Direct-seeding rapeseed production at high plant density raises the risk of lodging. We investigated the use of dwarf genes to improve rapeseed plant architecture to balance yield and lodging. Three genotypes with different plant architectures (dwarf sca^HS5, semi-dwarf +/sca^HS5, and tall HS5) were evaluated under varying nitrogen rates (N1, N2, and N3: 120, 240, and 360 kg N ha⁻¹) and plant densities (D1, D2, and D3: 15, 45, and 75 plants m⁻²) from 2019 to 2022. The results showed that increasing N rate positively influenced yield while decreasing lodging resistance in all genotypes. Increasing plant density (D2–D3) enhanced lodging resistance and yield in sca^HS5 and +/sca^HS5, but reduced yield in HS5. Compared to the two parents, +/sca^HS5 exhibited moderate expressions of IAA3, GH3.15, and SAUR30 in stems under N2D3, resulting in reduced plant height and increased compactness. Additionally, +/sca^HS5 had a thicker silique layer than HS5 by 14.7%, and it had a significant correlation between branch height/angle and yield. Increasing N rate led to increased lignin and pectin contents, while cellulose content decreased. Increasing plant density resulted in greater stem cellulose content and CSLA3/7 expression in sca^HS5 and +/sca^HS5, but decreased in HS5. Compared to HS5, +/sca^HS5 exhibited higher expressions of ARAD1 and GAUT4, along with a 51.1% increase in pectin content, leading to improved lodging resistance under N2D3. Consequently, +/sca^HS5 showed a 46.4% higher yield and 38.9% lodging resistance than HS5 under N2D3, while sca^HS5 demonstrated strong lodging resistance but lower yield potential. Overall, this study underscores the potential of utilizing auxin dwarf genes to optimize the trade-off between yield and lodging resistance in rapeseed and the possibility of maximizing yield potential by optimizing the plant architecture of +/sca^HS5 through nitrogen reduction and dense planting.

Biological yield indicates the potential for increasing yield. Leaf carbon metabolism plays an important role in the biomass accumulation of rapeseed (Brassica napus L.). Field experiments with the hybrid HZ62 (with a conventional plant architecture) grown in 2016–2017, and HZ62 and accession 1301 (with a compact plant architecture) grown in 2017–2018 were conducted to characterize the physiological and proteomic responses of leaf photosynthetic carbon metabolism to density and row spacing configurations. The densities were set at 15×10⁴ ha⁻¹ (D1), 30×10⁴ ha⁻¹ (D2), and 45×10⁴ ha⁻¹ (D3) (main plot), with row spacings of 15 cm (R15), 25 cm (R25), and 35 cm (R35) (subplot). Individual and plant population biomass accumulation was greatest at R25, R15, and R15 for D1, D2, and D3, respectively, for both genotypes. In comparison with D1R25, the individual aboveground biomass of HZ62 decreased by 60.2%, whereas the population biomass increased by 31.9%, and the individual biomass of genotype 1301 decreased by 54.0% and the population biomass increased by 53.9% at D3R15. Leaf carbon metabolic enzymes varied between genotypes at flowering stage. In contrast to D1R25, at D3R15 the activities of ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) and sucrose phosphate synthase (SPS) and the contents of starch, sucrose and soluble sugars in leaves were significantly decreased in HZ62 and increased in genotype 1301. The activities of fructose-1, 6-bisphosphatase (FBPase) decreased, in consistency with the abundance of fructose-bisphosphate aldolase in HZ62. In contrast, sucrose synthase (SuSy) activity appeared to decrease in both genotypes, but a significant increase in abundance of a protein with sucrose synthase was found in the 1301 genotype by proteomic analysis. With increased density and reduced row spacing, the expression of most key proteins involved in carbon metabolism was elevated, and enzyme activity and carbon assimilate content were increased in 1301, whereas HZ62 showed the opposite trend, indicating that the compact plant type can accumulate more population biomass with denser planting.