Graphene Oxide‐Coated Zinc Sulfide NPs Stimulate the Antioxidant Activity and Biochemical Traits of <i>Thymus daenensis</i> Celak Under Salinity Stress: An In Vitro Assay

In the present study, the impact of synthesized graphene oxide-coated zinc sulfide nanoparticles (graphene oxide [GO]-zinc sulfide [ZnS] nanoparticles [NPs]) (100, 300, and 450 µg mL^-1) as a novel elicitor was evaluated on different biochemical traits in callus culture of Thymus daenensis Celak under salinity stress (150 mM NaCl) and non-saline (0 mM NaCl) conditions. Elicitation with GO-ZnS NP showed an increasing effect on H₂O₂, enzymatic, and nonenzymatic antioxidant activity under saline and non-saline conditions. The concentration of 100 µg mL^-1 GO-ZnS NPs produced the highest total phenolics content (TPC) (4.67-fold), phenylalanine ammonia-lyase activity (PAL) (9.85-fold), ascorbate peroxidase (APX) (3.07-fold), catalase (CAT) (1.68-fold), and antioxidant activity (Ferric Reducing Antioxidant Power [FRAP] assay) (7.42-fold), compared to the control treatment (non-salinity and non-GO-ZnS NPs). The total flavonoids (TFD) (2.26-fold) and thymol (17.33-fold) contents showed the highest increase under 300 µg mL^-1 GO-ZnS NPs and salinity stress. Parallel with the significant increase in H₂O₂ activity (4.86 µmol g^-1 FW), the antioxidant activity (2,2-diphenyl-1-picrylhydrazyl [DPPH] assay) increased the most (91.42%) under the combined effects of 450 µg mL^-1 GO-ZnS NPs and salinity stress. The results suggest that 100 µg GO-ZnS NPs mL^-1 might be used as a new elicitor to increase the biosynthesis of total phenolics, thymol, and activity of PAL, APX, and CAT in T. daenensis through callus culture under salinity stress.