Objectives The optimal proportion of organic fertilizer to chemical nitrogen fertilizer for strong-gluten and weak-gluten wheat cultivars were studied from the point of flag leaf senescence, grain filling characteristics and yield formation.

Methods From 2022 to 2024, a localized field experiment was conducted in Shangqiu City, Henan Province, using a high-gluten wheat cultivar Xinmai 45 and a low-gluten wheat cultivar Zhengmai 113 as experimental materials. Under a constant total nitrogen rate of 240 kg/hm², five treatments were established: farmers’ chemical fertilization (FP), and 20%, 30%, 40%, and 50% substitution of mineral nitrogen with organic fertilizer (denoted as MS20, MS30, MS40, and MS50, respectively). The flag leaf area index (LAI) and relative chlorophyll content (SPAD value) were determined at the jointing, booting, and anthesis stages and at 7, 14, 21, 28, and 35 days after anthesis, and thousand-kernel weight was measured at 7, 14, 21, 28, and 35 days after anthesis for calculating the post-anthesis flag leaf senescence and grain-filling parameters. A structural equation model (SEM) was used to analyze the causal pathway linking “senescence−grain filling−yield”.

Results Compared with FP, the MS30 treatment significantly increased the post-anthesis SPAD values and leaf area index (LAI) of the flag leaves. Based on the two-year mean results, the MS20 and MS30 treatments prolonged the total chlorophyll duration of the flag leaves of Xinmai 45 by 2.00 d, and that of Zhengmai 113 by 1.7 d and 2.1 d, respectively. Moreover, the average senescence rate of the flag leaves decreased with increasing organic fertilizer substitution ratio. Multiple regression analysis revealed that LAI was the dominant factor regulating chlorophyll decline, and SPAD and LAI exhibited a significant synergistic effect on shortening the rapid senescence phase of leaves. The strong-gluten wheat cultivar Xinmai 45 showed an overall stronger stay-green capacity than the weak-gluten wheat cultivar Zhengmai 113. Both the grain filling of the two cultivars followed a typical “slow−fast−slow” S-shaped curve, Xinmai 45 had a higher mean grain-filling rate and reached the maximum grain-filling rate earlier than Zhengmai 113. Compared with other treatments, the MS30 resulted in significantly higher maximum grain-filling rate and maximum grain weight accumulation ( Y_\max ), and the MS20 treatment resulted significantly higher Y_\max than FP, MS40, and MS50, respectively. Both MS30 and MS20 were recorded similar but significantly higher yield of the two cultivars than the other treatments. From yield components, Xinmai45 had an advantage in grain number per spike over Zhengmai113, whereas Zhengmai 113 had an advantage in 1000-grain weight over Xinmai 45. Under the MS30 treatment, the grain number per spike of Xinmai45 was on average 11.52% higher than that of Zhengmai 113, while the 1000-grain weight of Zhengmai 113 was on average 21.11% higher than that of Xinmai 45, indicating clear differences in yield components between the two wheat cultivars. SEM analysis showed that leaf photosynthetic traits (total effect of SPAD on yield = 1.156) and grain-filling capacity (direct effect of thousand-kernel weight on yield = 0.742) were the two key links determining yield formation.

Conclusions In the wheat-growing areas of eastern Henan, replacing 30% of nitrogen fertilizer with organic fertilizer could significantly increase the SPAD value and leaf area index (LAI) of flag leaves from 21 to 35 days after anthesis and enhance the maximum and mean grain-filling rate of the two wheat cultivars. The high LAI and SPAD synergistically shorted the rapid chlorophyll degradation phase, and markedly delayed the senescence of flag leaves, ensuring a prolonged grain-filling period and increased yield. The strong-gluten cultivar Xinmai 45 exhibited higher grain number per spike, chlorophyll retention duration, and photosynthetic maintenance capacity, resulting in an overall higher yield than Zhengmai 113. However, Xinmai 45 showed a faster rate of chlorophyll degradation and leaned more towards a grain number-oriented yield increase strategy. The weak-gluten cultivar Zhengmai 113 had a slower decline in grain-filling rate during the later filling stage, and a more pronounced advantage in thousand-grain weight, leaning more towards a grain weight-oriented yield increase strategy.