Effects of Wide–Narrow Row Spacing and Planting Density on Canopy Structure, Photosynthetic Performance, and Yield of Brewing Sorghum in Slightly Saline–Alkali Soils

Slightly saline–alkali soils represent an important but underutilized land resource in northern China, and optimizing planting patterns is essential for improving sorghum productivity under such marginal conditions. This study aimed to evaluate the effects of wide–narrow row spacing combined with different planting densities on the canopy structure, photosynthetic performance, and grain yield of brewing sorghum. A field experiment was conducted from 2022 to 2024 at the Yulin Experimental Station in Shaanxi Province, China, using the brewing sorghum cultivar Liaonuo 16. Four planting treatments were established: wide–narrow row spacing (80/60 cm) with three planting densities (105,000, 112,500, and 120,000 plants ha−1) and uniform row spacing (60 cm) with 112,500 plants ha−1 as the control. Wide–narrow row spacing combined with higher planting density significantly improved canopy structure and light interception. The treatment with 120,000 plants ha−1 increased light interception in the middle and lower canopy layers during flowering and grain filling by 8.7% and 25.58%, respectively, and enhanced total canopy light interception by 3.33% and 1.96%. Moreover, the leaf area index and photosynthetic capacity were improved, resulting in a 10.1% increase in grain yield compared with the uniform row spacing treatment. Wide–narrow row spacing combined with a planting density of 120,000 plants ha−1 effectively optimizes canopy structure and enhances sorghum productivity in slightly saline–alkali soils, providing a practical cultivation strategy for improving resource use efficiency in marginal farmlands.