Effect of Sowing Date and Low-Temperature Seed Germination on Rapeseed Yield

Abstract

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.

1. Introduction

Rapeseed (Brassica napus L.) is a major oilseed crop, with the largest sown area of 7.8 million hectares and annual production of 16.3 million tons in China. It plays a crucial role in ensuring national edible oil security and farmers’ income, contributing significantly to the agricultural economy [1]. The Chinese Government’s No. 1 Central Document (2021–2025) emphasized expanding rapeseed cultivation on winter fallow fields to enhance oil self-sufficiency and rural revitalization. In the Yangtze River Basin in China, over 4.2 million hectares of winter idle fields can be used for rapeseed cultivation [2].

With the popularization and application of direct seeding technology and an increase in labor cost, the cultivation pattern has gradually shifted from transplanting to direct seeding of rapeseed [3,4]. Due to the late harvest of rice, the sowing time of direct-seeded rapeseed is delayed by about 20 days compared to traditional transplanting [5]. This overlap between rice harvest and rapeseed establishment has created a significant production bottleneck in the Yangtze River Basin, where farmers often face a trade-off between completing rice harvests and ensuring timely rapeseed sowing [6].

Low temperatures during the sowing period are one of the most critical environmental constraints for rapeseed establishment in this region. The temperature at sowing in the Yangtze River Basin is generally below 10 °C, which often hinders germination and seedling growth [3,7,8,9,10]. Late sowing reduces yield by decreasing survival rate, vegetative growth, and biomass accumulation because of delayed reproductive growth and increased exposure to low-temperature stress during the seedling stage [11]. Existing studies show that most current rapeseed varieties have limited tolerance to low temperatures [12,13], highlighting the need for new germplasms with improved cold tolerance and stable yield potential under delayed sowing conditions.

Although many previous investigations have independently examined either low-temperature germination behavior [3,7,8,10] or the influence of sowing date on field yield performance [5,12], very few have integrated both components within a single experimental framework that spans multiple genotypes [14,15]. Understanding how early germination traits under low temperatures translate into field performance can provide practical insights for breeding and management strategies aimed at stabilizing production under late-sowing and cold-stress conditions.

Therefore, this study was designed to bridge that gap by jointly evaluating the low-temperature germination traits and field performances of 30 rapeseed accessions across multiple sowing dates and two growing seasons. Specifically, the objectives were to (i) analyze seed germination characteristics of 30 rapeseed genotypes under optimal (20/14 °C) and low-temperature (12/6 °C) conditions, (ii) evaluate the influence of delayed sowing on overwintering survival and yield across two field seasons, and (iii) examine the relationship between early-stage low-temperature tolerance and subsequent field performance.

We hypothesized that genotypes demonstrating strong germination and seedling vigor under low-temperature laboratory conditions would also maintain higher overwintering survival and yield when sown late in the field. This integrated approach provides both physiological and agronomic perspectives to guide the selection and management of cold-tolerant rapeseed varieties suited for direct-seeding systems in the Yangtze River Basin.

2. Materials and Methods

2.1. Materials

A total of 30 rapeseed (Brassica napus L.) accessions were used in this study (

Table 1. Information for the 30 rapeseed resources.

No.	Name	Origin	No.	Name	Origin	No.	Name	Origin
1	Huyou21	Shanghai	11	Fengyou9	Henan	21	Fengyou10	Henan
2	ZS11	Hubei	12	D4818	Guizhou	22	Zheyou50	Zhejiang
3	3406	Shanghai	13	98033	Jiangshu	23	Qingza4	Qinghai
4	3409	Shanghai	14	Zhong11-P073	Hubei	24	3453	Shanghai
5	Qinyou7	Shanxi	15	3439	Shanghai	25	HF08	Shanghai
6	3417	Shanghai	16	M417	Zhejiang	26	3456	Shanghai
7	3426	Shanghai	17	Zheza0903	Zhejiang	27	3462	Shanghai
8	YH800	Jiangshu	18	86155	Hubei	28	3465	Shanghai
9	P7089	Hubei	19	Huyou039	Shanghai	29	3471	Shanghai
10	Qinyou99	Shanxi	20	3445	Shanghai	30	ShuYJ-3	Jiangshu

). Seeds were provided by the Shanghai Academy of Agricultural Sciences (SAAS, Shanghai, China). Fifty seeds of each accession were selected per replicate, with three replicates for each treatment.

2.2. Seed Germination

The germination experiment followed Zhu et al. [3] and Zhang et al. [16], with modifications. Seeds of all 30 rapeseed accessions were germinated in Petri dishes (diameter 9 cm) lined with two layers of filter paper. Each dish contained 50 seeds and 15 mL of distilled water. The following two temperature treatments were applied using a growth chamber (MGC-450HP, Bluepard Instruments, Shanghai, China):

• Normal temperature (CK): 20 °C/14 °C (11 h/13 h dark/dark).
• Low temperature (LT): 12 °C/6 °C (11 h/13 h dark/dark).

These temperature regimes represent the typical optimal and sub-optimal thermal conditions encountered during autumn sowing in the Yangtze River Basin. Petri dishes were arranged in a completely randomized design within the growth chamber and rotated daily to minimize position effects. Three replicates were arranged in a completely randomized design. Germinated seeds were counted daily after imbibition. A seed was considered germinated when the radicle length exceeded 2 mm.

• Germination potential (GP, %) was calculated as the ratio of the number of germinated seeds on the third day after imbibition to the total number of seeds.
• Germination rate (GR, %) was calculated as the ratio of the number of germinated seeds on the tenth day after imbibition to the total number of seeds.
• The root length (RL) and shoot length (SL) were measured as the mean of 10 randomly selected seedlings per Petri dish after 10 days of incubation.

All germination data were analyzed for significant differences between CK and LT using Student’s t-test (p < 0.05).

2.3. Field Experimental Design and Management

A two-year field experiment was conducted from October 2020 to May 2022 at the Experimental Base of the Shanghai Academy of Agricultural Sciences, Shanghai, China (30°53′ N, 121°22′ E; altitude 5 m). The soil at the site was classified as rice-derived clay loam, and the previous crop was maize. Before sowing, soil samples (0–20 cm depth) were analyzed for basic fertility, showing 1.8% organic matter, 98 mg kg⁻¹ available nitrogen, 21 mg kg⁻¹ available phosphorus, and 97 mg kg⁻¹ available potassium. The field has a pH of 6.7, typical of alluvial soils in the lower Yangtze Basin.

The region has a humid subtropical monsoon climate typical of the Yangtze River Basin. According to records from the Shanghai Meteorological Bureau, total precipitation during the rapeseed growth period (15 October–18 May) was 394.2 mm in 2020–2021 and 350.1 mm in 2021–2022. Mean daily maximum and minimum air temperatures were 31 °C and −8 °C in 2020–2021 and 29 °C and −4 °C in 2021–2022, respectively. Soil temperature at 5 cm depth was monitored throughout the emergence stage using in situ probes, averaging 15.4 °C in mid-October (S1) and declining to 7.8 °C by early December (S6). These seasonal patterns are illustrated in Figure 1, showing daily precipitation and temperature variations throughout both years. Climate data were obtained from the Shanghai Meteorological Bureau to ensure accuracy and consistency with regional meteorological observations.

Thirty rapeseed (Brassica napus L.) accessions were evaluated to determine the effects of sowing date on yield performance. A split-plot experimental design was employed, with sowing date as the main plot factor and accession as the sub-plot factor. Six sowing dates were tested each year: 15 October (S1), 25 October (S2), 4 November (S3), 14 November (S4), 24 November (S5), and 4 December (S6). Each treatment was replicated three times in a completely randomized layout.

Each plot measured 2 m × 1.5 m and contained four rows spaced 44 cm apart. Fifty seeds were sown per row by manual drilling, maintaining a plant spacing of 12 cm (133,000 plants ha⁻¹). Immediately after sowing, furrow irrigation was applied to ensure uniform emergence. Subsequent irrigation was managed using shallow flooding during seedling establishment and supplemental furrow irrigation every 10–15 days before winter to maintain 70–80% field capacity. Irrigation was withheld during cold months and resumed at the stem-elongation stage.

Fertilization: A basal application of 300 kg ha⁻¹ compound fertilizer (N:P₂O₅:K₂O = 20:5:10) was made before sowing, followed by 150 kg ha⁻¹ of the same formulation as top-dressing at the bud stage.

Crop management: Standard agronomic practices, including weeding, irrigation, pest, and disease control, were applied uniformly throughout the experiment.

Phenological observations: Seedling establishment and overwintering survival were assessed on 4 February 2021 and 4 February 2022, corresponding to the onset of spring (“Lichun”). The overwintering survival rate was calculated as the percentage of established seedlings that survived to spring. The flowering period was defined as the number of days from sowing to 50% flowering.

At physiological maturity, all plants from the two inner rows of each plot were harvested manually. Samples were air-dried and threshed, and the following yield traits were determined: above-ground biomass per plant, seed yield per plant, 1000-seed weight, and total seed weight per plot.

Randomization and climate summary: All treatments were randomized each season using a computer-generated layout to avoid systematic bias and edge effects.

Table 2. Climatic characteristics at the experimental site during the two growing seasons (2020–2021 and 2021–2022).

Season (Year)	Total Rainfall (mm)	Mean Air Temp (°C)	Min Air Temp (°C)	Max Air Temp (°C)	Mean Soil Temp at 5 cm (°C) *
2020–2021	394.2	12.4	–8	31	15.4 (October → 7.8 December)
2021–2022	350.1	13.1	–4	29	15.9 (October → 8.1 December)

* Soil temperature recorded at 5 cm depth during emergence.

summarizes the key climatic variables for the two experimental years, showing precipitation, air, and soil temperature ranges during the rapeseed growth period.

Climate data were obtained from the Shanghai Meteorological Bureau and in situ soil probes at the experimental site.

2.4. Statistical Analysis

All recorded data, including seed germination traits, mean yield per plant, and other agronomic variables, were entered and organized in Microsoft Excel 2010. Differences between the CK (20 °C/14 °C) and LT (12 °C/6 °C) treatments for seed germination traits were analyzed using Student’s t-test (p < 0.05).

Because climatic conditions differed significantly between the two experimental years, data were analyzed separately for each year. Analysis of variance (ANOVA) was conducted using SAS 9.2 software to test the main and interactive effects among factors. The least significant difference (LSD) test was applied at the 0.05 probability level to compare means.

A two-way ANOVA (sowing date × accession) and three-way ANOVA (sowing date × accession × year) were performed to evaluate the effects and interactions of experimental factors on yield and related quality traits within and across years. Pearson’s correlation coefficients were computed to examine the relationships between yield and germination parameters at a significance level of 0.05.

Additionally, Welch’s ANOVA was performed using the R software packages agricolae via the CNSknowall web platform (http://cnsknowall.com, accessed on 11 August–6 September in 2025), a comprehensive online tool for statistical analysis and visualization. All graphical plots and figures were generated using the CNSknowall visualization module.

3. Results

3.1. Effects of Temperature Treatments on Rapeseed Germination

There was significant difference in rapeseed germination between the CK and LT treatments, with mean germination rates of 97.6% and 88.0%, respectively (Figure 2). Seed germination potential (GP), root length (RL), and shoot length (SL) of these rapeseed resources were significantly reduced in the LT compared to CK. The mean GP under CK was 95.7%, which dropped to 41.9% in LT (Figure 2). Similarly, the RL and SL of these rapeseed resources was 7.63 cm and 5.02 cm in the CK conditions, respectively, but decreased to 1.47 cm and 0.48 cm in the LT conditions, respectively (Figure 2).

Most materials in this study demonstrated similar germination rates (>90%) within ten days after imbibition in LT and CK conditions (Figure 2 and Figure 3). But the traits of GR, RL, and SL in LT for all materials were lower than that in CK (Figure 3). At LT, only 7 resources, including Zheza0903 (No.17), 86155 (No.18), Zheyou50 (No.22), 3409 (No.4), 3462 (No.27), 3445 (No.20), and M417 (No.16), exhibited excellent tolerance to cold stress, with a mean GP higher than 80%, while the GP under LT for most of the rest resources was lower than 50% (Figure 3). According to these resources in the study, 3409 (No.4) and Zheza0903 (No.17) showed a high development level with the highest length of root and shoot, respectively, under different temperatures treatment (Figure 3). Specifically, the maximum SL in LT condition was observed in Zheza0903 (No. 17); the next was 3409 (No.4). Meanwhile, the maximum RL was observed in 3409 (No.4), followed by Zheza0903 (No. 17) (Figure 3).

To summarize the genotypic variation in cold tolerance observed in Figure 2 and Figure 3, seven accessions, 3409 (No. 4), M417 (No. 16), Zheza0903 (No. 17), 86155 (No. 18), 3445 (No. 20), Zheyou50 (No. 22), and 3462 (No. 27), maintained high germination potential and relatively greater root and shoot growth under low-temperature (LT) conditions (

Table 3. Germination traits of the seven cold-tolerant rapeseed genotypes under normal (CK) and low-temperature (LT) conditions.

Genotype (No.)	GP (%) CK	GP (%) LT	RL (cm) CK	RL (cm) LT	SL (cm) CK	SL (cm) LT	GP Reduction (%)
3409 (4)	96 ± 2	85 ± 3	8.1 ± 0.4	2.5 ± 0.3	5.0 ± 0.2	0.6 ± 0.1	11.5
M417 (16)	97 ± 1	82 ± 4	7.4 ± 0.5	2.1 ± 0.2	4.8 ± 0.3	0.5 ± 0.1	15.5
Zheza0903 (17)	98 ± 1	88 ± 2	7.9 ± 0.3	1.9 ± 0.2	5.5 ± 0.2	0.8 ± 0.1	10.2
86155 (18)	95 ± 2	84 ± 3	7.1 ± 0.3	1.6 ± 0.2	5.0 ± 0.3	0.6 ± 0.1	11.6
3445 (20)	97 ± 1	80 ± 3	7.0 ± 0.4	1.7 ± 0.2	4.9 ± 0.3	0.5 ± 0.1	17.5
Zheyou50 (22)	98 ± 2	83 ± 3	8.0 ± 0.4	1.8 ± 0.2	5.2 ± 0.2	0.6 ± 0.1	15.3
3462 (27)	96 ± 1	81 ± 4	7.6 ± 0.5	1.5 ± 0.2	5.1 ± 0.3	0.5 ± 0.1	15.6

). These genotypes exhibited mean germination potentials above 80% at 12/6 °C, with only 10–18% reductions compared to the control (CK). Root and shoot lengths under LT remained approximately 20–30% of those under CK, indicating better early-stage vigor compared with the other accessions. The overall trends confirm that specific cultivars within the population possess a stronger tolerance to low-temperature stress during the germination and seedling stages.

3.2. Overwintering Survival Rates of Rapeseed in Response to Sowing Dates

Overwintering survival declined, as sowing was delayed in both years (Figure 4). Differences among the October sowing dates were small and mostly not significant, but postponing sowing to November caused a significant drop in survival, especially under extreme weather. In 2020–2021, a cold wave in late December and early January (Figure 1) reduced survival of most accessions sown after November to <20%. Across all accessions relative to S1, survival decreased by 5.8%, 65.7%, 63.8%, 81.6%, and 84.3% for S2–S6 in 2020–2021 and by 0.2%, 1.3%, 3.8%, 7.5%, and 7.9% in 2021–2022, respectively (Figure 4). Based on these two seasons at this lower Yangtze River Basin site, the latest safe sowing window was approximately late October; sowing in November markedly increased winter mortality.

Because only 24, 27, 7, and 2 lines survived under S3, S4, S5, and S6 in 2020–2021, respectively, flowering and yield traits for those sowing dates were only evaluated for the surviving lines and are not directly comparable across all accessions. Consequently, datasets from late-November sowings in 2020 (very low survival) were excluded from some of the comparative analyses described below.

3.3. The Responses of Rapeseed Flowering Stage to Sowing Dates

The duration from sowing to 50% flowering (STF) shortened consistently with delayed sowing (Figure 5). In 2021–2022, STF was shorter than S1 by 5, 13, 18, 22, and 26 days for S2–S6, respectively. In 2020–2021, STF was shorter than S1 by 5, 13, 19, 22, and 28 days for S2–S6, respectively. Across years, STF at the same sowing date was similar (e.g., S1 ≈142 days and S2 ≈137–138 days in both seasons), indicating that sowing time was the primary driver of flowering timing while inter-annual effects were comparatively small (Figure 5).

3.4. Yield per Plant in Response to Sowing Dates

Seed yield declined with delayed sowing in both seasons (Figure 6). In 2021–2022, yield under S1 exceeded S2–S6 by 5.5%, 8.5%, 13.9%, 14.0%, and 23.3%, respectively (p < 0.05). In 2020–2021, yield at S1 was similar to S2 (p > 0.05), but was 6.3–22.8% higher than S3–S6 (p < 0.05). Above-ground biomass followed the same pattern as yield within each year (Figure 6).

Two-way ANOVA indicated significant main effects from sowing date and variety for both traits in both years, with a significant sowing date × variety (SD × V) interaction (Supplementary Table S1). The year × variety interaction was significant for above-ground biomass, and mean yield and biomass were higher in 2021–2022 than in 2020–2021 (Supplementary Table S1). At S1, interactions were not significant for some traits, consistent with reduced stress at the earliest sowing date (Supplementary Table S1).

Variety × sowing highlights. Consistent with Section 3.1, 3409 and Zheza0903 ranked among the top-yielding accessions at S1–S2, whereas 86155 and M417 performed relatively better than the population mean when sowing was moderately delayed (S3–S4). Because only 24, 27, 7, and 2 lines survived under S3, S4, S5, and S6 in 2020–2021, respectively, yield and biomass for those sowing dates reflect the surviving subset and are not strictly comparable across all accessions.

Patterns in 1000-seed weight paralleled those of seed yield (Figure 7). The trait was significantly affected by sowing date and variety in both years, with a significant sowing date × variety (SD × V) interaction (Supplementary Table S2). The year effect was also significant, and the year × variety interaction varied by sowing date (Supplementary Table S2). In both 2020–2021 and 2021–2022, 1000-seed weight decreased with delayed sowing (p < 0.05), with the highest values at S1 in 2021–2022. Together with the yield results, these findings confirm that selecting an earlier sowing window in combination with appropriate varieties under prevailing climatic conditions is critical to secure grain size and final yield.

3.5. Correlation Analysis of Rapeseed Yield and Seed Germination

Correlations between germination traits (measured under CK and LT) and seed yield at each sowing date are shown in Figure 8. In 2020–2021, yield at S1, S4, and S5 was positively correlated with LT germination potential (GP) (p < 0.05). In the same year, yield at S4 and S5 also showed a positive correlation with LT root length (RL) (p < 0.01). In contrast, in 2021–2022, the yields at S1, S2, S4, and S6 were negatively correlated with CK shoot length (SL) (p < 0.05).

These patterns suggest that low-temperature germination performance (high LT-GP and longer LT-RL) is more predictive of yield when winters are harsher (2020–2021), whereas CK traits (e.g., CK-SL) are not reliable yield predictors, and can even negatively relate to yield under milder conditions (2021–2022).

Across both seasons, later sowing reduced overwintering survival, seed yield, and 1000-seed weight, but effects were stronger in 2020–2021, when a late-December to early-January cold wave occurred (Figure 1). In that colder year, survival for sowings after November fell below 20% for most accessions, and yield penalties relative to S1 reached 6.3–22.8% (Figure 4 and Figure 6), with parallel declines in 1000-seed weight (Figure 7). In 2021–2022, survival losses and yield penalties with delay were smaller (S1 exceeding S2–S6 by 5.5–23.3%), although the direction of responses remained the same (Figure 6). Consistent with these differences, LT germination traits showed clearer positive associations with yield in the colder season, whereas correlations were weaker or mixed in the milder season (Figure 8; Supplementary Tables S1 and S2).

4. Discussion

Late direct seeding has expanded in the Yangtze River Basin with rising labor costs and tighter rice-rapeseed rotations, which exposes seeds and seedlings to sub-optimal temperatures at establishment [5,13]. Consistent with earlier studies showing that low temperature restricts Brassica napus germination and early growth [10,12,17], we observed large reductions in germination potential, root length, and shoot length under low temperatures relative to control. Such inhibition results from reduced membrane fluidity, decreased enzyme activity, and slower carbohydrate mobilization, which limit radicle protrusion and hypocotyl elongation [9,12,13,17,18]. Moreover, low temperatures cause transient oxidative stress and imbalance in ABA/GA signaling, further delaying germination and early seedling expansion. The stronger reduction in root length than shoot length agrees with prior reports [12,17], indicating that root development is particularly sensitive to metabolic inhibition under cold stress and directly influences early nutrient and water uptake efficiency.

In addition to physiological limitations, the observed genotypic variation highlights intrinsic differences in cold-response capacity. Seven genotypes, 3409, M417, Zheza0903, 86155, 3445, Zheyou50, and 3462, maintained >80% germination potential and comparatively longer roots and shoots at 12/6 °C. These cultivars likely possess superior membrane stability and stronger antioxidative systems that sustain cell expansion at low temperatures. For instance, 3409 and Zheza0903, which exhibited both high germination vigor and top yield performance at early sowing dates, appear to combine rapid carbohydrate mobilization with efficient root elongation under stress. M417 and 86155, which performed well after moderate delays (S3–S4), may maintain better cold acclimation and osmotic adjustment during establishment. Zheyou50, 3445, and 3462 also retained stable yield components, suggesting genetic backgrounds that confer both seedling vigor and resilience during winter hardening. These differences show that cold-tolerant germination traits can serve as early indicators of stable yield under variable field conditions.

Timely sowing helps secure crop growth and productivity and mitigates yield losses under adverse climatic conditions [16,19,20]. Overwintering survival is a direct indicator of late-sowing tolerance. In our study, survival declined as sowing was delayed, which agrees with previous work conducted in the Yangtze River Basin [5]. Physiologically, late sowing shortens the rosette formation period, limiting carbohydrate and proline accumulation, which normally supports freezing tolerance. Smaller rosettes also have reduced photosynthetic capacity and lower leaf area index before winter, decreasing the energy available for stress repair [5]. Field evidence from the lower reaches of the Basin indicates that a latest “safe” window is approximately late October; postponing to November often reduces mean overwintering survival below ~30% [5]. Our results corroborate this, showing that S1–S2 averaged > 80% survival whereas S3–S6 dropped below ~23% in the colder year (2020–2021). These patterns reflect classic cold-hardening limitations also observed in European winter oilseed rape, where earlier sowing enhances accumulation of soluble sugars, proteins, and protective metabolites [21,22].

A main contribution of this study is the link between laboratory low-temperature tolerance and field outcomes under delayed sowing. In the colder season of 2020–2021, yield at S1, S4, and S5 was positively correlated with low-temperature germination potential, and yield at S4–S5 was positively correlated with low-temperature root length. This indicates that genotypes expressing high germination potential and root elongation at 12/6 °C can establish stronger stands and accumulate sufficient biomass before winter, thereby sustaining yield formation despite delayed sowing. In the milder season of 2021–2022, correlations were weaker or even negative for warm-temperature traits (e.g., CK-SL), supporting earlier conclusions that laboratory low-temperature performance, not general vigor, is a better predictor of field yield when thermal stress dominates establishment [10,12,13,17].

Yield and 1000-seed weight both declined as sowing was delayed in the two seasons. Later sowing reduces thermal time and shifts reproductive stages into cooler, shorter-day periods, lowering branching, silique number, and seed-filling duration [5,23,24,25,26]. From a mechanistic viewpoint, cold-limited photosynthesis reduces assimilate supply, while shortened source activity curtails carbon allocation to developing seeds, directly reducing 1000-seed weight [22,27]. The close alignment between yield, above-ground biomass, and 1000-seed weight observed here suggests that late-sown yield penalties are primarily due to diminished source strength rather than sink capacity. Similar patterns have been reported in temperate oilseed rape, cotton, and wheat [24,25,26], emphasizing the importance of sufficient thermal accumulation before flowering.

From a breeding and management perspective, the superior genotypes identified in this study provide practical targets for improving low-temperature establishment. Zheza0903 and 3409 can serve as donors for high vigor and early-season robustness, whereas M417 and 86155 may contribute traits linked to sustained growth under fluctuating winter conditions. Farmers should prioritize sowing by late October and ensure adequate soil moisture and basal fertilization to promote rapid emergence and carbohydrate accumulation before cold onset. In breeding programs, screening under 12/6 °C germination conditions offers a cost-effective proxy for identifying accessions that combine strong emergence, overwintering ability, and yield stability.

Finally, these results hold broader implications for climate resilience. Even with warming trends, short-term cold spells continue to occur in East Asia, posing episodic threats to direct-seeded rapeseed. Selecting cultivars with superior low-temperature germination, efficient energy use, and robust early root systems can buffer against such variability. Integrating physiological selection with optimized agronomy thus provides a dual pathway for sustaining rapeseed production under current and future climatic regimes [6,10,12,13,16,17,20,21].

5. Conclusions

Across two seasons, delayed sowing reduced overwintering survival, yield, and 1000-seed weight. In the colder season, survival for sowings after November fell below 20% for most accessions, while S1–S2 exceeded 80%. Yield penalties with delay were 6.3–22.8% relative to S1 in 2020–2021 and 5.5–23.3% in 2021–2022. This corresponds to an average yield decline of approximately 8–10% for every 10-day delay beyond mid-October, highlighting the steep sensitivity of rapeseed productivity to postponed planting under lower Yangtze conditions. Low-temperature germination traits, especially LT germination potential (GP) and LT root length (RL), were positively associated with yield under harsher winter conditions, indicating that LT screening is useful for identifying late-sowing-tolerant material. Seven accessions (3409, M417, Zheza0903, 86155, 3445, Zheyou50, and 3462) are promising for production and breeding. For practical cultivation in this region, the evidence supports late October as the latest safe sowing window, with November sowing carrying markedly higher risk of yield and survival loss.

Future research should validate these thresholds across additional sites and years, quantify the physiological drivers of cold tolerance (e.g., soluble sugars, proline accumulation, membrane stability, and hormonal balance), and integrate these traits with QTL and marker-based selection pipelines to accelerate breeding of robust, late-sowing-tolerant cultivars.