Fulvic Acid-releasing Chitosan Nanoparticles Promote the Growth and Drought Stress Tolerance of Rice Plants

Drought stress is a major limiting factor for rice production globally. The current climatic changes have further increased the incidence and duration of droughts worldwide. On the other hand, nanotechnology can revolutionize agriculture by offering novel tools and materials for various applications. Here, we synthesized fulvic acid-releasing chitosan nanoparticles (Ch-FANPs) and investigated their potential for promoting rice growth under extended drought conditions. In a hydroponic screening experiment, 0.1 mM Ch-FANPs gave the best results by supporting maximum shoot and root lengths as well as fresh and dry weights. In soil-based experiments, this concentration promoted rice growth under irrigated (control) and during a 10-day drought stress regime by supporting the maximum above and below-ground growth parameters such as root and shoot lengths, leaf area, and relative water content, compared to fulvic acid (FA) and chitosan (Ch) alone. Furthermore, Ch-FANPs mitigated drought dress by modulating stomatal movement and chlorophyll content resulting in maximum biomass. Ch-FANPs protected against drought stress by activating antioxidant defense as indicated by the lowest H₂O₂ and MDA contents and the highest ascorbate peroxidase (APX) activity and soluble protein content in Ch-FANP-treated plants. In addition, Ch-FANPs treatment also resulted in significantly higher expression of drought marker genes OsDREB1A, OsCIPK3, and OsNAC77 under irrigated and drought stress conditions. These results indicate the Ch-FANPs play a key role in mitigating the adverse effects of water scarcity and can be further tailored to mitigate the effects of other stresses.