Self-Compatibility Alleviates Inbreeding Depression and Maintains Yield and Quality in Winter Turnip Rape (Brassica rapa L.)

Winter turnip rape (Brassica rapa L.) is an important winter oilseed crop in the arid and cold regions of northern China due to its strong cold tolerance and wide adaptability. However, its prevalent self-incompatibility readily causes inbreeding depression, leading to declines in yield and quality and creating difficulties for cultivar breeding and pure-line propagation. To clarify how self-compatibility affects growth, yield, quality, and physiological traits, highly self-compatible (HSC), self-compatible (SC), and self-incompatible (SI) lines were compared under self-pollination and open pollination, and leaf antioxidant enzyme activities and endogenous hormone contents were measured. The results showed that the effects of selfing on seedling vegetative growth, agronomic traits, quality, and biomass were far smaller in HSC and SC lines than in SI lines, with inbreeding depression markedly alleviated. The mean inbreeding depression rates (IDRs) of major traits in HSC, SC, and SI lines were 4.32%, 8.14%, and 20.05%, respectively, the IDRs of effective silique number per plant were 3.03%, 5.22%, and 20.53%, and those of seed yield were 4.69%, 9.39%, and 27.90%. Line-level ANOVA and Tukey tests (three lines per group) confirmed that the IDRs of SI lines were significantly higher than those of HSC lines for all six core traits, with SC lines intermediate. Physiological and biochemical analyses indicated that HSC lines maintained higher SOD activity, and the contents of growth-promoting hormones, such as gibberellin (GA), cytokinin (CTK), and auxin (IAA), were generally higher than in SI lines, whereas CAT activity did not show a consistent advantage: in HSC lines CAT did not differ significantly between SP and OP (the two shared a significance group), and under open pollination SI-OP was grouped with HSC-OP and SC-OP. In contrast, after selfing, SI lines accumulated more abscisic acid (ABA), whereas the ethylene (ETH) differences were not consistently significant and the coordination of their antioxidant enzymes was altered rather than disrupted. Correlation analysis showed that the contents of growth-promoting hormones (GA, CTK, IAA, and BR) were significantly or highly significantly positively correlated with yield traits, such as seed yield per plant, effective silique number per plant, and thousand-seed weight, whereas ABA and ETH contents were highly significantly negatively correlated. A structural equation model, based on composite scores of hormones, enzymes, growth, and yield and quality, indicated that the hormone composite was strongly associated with the growth composite (standardized path coefficients of hormone → growth and growth → yield and quality were 0.816 and 0.535, respectively), whereas the enzyme composite was directly associated with yield and quality (0.291). These paths reflect statistical associations among the composite scores rather than confirming the specific underlying mechanisms, and vegetative growth was the main mediator linking the hormone composite to yield and quality. To our knowledge, self-compatibility in winter turnip rape can maintain relatively stable plant growth and development as well as yield and quality by alleviating inbreeding depression and maintaining physiological metabolism and hormone homeostasis. This study provides a theoretical basis for the screening and utilization of self-compatible germplasm and for high-yield breeding of winter turnip rape; because stress tolerance was not directly measured, any extrapolation to stress-resistant breeding requires dedicated stress-tolerance experiments in future work.