Optimizing Sulfur Fertilization for Enhanced Physiological Performance, Grain Filling Characteristics, and Grain Yield of High-Yielding Winter Wheat Under Drip Irrigation

The North China Plain is one of the major wheat cultivation regions. As a cornerstone of global food security, wheat makes the enhancement of its yield critically important. Sulfur critically regulates photosynthesis, antioxidant defense, and grain filling dynamics. To elucidate the physiological mechanisms of S in wheat grain filling and guide field practices, a two-year field experiment (2022–2023 and 2023–2024) was conducted in the North China Plain using two dominant cultivars, Jimai 20 (JM20) and Yannong 999 (YN999). Four sulfur (ammonium sulfate) gradients (S1: 15 kg ha −1 ; S2: 30 kg ha −1 ; S3: 45 kg ha −1 ; S4: 60 kg ha −1 ) and a control (S0) were applied at the jointing stage via a drip fertigation system. The key findings reveal that optimal S application (YN999: 45 kg ha −1 ; JM20: 30–45 kg ha −1 ) enhanced post-anthesis photosynthetic capacity by increasing flag leaf SPAD values and superoxide dismutase (SOD) activity while reducing malondialdehyde (MDA) accumulation, thereby delaying leaf senescence. These improvements translated into optimized grain filling parameters: YN999 and JM20 exhibited 2.27–5.62% and 13.20–13.86% increases in mean grain filling rate, 3.92–4.73% and 2.11–4.36% extensions in grain filling duration, and 7.62–7.83% and 9.55–10.23% boosts in thousand grain weight, respectively. Consequently, yield increased by 0.58–1.54 t ha −1 for YN999 and 1.36–1.49 t ha −1 for JM20. Under drip fertigation conditions in the North China Plain, sulfur application at 30–45 kg ha −1 effectively enhances wheat yield. These findings provide fertilization guidance for the development of precision agriculture and can help alleviate the local soil sulfur deficiency trend.