Proline-Functionalized Graphene Oxide Nanoparticles (GO-Pro NPs) Mitigate Salt-Induced Adverse Effects on Morpho-Physiological Traits and Essential Oils Constituents in Moldavian Balm (Dracocephalum moldavica L.)

Many methodologies have been established to lessen negative impacts of salinity on plants. Of those methodologies, nanoparticles (NPs) application has achieved great importance thanks to their unique physico-chemical properties. Consequently, formerly respecting encouraging impacts of graphene oxide (GO) and proline (Pro) on different plant processes under non-stress and stress conditions, proline-functionalized graphene oxide nanoparticles "GO-Pro NPs" were synthesized and characterized. Graphite powder, as starting material, was used to synthesize GO using modified Hummers method followed by functionalization of its surface by proline in basic media. Afterward, GO-Pro NPs, GO and Pro, each at 0, 50 and 100 mg L-1 concentrations with three replications, were applied on Moldavian balm (Dracocephalum moldavica L.) plants to assay their effects under non-stress (0 mM) and salt stress (50 and 100 mM) conditions. GO-Pro NPs and Pro effectively alleviated negative effects of salinity through increasing morphological parameters, photosynthetic pigments, chlorophyll fluorescence parameters, chlorophyll index (SPAD), and membrane stability index (MSI) and decreasing hydrogen peroxide and malondialdehyde, as well. Also application of GO-Pro NPs enhanced proline, antioxidant enzymes activities, and most dominant constituents of essential oil. The highest MSI (48.87%) and proline content (15.36 mu M g(-1) FW) were observed in plant treated with GO-Pro NPs (50 mg L-1) under 100 mM NaCl salinity stress. The GO-Pro NPs treatment at lower dose (50 mg L-1) could be introduced as the best preservative treatment for Moldavian balm under salt stress. GO application mostly had no effect on the measured parameters announcing it as carrier for Pro to enhance its efficiency. In conclusion, GO-Pro NPs application could promote Moldavian balm performance and essential oil under salinity presenting GO-Pro NPs as new treatment against stress conditions.