A Climate-Smart Biostimulant to Stabilize Soybean Yields Under Irregular Rainfall: Insights From Physiology to Production

Chitosan-based biostimulants offer potential to improve crop performance under water deficit, yet their physiological modes of action in field-grown soybean remain poorly defined. Here, we evaluated FF-BR (patent number US 9,868,677 B2), a foliar formulation combining chitosan and pyroligneous acid, across two growing seasons and soybean cultivars under contrasting rainfall regimes in southern Brazil. In 2023/2024, treatments consisted of two FF-BR applications at an interval of 17 days at R1-R2 (beginning-full bloom) phase at doses of 0, 0.75, 1.0, and 1.25% (v/v). In 2024/2025, they were three and four applications of FF-BR at an interval of 18 days in the same phase. In 2023/2024, FF-BR applications in the mid-maturity cultivar Soytech ST 641 I2X increased linearly intrinsic water use efficiency (iWUE) through partial stomatal closure, maintaining carbon assimilation under terminal drought. These responses, likely mediated by abscisic acid (ABA) signaling and H₂O₂ accumulation, enhanced reproductive performance and increased yield by up to 481 kg ha⁻¹ (18.5% over control). In 2024/2025, early-maturing Neogen 610 was exposed to a moderate prolonged water deficit. Repeated FF-BR applications reduced carbon isotope discrimination (Δ¹³C), indicating sustained improvements in iWUE. Grain yield increased by 335 to 682 kg ha⁻¹ (7.3 to 15% over control), driven primarily by enhanced grain number. Heat map analysis of pod development revealed increased locule filling from node two upward, reflecting maintained assimilate allocation under stress. These results suggest that FF-BR application should target periods when stomatal regulation begins to limit carbon assimilation but precedes irreversible reproductive loss. This timing aligns with recent findings that priming, rather than boosting, is the core mechanism of effective biostimulants under abiotic stress. Overall, FF-BR represents a climate-adaptive, field-validated strategy to enhance drought resilience and stabilize soybean yields under irregular rainfall conditions.