Nano-encapsulation of peppermint essential oil in chitosan nanoparticles improves salinity tolerance in lettuce

Nanotechnology-based delivery systems are emerging as efficient tools for mitigating salinity-induced constraints in plants through the targeted and controlled release of bioactive compounds. However, the mechanistic role of peppermint essential oil-loaded chitosan nanoparticles (P-EO-CSNPs) in regulating salinity stress responses remains poorly understood. Accordingly, this study investigated the physiological, biochemical, and nutritional responses of lettuce ( Lactuca sativa L.) to foliar application of P-EO-CSNPs under salinity stress. A factorial experiment was conducted at three salinity levels (0, 25, and 50 mM NaCl) and five foliar treatments: distilled water (control), P-EO-CSNPs at 500 and 1000 ppm, peppermint essential oil at 1000 ppm, and chitosan nanoparticles at 0.5%. Salinity stress at 50 mM NaCl significantly reduced growth traits and photosynthetic pigment accumulation while increasing oxidative stress markers, including hydrogen peroxide and malondialdehyde. In contrast, foliar application of P-EO-CSNPs, particularly at 500 ppm, significantly alleviated stress effects by improving nitrogen, phosphorus, and potassium uptake by 94%, 230%, and 76%, respectively. This improvement was accompanied by enhanced antioxidant enzyme activities, including ascorbate peroxidase, guaiacol peroxidase, and superoxide dismutase, which increased by 219%, 225%, and 165%, respectively. Furthermore, levels of total phenolics, total flavonoids, ascorbic acid, and soluble proteins were markedly elevated, indicating strengthened metabolic and antioxidant capacity. These coordinated responses led to a reduction in hydrogen peroxide and malondialdehyde by 163% and 62%, respectively, reflecting improved redox homeostasis under salinity stress. Importantly, these physiological and biochemical improvements translated into enhanced plant performance, including increased root and shoot biomass, leaf number, stem diameter, and lettuce head fresh weight. P-EO-CSNPs at 500 ppm were more effective than the separate applications of peppermint essential oil and chitosan nanoparticles, mitigating salinity-induced damage in lettuce by enhancing antioxidant defense, improving osmotic regulation, and maintaining nutrient homeostasis. These findings highlight the potential of nano-encapsulation as an efficient strategy to improve plant tolerance to salinity stress. However, these results were obtained under greenhouse conditions and require further validation in the field.