Increasing wheat (Triticum aestivum L.) planting density is the most effective production management method for increasing yields; however, excessive crop populations under high planting densities may experience elevated risk of stem lodging. We conducted this study to assess the relationship between reduced lodging and increased yield, investigate the effects of planting density on wheat population structure, stem strength, and material transport, and provide a basis for rationale planting densities. The experiments were carried out using a split-plot design with three replicates. The main plots contained two wheat varieties: Bainong 5819 (BN5819) and Bainong 4199 (BN4199), and the sub-plots contained four planting density treatments: 90 × 104 plants/ha (D1), 180 × 104 plants/ha (D2), 270 × 104 plants/ha (D3), and 360 × 104 plants/ha (D4). A two-year field trial was conducted in 2021–2023. The relationships between population structure characteristics, changes in stem strength, activation, and retransport of stem material after anthesis, and achievement of high and stable yields were investigated at the different planting densities. When the planting density of wheat increased from D1 to D4 treatment, the activity of fructan hydrolase was significantly increased. Compared with D1 treatment, the highest activity of fructan hydrolase was increased by 457.47 μg/h/g under D4 treatment. At the same time, the increase of density also increased the contribution rate of dry matter accumulation (CDMA) to grain after anthesis increased, with the highest increase in CDMA at 33.67%, which significantly reduced stem strength. Correlation analysis revealed a significant negative association between CDMA and stem strength. Specifically, CDMA levels were significantly lower with the D3 treatment than the D4 treatment, while stem strength remained higher after anthesis as an adaptive response to mitigate lodging risk. Stem storage compounds can promote grain filling and a weight increase in inferior grains. The number of spikes per unit area increased significantly with increasing planting density, but the number of grains per spike and 1000-grain weight decreased significantly. In two years, the number of spikes in D3 treatment increased by a maximum of 211.67 × 104 ha−1 and 99.17 × 104 ha−1, respectively, compared to D1 and D2 treatments. The number of grains per spike was significantly higher than that of D4 treatment, the highest being 3.68 grains. Therefore, in the North China Plain with suitable water, fertilizer, and temperature, the sowing density of 270 × 104 plants/ha established population structure, significantly reduced CDMA, maintained post-anthesis stem strength, enhanced resilience of stems against post-anthesis lodging, and resulted in high yields by stabilizing the number of grains per spike and increasing the number of wheat spikes.
