Rapeseed, a major oil crop in the world, is easily affected by low-temperature stress. A low temperature delays seed germination and increases seedling mortality, adversely affecting rapeseed growth and production. In the present study, a tolerant cultivar (Huyou21) was crossed with a susceptible genotype (3429) to develop a mapping population consisting of 574 F₂ progenies and elucidate the genetic mechanisms of seed germination under low temperatures. Two quantitative trait loci (QTL) for low-temperature germination (LTG) were detected, one on chromosome A09 (named qLTGA9-1) and the other on chromosome C01 (named qLTGC1-1), using the QTL-seq approach and confirmed via linkage analysis in the mapping population. Further, qLTGA9-1 was mapped to a 341.86 kb interval between the SSR markers Nys9A212 and Nys9A215. In this region, 69 genes including six specific genes with moderate or high effect function variants were identified based on the Ningyou7 genome sequence. Meanwhile, qLTGC1-1 was mapped onto a 1.31 Mb interval between SSR markers Nys1C96 and Nys1C117. In this region, 133 genes including five specific genes with moderate effect function variants were identified. These specific genes within the two QTL could be used for further studies on cold tolerance and as targets in rapeseed breeding programs.

Seed density per silique (SDPS) and valid silique length (VSL) are two important yield-influencing traits in rapeseed. SDPS has a direct or indirect effect on rapeseed yield through its effect on seed per silique. In this study, a quantitative trait locus (QTL) for SDPS was detected on chromosome A09 using the QTL-seq approach and confirmed via linkage analysis in the mapping population obtained from 4263 × 3001 cross. Furthermore, one major QTL for SDPS (qSD.A9-1) was mapped to a 401.8 kb genomic interval between SSR markers Nys9A190 and Nys9A531. In the same genomic region, a QTL (qSL.A9) linked to VSL was also detected. The phenotypic variation of qSD.A9-1 and qSL.A9 was 53.1% and 47.6%, respectively. Results of the additive and dominant effects demonstrated that the expression of genes controlling SDPS and VSL were derived from a different parent in this population. Subsequently, we identified 56 genes that included 45 specific genes with exonic (splicing) variants. Further analysis identified specific genes containing mutations that may be related to seed density as well as silique length. These genes could be used for further studies to understand the details of these traits of rapeseed.

Direct seeding of winter rapeseed in the Yangtze River Basin often coincides with low temperatures during establishment. The aim of this study was to test whether low-temperature (LT) germination performance predicts overwintering survival and yield under delayed sowing. Thirty accessions were evaluated in controlled germination at 20/14 °C (CK) and 12/6 °C (LT) and in two field seasons (2020–2021 and 2021–2022), with six sowing dates from 15 October to 4 December. Mean germination rate was 97.6% in CK and 88.0% in LT. Germination potential (GP) averaged 95.7% in CK and 41.9% in LT. Root and shoot length decreased from 7.63 and 5.02 cm in CK to 1.47 and 0.48 cm in LT. Overwintering survival declined with later sowing. In the colder season (2020–2021), survival for sowings after November fell below 20% for most accessions, whereas S1–S2 averaged above 80%. Yield decreased with delay. In 2021–2022, yield under S1 exceeded S2–S6 by 5.5%, 8.5%, 13.9%, 14.0%, and 23.3%. In 2020–2021, S1 was similar to S2, but 6.3–22.8% higher than S3–S6. Thousand-seed weight followed the same trend. LT GP and LT root length were positively correlated with yield at several sowing dates in the colder season, indicating that LT germination traits are predictive of late-sown performance under harsher winters. Seven accessions (3409, M417, Zheza0903, 86155, 3445, Zheyou50, and 3462) showed superior LT germination and comparatively better field performance. For the lower Yangtze site, a practical latest safe sowing window is late October, based on two seasons; November sowing substantially increases winter mortality and yield risk. Selecting genotypes with strong LT germination and managing for rapid autumn establishment can stabilize 1000-seed weight and yield when sowing is delayed.

Rapeseed is a major oil crop in the world, which is easily affected by low-temperature stress. Low-temperature delays seed germination and increases seedling mortality that adversely affects rapeseed growth and production. To understand the genetic mechanisms of seed germination under low-temperature in rapeseed, we crossed a tolerant cultivar with a susceptible genotype to develop a mapping population of 574 F₂ progenies. Two quantitative trait loci (QTLs) for low-temperature germination (LTG) were detected on chromosome A09 (named qLTGA9-1) and C01 (named qLTGC1-1) using QTL-seq method, and confirmed via linkage analysis in the mapping population. qLTGA9-1 was mapped to a 341.86-kb interval between SSR markers Nys9A212 and Nys9A215. In this region, 69 genes including six specific genes with moderate or high effect function variant were identified based on Ningyou7 genome sequence. qLTGC1-1 was mapped a 1.31-Mb interval between SSR markers Nys1C96 and Nys1C117. In this region, 133 genes including five specific genes with moderate effect function variant were identified based on Ningyou7 genome sequence. These specific genes within the two QTLs could be targets in rapeseed breeding programs and further studies for cold tolerance.

To develop innovative, high-value utilization models for green manure rapeseed, this study investigated the effects of planting density (PD) and nitrogen (N) application rate on the yield and quality of rapeseed flowering stalks harvested at different plant heights. A factorial experimental design was employed, incorporating three factors, PD, N application, and harvesting height, each at three levels. Morphological traits, including stem diameter, fresh weight per plant, flowering stalk yield, and fresh biomass yield after stalk harvesting, were evaluated, together with biochemical traits comprising cellulose, soluble protein, soluble sugar, and vitamin C concentrations in the flowering stalks. The results showed that stem diameter, individual plant fresh weight, and flowering stalk yield generally declined with increasing PD and delayed harvesting height, whereas responses to N application varied depending on PD level. Flowering stalk yield ranged from low to high values across treatments, with the highest yields consistently recorded at a PD of 202,000 plants ha⁻¹ (PD2) across all N levels and at 303,000 plants ha⁻¹ (PD3) combined with 17.25 kg N ha⁻¹ (N1). Variations in PD and N application significantly influenced both yield performance and biochemical composition of the flowering stalks. Elevated concentrations of soluble protein, soluble sugar, and vitamin C were observed at a harvesting height of 25 cm under PD3N1 and at 30 cm under PD2N1 and PD2N2, whereas lower cellulose concentrations were detected at 30 cm under PD2N1 and PD2N2. Overall, these findings demonstrate that coordinated optimization of planting density, nitrogen input, and harvesting height can effectively balance yield and quality traits, providing a practical dual-purpose production strategy for rapeseed cultivated as both a vegetable and green manure in the Shanghai region.