Are applications of a biostimulant based on Ascophyllum nodosum able to reduce lactofen phytotoxicity in soybean plants?

Abstract The herbicide lactofen, a protoporphyrinogen oxidase (PPO) inhibitor, induces phytotoxicity in soybean ( Glycine max ), which is typically characterized by the accumulation of reactive oxygen species (ROS) and subsequent degradation of the photosynthetic apparatus. This study evaluated the potential of an Ascophyllum nodosum -based biostimulant to mitigate lactofen-induced phytotoxicity in soybean. A greenhouse-based pot trial using a completely randomized experimental design was employed with five treatments: control, biostimulant only, lactofen, lactofen + biostimulant (simultaneous), and lactofen followed by biostimulant application 6 days after herbicide application (6 DAA). Photosynthetic pigment content, gas exchange, chlorophyll a fluorescence, ascorbate peroxidase (APX) activity, and phytotoxicity were assessed at the V4 phenological stage, and grain yield was obtained at the R8 phenological stage. Simultaneous application of lactofen and the biostimulant minimized total chlorophyll loss by 61.45% and significantly enhanced the effective quantum yield of photosystem II ( ϕ PSII ) and carboxylation efficiency compared to the herbicide alone. The treatment led to a 26.76% reduction in H 2 O 2 concentration, driven by an upregulation of APX activity, which preserved leaf area and increased the 100-grain weight by 28.31%. These findings suggested that biostimulant stabilized photosynthetic complexes and improved redox homeostasis in treated leaves. Conversely, the delayed application of the biostimulant (6 DAA) proved ineffective, indicating that mitigation success was highly dependent on the application timing. Thus, simultaneous foliar application of A. nodosum -based biostimulant with lactofen serves as a robust strategy for mitigating chemical stress, ensuring metabolic resilience, and stabilizing productivity in soybean cultivation.