Development of late-bolting F 1 hybrids of Chinese cabbage ( Brassica rapa L.) allowing early spring cultivation without heating

We developed new F 1 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 F 1 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 F 1 cultivars flowered. Secondly, an investigation of the three plantings showed that our F 1 hybrids formed heads when seeds were sown from mid-February to early March, whereas the conventional F 1 cultivar did not form heads because of premature bolting. Thirdly, we identified some F 1 hybrids with extremely late bolting during early spring cultivation in an investigation of many F 1 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 F 1 of MS02 × 12-04 which had a high commercialization rate on average (92%), whereas the rates of three conventional F 1 cultivars were only 0–2%. In the near future, these F 1 hybrids will be valuable late-bolting cultivars despite climate change, permitting stable cultivation and harvest over wide regions.