Under reduced nitrogen fertilizer conditions, increasing wheat planting density to enhance sink capacity for achieving high productivity

The practice of reducing nitrogen while increasing planting density effectively maintains yield while improving nitrogen use efficiency. Sink limitation predominantly occurs under non-nitrogen-limiting conditions. Increasing plant density in conjunction with reduced nitrogen application significantly boosts sink capacity and the grain-to-leaf ratio. An elevated grain-to-leaf ratio facilitates greater post-anthesis dry matter accumulation and enhances nitrogen remobilization. Nitrogen reduction is an effective strategy for improving nitrogen use efficiency (NUE) in crops, but it often leads to a decrease in grain number per unit area. Increasing planting density can enhance sink capacity by raising the number of grains per unit area. However, it remains unclear whether the combined strategy of reducing nitrogen input while increasing planting density can sustain or improve wheat yield, as well as the underlying source-sink regulatory mechanisms. In this study, two wheat cultivars with contrasting sink characteristics were selected: the multi-spike cultivar XN20 (Xinong 20) and the large-spike cultivar LKAZ8 (Lankaoaizao 8). A split-plot experimental design was adopted, involving three nitrogen levels and two planting densities. The results showed that the combined strategy of reducing nitrogen and increasing density significantly enhanced aboveground dry matter, grain yield, and NUE. While nitrogen reduction decreased the leaf area index (LAI) by 1.61–22.39%, increased planting density raised LAI by 2.99–14.13%, sink capacity by 3.08–27.58%, and improved the grain-to-leaf area ratio (GN-LAR). GN-LAR was significantly positively correlated with post-anthesis dry matter accumulation and nitrogen remobilization. Compared with conventional nitrogen and density management, the integrated strategy enhanced source supply, improved the source-sink relationship by increasing sink capacity and optimizing GN-LAR, and thereby promoted post-anthesis dry matter accumulation and nitrogen remobilization, strengthening the coordination between source supply and sink demand. These findings provide new insights into the regulatory mechanisms of the source-sink relationship in wheat under conditions of reduced nitrogen and increased planting density, offering a scientific basis for achieving a balance between high yield and high NUE in wheat production.