Carbon quantum dots and mycorrhizal fungi improve cadmium stress tolerance and essential oil production in grapefruit mint (Mentha suaveolens × piperita)

Cadmium (Cd) is a non-essential, highly phytotoxic element and a widespread pollutant in agricultural soils. Employing biofertilizers and nanoparticles (NPs) has been identified as an effective strategy to alleviate the detrimental impacts of heavy metal (HM)-contaminated soils. Previous studies have provided partial insights into the individual roles of CQD‑NPs and AMF under drought stress; however, information on their combined effects is still limited and fragmented. Therefore, the present study aimed to evaluate the individual and synergistic effects of CQD-NPs and AMF on the yield and EO quality of grapefruit mint under Cd-induced stress. To evaluate this, a factorial experiment was conducted in a completely randomized design (CRD) with 32 treatments and three replications. The first factor consisted of four Cd concentrations (0, 10, 20, and 40 mg Cd kg −1 dry soil), the second factor included CQD-NPs at four levels (0, 5, 10, and 20 mg L −1 ), and the third factor was inoculation versus non-inoculation with AMF. The results indicated that increasing Cd stress significantly reduced dry weight (DW), chlorophylls (Chl), relative water content (RWC), root colonization, and total soluble protein (TSPC) content. However, the synergistic effect of CQD-NPs and AMF enhanced DW, RWC, TSPC, and Chl regeneration. This combination also increased essential oil (EO) content, decreased malondialdehyde (MDA) and hydrogen peroxide (H 2 O 2 ) levels. It modulated the activities of antioxidant enzymes, including superoxide dismutase (SOD), ascorbate peroxidase (APX), and guaiacol peroxidase (GPX), under both non-stress and Cd-stress conditions. Notably, the maximum EO content (1.90%) was recorded under moderate Cd stress when plants were treated with AMF + CQD-NPs 20 . GC–MS analysis of EO revealed that linalool (48.50%) and linalyl acetate (44.72%) were the predominant volatile constituents of grapefruit mint. The highest amount of linalool and linalyl acetate was obtained under severe and moderate Cd stress, respectively, in plants treated with AMF + CQD-NPs 20 . Overall, under severe and moderate stress conditions, the combined application of CQD-NPs 20 and AMF increased the contents of linalool and linalyl acetate by 34.23% and 32.81%, respectively, compared with the control treatment. These findings suggest that the combined use of CQD-NPs and AMF can alleviate Cd-induced stress and serve as a sustainable strategy for the eco-friendly production of medicinal plants.