Optimizing Planting Density to Improve Source-Sink Relationship and Yield of Hybrid Wheat Under Late-Sowing Conditions

Increasing planting density is an effective measure to mitigate the negative impacts of late-sowing on yield formation in winter wheat. However, the physiological mechanisms underlying source-sink coordination and high-yield performance through density regulation in hybrid wheat with high yield potential remain unclear. A two-year field experiment was conducted using the hybrid variety Jingmai 17 and conventional variety Jimai 22 as experimental materials, with three planting densities: 150 plants·m−2 (M1), 300 plants·m−2 (M2), and 450 plants·m−2 (M3). The effects of planting density on the source-sink relationship and yield were systematically investigated. The results showed that both Jingmai 17 (2.4–9.7%) and Jimai 22 (1.4–10.6%) exhibited the most significant yield increases under the M2 treatment. This density maintained photosynthetic capacity during the mid-to-late grain-filling stage, delayed leaf senescence, promoted assimilate translocation to the grains, and simultaneously improved grain number per spike and thousand-grain weight by optimizing source-sink coordination efficiency. Compared with Jimai 22, the hybrid wheat Jingmai 17 demonstrated a significant yield advantage (8.2–10.1%), which was attributed to its stronger and more persistent source function, larger and more stable sink capacity, and higher source-sink coordination efficiency. In conclusion, under late-sowing conditions, the hybrid variety Jingmai 17 at a density of 300 plants·m−2 achieved the most effective optimization of the source-sink relationship, fully exploited its yield potential, and achieved a balance between high and stable yield. This study provides a theoretical and practical cultivation reference for the selection of hybrid wheat varieties in this region.