Slow‐release fertilizer with brassinolide improves grain appearance quality of main‐season ratoon rice by altering tartaric and oxalic acid concentrations in root bleeding sap and enhancing leaf antioxidant capacity

The combination of slow‑release fertilizer (SRF) and plant growth regulators is increasingly adopted in rice ( Oryza sativa L.) production, yet its physiological impacts on grain appearance quality in main‑season ratoon rice remain poorly understood. A soil‐grown experiment was carried out with three rice cultivars and four fertilization treatments: conventional urea (Control [CK]), sole SRF application, SRF combined with brassinolide (BR), and SRF combined with 1‑naphthaleneacetic acid (NAA). Leaf photosynthetic attributes, senescence‑related physiological parameters, organic acid concentrations in root bleeding sap, grain‑filling characteristics, and chalkiness indices were systematically evaluated. Within each year, SRF + BR consistently outperformed the other treatments, while SRF + NAA only induced moderate improvements in grain appearance quality. Relative to CK, SRF + BR significantly boosted leaf photosynthesis, increasing net photosynthetic rate and soil and plant analyzer development value by 28.10% and 10.32%, respectively. It also enhanced antioxidant enzyme activities (superoxide dismutase, peroxidase, and catalase) and non‑enzymatic antioxidants (ascorbic acid and reduced glutathione), thereby reducing malondialdehyde (MDA) accumulation and alleviating oxidative damage. At the root level, SRF + BR altered bleeding sap organic acid concentrations by decreasing oxalic acid and elevating tartaric acid contents. Exploratory structural equation modeling further suggested no statistically significant association between tartaric acid and grain‑filling progress, whereas oxalic acid was negatively associated with grain filling. Such coordinated root‑shoot physiological improvements were associated with maintained higher leaf function during grain filling, accelerated grain‑filling processes, and reduced grain chalkiness. Correlation analysis verified that chalkiness was positively correlated with MDA and oxalic acid but negatively correlated with antioxidant capacity, tartaric acid, and grain‑filling traits. Taken together, SRF + BR altered tartaric and oxalic acid concentrations and sustained leaf photosynthetic‑antioxidant function, which may contribute to improved carbon‑nitrogen balance and reduced chalk formation. This work offers a promising agronomic strategy for high‑quality main‑season ratoon rice production.