Enhanced salinity stress tolerance and growth promotion in Melissa officinalis using caffeic acid nanocomposites as a promising nano-bio-based strategy

The application of caffeic acid (CA) and its nano-composite form (CA-NCs) has great promise for improving growth and abiotic stress tolerance because of the nano-composite formulation’s higher stability, bioavailability, and targeting. Limited research exists on caffeic acid nanocomposites in salinity-sensitive M. officinalis, with unexplored comparative efficacy against free caffeic acid in regulating photosynthesis, osmotic adjustment, and antioxidant activity. Thus, we assessed the potential of CA-NCs on growth and stress tolerance in Melissa officinalis grown during salinity compared to the free form of CA. The CA-NCs were synthesized and characterized through FTIR, TGA, and SEM analyses. CA-NCs demonstrated superior efficacy over free CA in mitigating NaCl stress in plants. Characterization confirmed their small size (99.8 nm), enhancing stability and bioavailability. Under salt stress, CA-NCs significantly improved root and shoot biomass, photosynthetic efficiency, and chlorophyll content. They effectively modulated osmotic stress by regulating relative water content and proline, while strongly reducing oxidative damage markers (MDA, H₂O₂). CA-NCs also boosted the activity of antioxidant enzymes and promoted the synthesis of protective secondary metabolites like flavonoids and anthocyanins. These synergistic effects highlight CA-NCs as a highly promising agricultural tool for enhancing plant growth and abiotic stress tolerance.