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.

Leaf traits are important factors affecting the photosynthetic capacity of crops. In Bainong 4199 (BN4199) and Aikang 58 (AK58) wheat, the role of leaf color and leaf thickness in improving wheat photosynthetic performance and its influence on material accumulation and yield were studied in the field environment. Compared with AK58, BN4199 has a deeper leaf color and thicker leaves. Further study on photosynthetic physiological characteristics showed that the photosynthetic capacity of BN4199 with deep color and thick leaves was higher than that of AK58 at flowering stage, 7 days after flowering, 15 days after flowering, and 20 days after flowering regardless of low light in the morning and evening or light at noon. During the flowering stage, the light saturation point and compensation point were 1% higher and 15.23% lower, respectively, in BN4199 than AK58. According to the diurnal variation in chlorophyll content in different growth stages, BN4199 was generally higher than AK58, and the chlorophyll content was the highest at each time point 7 days after flowering. The chlorophyll content was highest at each time point 7 days after flowering. Chlorophyll fluorescence parameters and light reflectance analyses indicated that BN4199 has significantly higher photosynthetic electron transport and population light energy absorption and utilization capacity than AK58. The 2-year field yields indicated significantly higher material accumulation in BN4199 than AK58. In summary, thick leaves with deep color were resistant to both strong light and weak light, thus, markedly increasing photosynthetic efficiency. Improvement in leaf color and leaf thickness might serve as an important index to enhance the photosynthetic performance of wheat, and achieve improvement and breeding of wheat with high light efficiency.