Nanotechnology has emerged as a transformative approach in agriculture, offering innovative solutions to mitigate abiotic stresses such as salinity, which significantly impact plant growth and productivity. Chitosan nanoparticles (CSNPs) are non-toxic and biodegradable stimulants of growth and secondary metabolite production, which offer new routes to combat plant stress. Therefore, this study aimed to identify the optimal concentration of CSNPs to mitigate salt stress and evaluate their effects on the physiological traits and performance of marigold (Calendula officinalis L.) plants. Conducted at the University of Maragheh research greenhouse, the factorial experiment employed a completely randomized design with twelve treatments, including three salinity levels (0, 25, and 50 mM NaCl) and four CSNP concentrations (0, 0.05%, 0.1%, and 0.2% w/v). The results demonstrated that CSNPs0.1 significantly enhanced antioxidant enzyme activity, including ascorbate peroxidase (APX), glutathione peroxidase (GPX), and superoxide dismutase (SOD), compared to other concentrations and controls. CSNPs0.1 also led to a 16.3% reduction in MDA content, indicating decreased oxidative stress. Under severe salt stress, CSNPs0.1 treatment resulted in the highest levels of proline and soluble carbohydrates, suggesting effective osmotic adjustment and improved cellular protection. Notably, the highest dry matter content, an increase of 6.8-fold compared to severe stress without CSNPs, was observed with CSNPs0.1 under non-stress conditions. These findings highlight that CSNPs0.1 not only mitigates salt-induced oxidative stress but also significantly enhances growth, stress tolerance, and medicinal quality. This underscores the pivotal role of CSNPs as a sustainable and environmentally friendly solution for reducing salinity stress, thereby advancing agricultural resilience and sustainability in saline environments
