Foliar Magnesium Modulates Photosynthetic Carbon Allocation and Antioxidant Homeostasis to Improve Yield and Dry Matter Remobilization of Upland Rice in Sichuan

Dry-sown and rain-fed rice cultivation is a core cropping mode characterized by dry tillage, mechanical precision direct seeding, and reliance on natural rainfall for water supply without irrigation during the growing period. To clarify the regulatory effects of foliar magnesium (Mg) application on yield, dry matter accumulation, and rice quality of upland rice in the hilly areas of Sichuan, four main cultivated hybrid rice varieties, namely Shenyou Yuehe Simiao, Byou 3611, Yahanyou 5287, and Hanyou 73, were used as materials. A two-factor split-plot design was adopted, with three magnesium sulfate concentrations: 0% (Mg0), 0.5% (Mg1), and 0.8% (Mg2), applied foliarly at the tillering and booting stages. The responses of yield components, dry matter, physiological indicators, and quality of upland rice to magnesium fertilizer were explored. The results showed that compared with Mg0, Mg1 significantly increased the number of effective panicles, filled grains per panicle, and seed setting rate, making it the optimal spraying concentration. Yahanyou 5287 achieved the highest yield of 7447.83 kg/hm² under Mg1 treatment in two years. Mg1 significantly increased the dry matter accumulation at the full heading and maturity stages, promoted the translocation of photosynthates from stems and leaves to grains, and improved the translocation rate and grain contribution rate. Under Mg1 treatment, the dry matter translocation amount, translocation rate, and contribution rate of stems and leaves of Yahanyou 5287 were 2687.23 kg/hm², 38.35%, and 22.24%, respectively. Meanwhile, Mg1 significantly increased the contents of soluble sugar and sucrose, the activities of SOD, POD, and CAT in leaves, reduced the MDA content, increased the leaf area index (LAI) and leaf magnesium content, thereby enhancing photosynthetic capacity and stress resistance. In terms of rice quality, Mg1 significantly increased the brown rice rate, milled rice rate, and head milled rice rate, reduced the chalkiness degree, and optimized processing and appearance quality. The contents of crude protein and amylose were dominated by genotype, with a slight regulatory effect of magnesium fertilizer. Among the tested varieties, Yahanyou 5287 was the most sensitive to magnesium fertilizer with the best comprehensive performance, while Shenyou Yuehe Simiao showed a weak response. In conclusion, foliar application of 0.5% magnesium sulfate at the tillering and booting stages can synergistically improve yield, optimize dry matter translocation, and enhance rice quality of upland rice in the hilly areas of Sichuan, providing theoretical and technical support for high-yield and high-quality cultivation of dry-sown and rain-fed rice in this region.