Photosynthesis-driven yield improvement in maize under sulfur application: role of young ear development and grain filling

Sulfur (S) deficiency is increasingly recognized as a limiting factor for maize productivity, yet the physiological mechanisms underlying sulfur-driven yield enhancement remain insufficiently understood. This study aimed to clarify how sulfur application influences maize ear development, grain filling, and photosynthetic performance. Field experiments were conducted during 2020–2021 on two contrasting soil types (black soil and sandy soil) with sulfur application rates ranging from 0 to 120 kg ha⁻¹. Moderate sulfur application (S60–S90) significantly increased maize grain yield (P < 0.05), with yield increases of approximately 10–27% in black soil and 7–26% in sandy soil, and peak yields of 11,843 kg ha⁻¹ and 8,780 kg ha⁻¹, respectively, at the S90 treatment. Yield improvement was primarily attributed to coordinated increases in kernel number per ear and thousand-grain weight. Kernel number per ear increased significantly by 2.7–6.8% in black soil and 2.0–7.4% in sandy soil, which was closely associated with enhanced nitrogen accumulation in young ears (12.1–46.1% higher than S0) and increased fertilization rates (1.0–2.3%). Thousand-grain weight was also significantly improved, increasing by 1.2–11.9% in black soil and 3.8–16.7% in sandy soil, supported by higher grain-filling rates (0.7–1.9 g day⁻¹ per thousand-grain) and a prolonged rapid grain-filling duration (0.5–1.9 days). In addition, sulfur application significantly maintained post-silking photosynthesis by increasing chlorophyll content and leaf area index, thereby ensuring sufficient assimilate supply for kernel development. Sulfur fertilization improves maize yield by coordinating pre-silking ear development and nitrogen uptake with post-silking photosynthetic activity to support grain filling. These findings provide a physiological framework for nutrient management strategies to optimize maize yield under variable soil fertility conditions.