Optimizing the interaction between planting density and nitrogen (N) application rate is critical for simultaneously improving grain yield and nitrogen use efficiency (NUE) in winter wheat (Triticum aestivum L.). However, the underlying regulatory mechanism remains poorly understood in the fluvo-aquic soil region of the southern Huang–Huai–Hai Plain. This study aimed to elucidate the physiological mechanism by which planting density and nitrogen application interactively regulate source–sink coordination to achieve synergistic high grain yield and high NUE, and to screen the optimal local cultivation combination for winter wheat in southeastern Henan. A two-year consecutive field experiment was conducted from 2018 to 2020 in Shangshui, Henan, using a split-plot design. Three planting densities (D1: 225 × 10⁴ plants ha⁻¹; D2: 375 × 10⁴ plants ha⁻¹; D3: 525 × 10⁴ plants ha⁻¹) and five N rates (N0: 0; N1: 180; N2: 240; N3: 300; N4: 360 kg N ha⁻¹) were established. Results demonstrated that planting density, N rate, and their interaction significantly regulated grain yield, NUE, and dry matter and N allocation, with consistent trends across both years. Increasing density enhanced total biomass and N accumulation, but dry matter and N partitioning to grains declined when density exceeded 375 × 10⁴ plants ha⁻¹. Grain yield exhibited a quadratic response to N rate; the optimal N rate for maximum yield decreased from 296.33 kg ha⁻¹ at low density (D1) to 237.50–245.38 kg ha⁻¹ at medium and high densities. The combination of 240 kg N ha⁻¹ and 375 × 10⁴ plants ha⁻¹ (D2N2) produced the highest average grain yield (8875.35 kg ha⁻¹), with simultaneous improvements in spike number and kernels per spike as well as superior dry matter and N partitioning to grains. This combination also maintained high nitrogen recovery efficiency (NRE) and nitrogen agronomic efficiency (NAE). Correlation analysis revealed that grain yield and NUE were significantly positively correlated with dry matter accumulation, N accumulation, and their partitioning proportions to grains. Overall, D2N2 achieved simultaneous high yield and high NUE by coordinately optimizing dry matter and N partitioning to grains. We therefore recommend reducing N fertilizer to approximately 240 kg ha⁻¹ combined with a moderate planting density of 375 × 10⁴ plants ha⁻¹ as the preferred strategy for sustainable and intensive winter wheat production in the fluvo-aquic soil region of southeastern Henan and adjacent areas.

Sowing date and seeding rate are key agronomic measures for regulating yield formation and quality accumulation in winter wheat, especially in the context of global warming, which has led to delayed sowing becoming the norm. Clearly identifying optimal combinations of sowing date and seeding rate under late-sowing conditions is crucial for achieving coordinated improvement in both high yield and quality. Therefore, this study used the strong-gluten wheat cultivar ‘Shaannong 33’ and conducted a two-factor split-plot field experiment in the southern North China Plain (Nanyang, Henan) during 2019–2021. Three sowing dates (early sowing: 27 October; intermediate sowing: 31 October–2 November; late sowing: 6–8 November) and four seeding rates (180 × 10⁴, 240 × 10⁴, 300 × 10⁴, and 360 × 10⁴ plants ha⁻¹) were tested to systematically analyze the effects of sowing date and seeding rate on wheat yield and its components, grain processing quality, and starch physicochemical properties. The results showed that sowing from late October to early November (approximately 25 October to 2 November in 2019–2020 and 25 October to 31 October in 2020–2021) combined with a seeding rate of 240–300 × 10⁴ plants ha⁻¹ maintained yield at a comparably high level across both growing seasons while improving processing quality indicators such as protein content, wet gluten content, and dough stability time for the strong-gluten cultivar ‘Shaannong 33’. However, the optimal sowing window varied between years: in 2019–2020, both D1 (27 October) and D2 (2 November) produced comparable yields, whereas in 2020–2021, D2 (31 October) was superior for both yield and quality. This inter-annual variability highlights the influence of climatic conditions on the optimal sowing window. Further delaying the sowing date to after 6 November led to decreases in yield and quality parameters; however, increasing the seeding rate to 360 × 10⁴ plants ha⁻¹ partially compensated for yield loss. In addition, late sowing significantly increased amylopectin content, swelling power, and pasting indicators such as peak viscosity and final viscosity while decreasing amylose content and pasting temperature, which is beneficial for improving cooking and eating quality. Correlation analysis indicated that the amylose/amylopectin ratio was the core factor determining starch pasting properties, while protein content showed a significant negative correlation with starch pasting indicators, reflecting the resource competition effect between grain protein and starch. Yield differences between the two years were mainly driven by changes in spike number per unit area, while inter-annual climate fluctuations (especially precipitation and temperature) significantly affected spike formation capacity. Overall, sowing date and seeding rate have a synergistic regulatory effect on yield and quality of late-sown wheat for the strong-gluten cultivar ‘Shaannong 33’. It is recommended to adopt a sowing date of 25–31 October with the seeding rate controlled at 240–300 × 10⁴ plants ha⁻¹ to achieve synergistic improvement of yield and quality under late-sowing conditions in the southern North China Plain for this cultivar.