Humic substances for abiotic stress resilience: current knowledge and perspectives

Humic substances (HS), encompassing humic and fulvic fractions as well as humic-like materials, are among the most adopted biostimulants to enhance plant resilience under abiotic stress. Yet their efficacy remains highly variable depending on the source, chemical composition, application dose, and experimental conditions. This review provides an overview of scientific studies addressing the application of HS to plants under different abiotic stresses. The chemical characterization of HS, including aromaticity, functional group distribution, and molecular size, affects their biological activity, though a direct and predictive structure-activity relationship has not yet been established. A convergent view across stress types indicates that HS influence plasma membrane function, ion transport, and redox balance, with the magnitude and direction of these effects depending on the chemical composition of the product, the application dose, and the stress conditions. However, the current review, considering studies evaluating the mode of action of HS during the years 2021-2025, reflects insufficient reporting of HS characterization and dose normalization and their corresponding biological effects. We also found a lack of high-throughput omics technologies to provide a deeper understanding of the mode of action of HS. Future studies should prioritize reporting clear standards for HS characterization and harmonized dosage recommendations. It is also critical to link the chemical properties of HS with their behaviors at the root-soil interface, the specific phase of stress being addressed, and the responses at various scales.