Salinity is a major abiotic stress that impairs plant growth and limits agricultural productivity. This study investigated the potential of rosemary essential oil-loaded chitosan nanoparticles (REO–CSNPs) to alleviate the detrimental effects of salt stress in lettuce (Lactuca sativa L.). The experiment was conducted in a greenhouse using a completely randomized design with three replications. Lettuce plants were exposed to varying salinity levels and treated with REO–CSNPs, chitosan nanoparticles, or rosemary essential oil. Salinity stress significantly reduced root and shoot biomass, chlorophyll content, and uptake of nitrogen, phosphorus, and potassium, while increasing levels of oxidative markers such as malondialdehyde (MDA) and hydrogen peroxide (H₂O₂). In contrast, foliar application of REO–CSNPs, particularly at 500 ppm, effectively improved plant growth and physiological status. This treatment enhanced nutrient accumulation, osmoprotectants (proline and soluble carbohydrates), and secondary metabolites (phenolics and flavonoids), and it significantly boosted the activities of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), and peroxidase (POX). Moreover, it reduced MDA and H₂O₂ levels, indicating decreased oxidative damage. In summary, these findings underscore the potential of REO–CSNPs as a sustainable biostimulant in saline environments and pave the way for future research into nanobiotechnology-based strategies to enhance crop resilience under abiotic stress conditions.
