Lower Bolting Height of Winter Rapeseed (Brassica napus L.) Enhances Cold Stress Tolerance and Adaptability to Arid–Frigid Regions in Northern China

Bolting height is a key genetic trait that affects the stress tolerance, environmental adaptation, and winter survival of Brassica napus winter rapeseed. It is particularly important for enhancing winter survival in the arid–frigid regions. This study aimed to elucidate the genetic relationship between bolting height and cold stress tolerance, thereby supporting breeding for enhanced cold tolerance. Ninety-five winter rapeseed accessions were used in this study. Through both spring and autumn sowing trials, the dynamic changes in bolting height under different environments were systematically analyzed, and the genetic stability of bolting height as well as its correlation with cold tolerance were clarified. Bolting height showed consistent variation trends between spring and autumn sowing trials, exhibiting high genetic stability. It displayed an extremely significant negative correlation with cold tolerance: genotypes with lower bolting height possessed stronger cold tolerance. The regulatory mechanism underlying low bolting and cold tolerance was revealed at cellular and molecular levels. Low bolting accessions exhibited flat and broad shoot apical meristems, with small and compact cells, a high nucleoplasmic ratio, and indistinct vacuoles. The gibberellin synthesis gene BnaA06g24070D was downregulated, while the key cold-tolerant gene BnCBF5 was upregulated. Exogenous hormone treatment preliminarily verified the causal regulatory effect of bolting height on cold tolerance. In both spring and autumn sowing trials, bolting height at the initial flowering stage showed an extremely significant positive correlation with vernalization index, with correlation coefficients of 0.80 and 0.78, respectively. Lower bolting height corresponded to a smaller vernalization index and stronger temperature sensitivity. Moreover, bolting height at the initial flowering stage showed an extremely significant negative correlation with comprehensive cold tolerance scores, with correlation coefficients of −0.77 and −0.80, respectively. Low-bolt materials had significantly higher overwintering rates and comprehensive cold tolerance scores, as well as a markedly lower semi-lethal temperature (LT50), compared with high-bolt accessions. Low-bolt accessions presented significantly prolonged bolting stage, bud stage, initial flowering stage, and whole growth durations, and their agronomic trait stability across years was significantly superior to that of high-bolt accessions. This study confirmed that low bolting height is a crucial breeding trait for the cold tolerance of winter rapeseed, and thus an important selection indicator for the cold tolerance improvement of winter rapeseed in arid–frigid regions in northern China.