Utilization of Chitosan Nanoparticles to Alleviate Chromium Stress and Improve Biochemical Properties and Essential Oil Profile in Thymus vulgaris

Background: Contamination of agricultural soils by heavy metals, particularly Chromium (Cr), poses significant threats to plant growth and food safety. Purpose: This study investigates the effects of chitosan nanoparticles (CSNPs) on Thymus vulgaris L. under Cr stress. Methods: Chitosan nanoparticles were appended to Cr-contaminated soil at concentrations of 0, 10, 20, and 40 mg kg⁻¹, with a range of concentrations of 0%, 0.05%, 0.1%, and 0.2% (w/v). Results: Findings indicated that Cr stress negatively impacted on essential oil (EO) quality and plant growth. Nevertheless, the use of CSNPs greatly reduced these adverse consequences. By boosting the capacity of polyphenol oxidase, phenylalanine ammonia-lyase, superoxide dismutase, guaiacol peroxidase, and peroxidase, the CSNPs improved plant resistance to Cr stress. Furthermore, there was an improvement in total antioxidant capacity as determined by 2,2-diphenyl-1-picrylhydrazyl and ferric reducing tests. Higher amounts of flavonoids, phenolic compounds, anthocyanins, ascorbic acid, and proline were also obtained by using CSNPs, while hydrogen peroxide levels were decreased. Gas chromatography–mass spectrometry (GC–MS) analysis revealed that CSNPs significantly improved the EO profile of thyme. The maximum concentrations of thymol and γ-terpinene were recorded in the Cr10-CSNPs0.1 treatment, while camphene and camphor were maximized in the Cr20-CSNPs0.2 treatment. Moreover, the Cr10-CSNPs0.05 treatment achieved the highest levels of p-cymene and 1,8-cineole. Conclusions: Overall, the most effective results were obtained with a 0.1% (w/v) concentration of CSNPs, which notably enhanced both the biochemical responses and the quality of thyme under Cr toxicity. Generally, applying 0.1% CSNPs is proposed as an efficient approach for enhancing the cultivation of medicinal plants in Cr-contaminated soils.