Exploring the stimulatory effects of graphene oxide-coated zinc oxide nanoparticles on Thymus daenensis Celak. callus under salinity stress: insights into biochemical and morphological changes

Graphene oxide-coated nanoparticles may modulate the combat against abiotic stress in plants. The chemical synthesis of graphene oxide-coated zinc oxide nanoparticles (ZnO-GO NPs) was confirmed by FT-IR, XRD, and SEM techniques. Then, the effects of ZnO-GO NPs (0, 100, 300, and 450 µg mL-1) on callus of Thymus daenensis were evaluated under non-saline (0 mM NaCl) and salinity stress (100 mM NaCl) for the first time. Salinity stress caused a significant increase (P< 0.01) in the content of all studied traits. Under the combined effects of salinity and ZnO-GO NPs treatments, the highest total phenolics content (TPC) (243.35 mg GAE g-1 DW), total flavonoids (TFD) content (54.92 mg QE g-1 DW), and glutathione activity (0.17 mg g-1 FW) were recorded at 100 µg mL-1, while the maximum glycine betaine (0.83 µmol g-1 DW) and catalase activity (0.194 U mg-1 protein) were found at 300 µg mL-1. The highest antioxidant activity (through DPPH assay) (85.52%) and malondialdehyde (3.92 µmol g-1 FW) content were found at 450 µg mL-1 ZnO-GO NPs. Under both non-saline and saline stress conditions, a dose-dependent trend was observed in calli cultures for TPC, TFD, glutathione, glycine, betaine, and catalase activity, attributed to oxidative damage caused by the toxic concentration of the NPs used. This study acknowledges the modulatory impacts of ZnO-GO NPs in eliciting the specialized metabolites biosynthesis (total phenolics and total flavonoids), osmoprotectants activity, and highlights their adverse effects on salinity tolerance. The novel findings of this study would provide useful contributions to additional studies on toxicity assessment of this new nanoparticle in phytochemical and industrial fields.