Optimizing application strategies of prohexadione-calcium·uniconazole composite regulator to improve lodging resistance and grain yield in rice cultivation in East China

Rice ( Oryza sativa L.) is a major staple crop worldwide. Yet, its grain yield is frequently constrained by lodging, which disrupts the canopy structure, reduces photosynthetic efficiency , lowers grain quality, and hampers mechanical harvesting . The application of plant growth regulators (PGRs) can mitigate lodging risks; however, the individual use of uniconazole and prohexadione-calcium presents certain limitations, such as panicle development issues and high costs, respectively. This study aimed to determine the optimal application timing and concentration of a composite PGR, prohexadione-calcium·uniconazole (PCU), to enhance stem traits and grain yield components, thereby improving rice lodging resistance and yield. A two-year field experiment was conducted to assess the effects of different PCU concentrations (90, 120, and 180 g ha⁻¹) applied at two critical growth stages: the tillering stage (I) and the jointing stage (when the first elongating internode of 50 % of plants reached 1 cm, designated as II). A water-treated control (PCU0) was included. The study evaluated plant height, stem characteristics, lodging index (LI), endogenous hormone levels, and grain yield parameters. Foliar application of PCU at 120 g ha⁻ 1 during the jointing stage (PCU120-Ⅱ) significantly reduced the LI by 25.0–36.5 % and increased grain yield by 8.1–9.3 %. Compared with PCU0-Ⅱ, PCU120-Ⅱ markedly shortened the basal internode length while increasing the culm diameter and culm wall thickness. Additionally, PCU120-Ⅱ significantly reduced endogenous gibberellin A 3 (GA 3 ) levels in the second internode (I-2), leading to a reduction in plant height and center of gravity height. Conversely, the elevated endogenous zeatin content promoted cell division in stem tissues, contributing to increased culm wall thickness in the basal internodes. Furthermore, PCU120-Ⅱ enhanced the accumulation of key cell wall components , including cellulose and lignin, thereby strengthening stem rigidity. According to the random forest importance ranking, culm wall thickness, lignin content, culm diameter, and cellulose content emerged as key traits for breeding lodging-resistant rice. Moreover, PCU120-Ⅱ significantly increased both the number of panicles per unit area and grain weight, which were the primary contributors to the observed yield improvement. Applying PCU at 120 g ha −1 during the jointing stage is the optimal strategy for improving rice lodging resistance and grain yield. This effect is achieved through modifications in stem morphology, hormonal balance, and grain yield components. This study elucidates the mechanism by which the novel composite PGR, PCU, enhances rice lodging resistance and optimizes its application strategies in terms of timing and dosage. The findings provide a promising approach to improving rice production efficiency and yield stability, offering valuable insights for both agronomic management and breeding programs. • Prohexadione-calcium·uniconazole (PCU) can improve lodging resistance of rice. • The decrease of field lodging rate improved the yield components, resulting in the increase of yield. • The application of 120 g ha −1 PCU at the jointing stage can improve lodging resistance and grain yield of rice. • Culm wall thickness, lignin content, culm diameter, and cellulose content are the key traits in lodging resistance breeding.