Humic acid (HA), a key component of soil organic matter, has emerged as a multifunctional biostimulant that enhances soil quality, nutrient availability, plant growth, and stress resilience. The complex supramolecular structure of HA influences soil physicochemical properties by improving aggregation, increasing cation exchange capacity, and enhancing water and nutrient retention. These changes facilitate greater uptake of essential macro- and micronutrients, supporting improved photosynthesis, root development, and biomass accumulation. At the physiological and molecular levels, HA modulates hormone signaling, activates antioxidant defenses, and primes stress-responsive pathways that enhance tolerance to drought, salinity, and other abiotic stresses. HA’s ability to regulate reactive oxygen species (ROS) homeostasis, maintain ion balance, promote osmolyte accumulation, and activate pathways such as Salt-Overly-Sensitive (SOS) or other stress-regulatory networks has been demonstrated across diverse crop species. Despite these benefits, variability arising from differences in HA source materials and extraction methods remains a major challenge for consistent application. Future research integrating multi-omics approaches, improved formulation strategies, and large-scale field validation will be essential for elucidating unknown HA’s regulatory mechanisms and maximizing its agricultural potential. Collectively, current evidence positions HA as a promising biostimulant capable of enhancing crop productivity and resilience within sustainable agricultural systems.

Drought is one of the environmental factors inhibiting plant productivity and growth, leading to oxidative damage. This study aims to identify the role of sodium hydrosulfide (NaHS) as a hydrogen sulfide (H2S) donor in drought stress tolerance in Brassica napus. Drought-induced stress symptoms appeared eight days after treatment, showing wilted leaves and a significant reduction of leaf water potential. Drought-induced increase of lipid peroxidation was significantly reduced by NaHS application. NaHS-treated plants mitigated stress symptoms under drought conditions by reducing hydrogen peroxide (H2O2) content, confirmed with H2O2 localization in situ. Furthermore, NaHS promotes photosynthetic activity by maintaining chlorophyll and carotenoid content, thereby supporting plant growth under drought conditions. Pyrroline-5-carboxylate and proline contents were significantly increased by drought but further enhanced by NaHS treatment, indicating the important roles of proline accumulation in drought stress tolerance. In conclusion, this study provides valuable insight into the roles of NaHS in alleviating drought stress by reducing oxidative stress and promoting proline accumulation. Therefore, NaHS may serve as an effective strategy to enhance crop production under drought-stress conditions.