The upper reaches of the Yangtze River represent one of China’s major rapeseed-producing regions, playing a pivotal role in ensuring the national supply of edible vegetable oil and improving the self-sufficiency rate of oil crops. However, the region is characterized by complex and variable climatic conditions, and traditional genetic improvement evaluation methods are highly susceptible to environmental interference, making it difficult to accurately track trends in varietal genetic potential. This study aimed to establish a method for tracing genetic improvement using annually top-yielding lines as representatives, thereby systematically revealing the genetic progress trajectory and agronomic trait evolution of rapeseed lines in the upper reaches of the Yangtze River from 2004 to 2023. The goal was to clarify the synergistic regulatory mechanisms of key agronomic traits in high-yield lines and provide theoretical support for high-yield and stable-yield rapeseed breeding.

Annual top-yielding lines from the National Winter Rapeseed Regional Trials conducted in the upper reaches of the Yangtze River from 2004 to 2023 were selected as the study objects. A mixed linear model was used to separate environmental effects via the best linear unbiased prediction (BLUP) model. Linear regression, Pearson’s correlation analysis, standardized path analysis, and principal component analysis (PCA) were integrated to comprehensively assess yield genetic progress trends and trait variation patterns over the past two decades, and to construct a multi-trait synergistic regulation network.

Both actual yield and BLUP-based yield of the rapeseed lines showed a significant upward trend from 2004 to 2023. The improvement of traits in high-yield lines exhibited clear stage-specific changes: from 2004-2013, breeding strategies emphasized compact plant type, with significant reductions in branching number, siliques per plant, and whole growth period; from 2014-2023, strategies shifted toward seed number type, with marked increases in siliques per plant and thousand-seed weight. Correlation and path analyses revealed that yield per plant is the core direct driving factor for enhancing population yield, while silique number per plant and branching number primarily contribute to population yield through indirect pathways via their effects on yield per plant. PCA revealed that the first five principal components all had eigenvalues greater than 1, cumulatively explaining 76% of the total variance. The PC1 axis, predominantly characterized by structural traits such as silique number per plant, branching number, and plant height, accounted for 29% of the variance, representing the primary dimension underlying inter-varietal differentiation, indicating a breeding trend from “single-trait breakthroughs” to “multi-factor synergy”.

The breeding focus for rapeseed in the upper Yangtze River has shifted from early-stage optimization of plant architecture toward late-stage enhancement of seed number. The study identified a high-yield model centered on yield per plant, supported by the coordinated improvement of branching number and siliques per plant, with balanced allocation to seed size traits. The lack of promotion area for high-yield varieties in the upper reaches of the Yangtze River from 2004 to 2013 indicates that the breeding direction in this region should comprehensively consider oil production and lodging resistance, in order to achieve sustained break throughs in rapeseed yield under different ecological and management conditions in the upper reaches of the Yangtze River.

It was found that there were significant genetic differences between Chinese and European Brassica napus lines, and the yield advantage of hybrid F₁ was obvious. To explore the effect of gibberellin (GA) on the growth and development of B. napus, the dynamic changes of GA during the developmental process of B. napus from different lines were measured. For explaining the role of GA in the formation of yield, the transcriptional levels of key enzyme genes involved in GA synthesis were analyzed, and the relationship between GA content and yield in parents and F₁ was clarified.

The dynamic changes of GA content in B. napus lines (15 Chinese lines and 15 European lines) with different sources from November 2020 to May 2021were determined using high performance liquid chromatography. Two F₁hybrid lines (YG2009×YC4, ZS11×YC4) with strong heterosis and their parents were used as materials to investigate the changes of GA content in different periods (D1, 15 January 2022; D2, 15 February 2022; D3, 15 March 2022)and measure growth indicators (plant height, root length, fresh weight, etc.), yield and component factors (silique number per plant, seed number per silique and thousand-seed weight), as well as photosynthetic indicators (photosynthetic rate, stomatal conductance, transpiration rate and intercellular CO₂ concentration).The change trend of GA content with temperature and the relationship between GA content and agronomic characters, photosynthetic characteristics and yield of parents and F₁hybrid from November 2021 to May 2022 were analyzed. Besides, the transcriptional levels of key enzyme genes involved in GA synthesis of the parents and F₁ at different stages (D1, D2, D3) also examined by quantitative real-time PCR.

The content of GA was closely related to the ambient temperature, and the GA content in the tested B. napus lines decreased first and then increased with the change of temperature. However, it was found that the average content of GA in B. napus lines from Europe was higher than that of Chinese lines. F₁ showed significant yield heterosis. Compared with the male and female parents, the yield of YG2009×YC4 and ZS11×YC4 increased by 18.06% and 10.35%, and 29.92% and 28.6%, respectively. There were significant differences in agronomic traits between parents and F₁, and the yield was correlated with GA content. The results also showed that the transcriptional levels of GA20ox4, GA3ox2 and SLR1 in parents and hybrid F₁ varied greatly.

The GA content of B. napus lines from Europe was more sensitive to temperature changes, but its average content of GA was higher than that from China. The yield of F₁ between European rape and Chinese rape lines showed strong heterosis, which was correlated with GA content. The transcriptional levels of GA20ox4 and GA3ox2 regulated the GA content.