Split Potassium Fertilization Modulates Endogenous Hormone Homeostasis to Optimize the Grain-Filling Process and Mitigate High-Temperature Damage in Rice

High temperature during flowering and grain filling severely reduces rice yield and grain quality. Split potassium (K) fertilization can mitigate such heat-induced damage, yet its mechanisms linking grain filling, endogenous hormones and grain performance remain unclear. Here, a two-year pot experiment was conducted to explore the effects of split K application on rice yield, quality and hormonal metabolism under high temperature. Four treatments included ambient temperature with full basal K (AT-K100), high temperature with full basal K (HT-K100), and two split K regimes under high temperature (HT-K70+30, HT-K30+70). Split K application decreased abscisic acid (ABA) levels at 5 days after anthesis (DAA), increased indole-3-acetic acid (IAA), zeatin riboside (ZR) and gibberellin A3 (GA3) at 5 DAA, and maintained higher IAA and GA3 levels until 20 DAA. The ratios of ABA/IAA and ABA/GA3 were also reduced at both 5 and 20 DAA. These hormonal alterations optimized grain-filling dynamics, prolonged active filling duration and improved middle- and late-stage filling rates, thereby promoting grain weight accumulation and suppressing chalkiness formation. Compared with HT-K100, HT-K70+30 increased yield by 8.75%, which was attributed to improved seed-setting rate and 1000-grain weight. HT-K30+70 enhanced spikelet number per panicle, seed-setting rate and 1000-grain weight, but significantly decreased effective panicles, resulting in no obvious yield advantage. Furthermore, split K application effectively reduced grain chalkiness, with a more pronounced effect at a higher panicle-stage K proportion. Under ongoing global warming, K management can be tailored to production goals: higher basal K is preferable for yield pursuit, while increasing panicle K topdressing effectively improves grain quality.