weight and with a flowering stalk less than 10 cm long. Then we identified a F1 of MS02 × 12-04 which had a high commercialization rate on average (92%), whereas the rates of three conventional F1 cultivars were only 0–2%. In the near future, these F1 hybrids will be valuable late-bolting cultivars despite climate change, permitting stable cultivation and harvest over wide regions.