Carbon nanotubes as novel plant elicitors are intensively studied in biotechnology due to their concentration-dependent effects on plant health. This emphasizes the importance of studying nanomaterials in the field of plant nanotoxicology and enables a better understanding of their advantages and disadvantages for plant health. Researchers examined how various concentrations of multi-walled carbon nanotubes (MWCNTs) affect growth and polyphenolic accumulation in the medicinal herb, Melissa officinalis. Two-month-old plant shoots were sprayed with various concentrations (0–250 mg L−1) of COOH- functionalized MWCNTs and harvested two and three weeks after elicitation. TEM images confirmed MWCNTs uptake into the mesophyll and the vessels of leaves. Low to moderate MWCNT concentrations (50–100 mg L−1) boosted growth indices and increased total amount of phenols, flavonoids, and phenolic acids, peaking three weeks after treatment with 100 mg L−1 MWCNTs, without destroying the cells and subcellular organelles. HPLC analysis showed this treatment yielded the highest content of rosmarinic acid, salvianolic acid B, apigenin, and kaempferol. However, 250 mg L−1 MWCNTs damaged cells without boosting metabolite production. We suggest that elicitation with low to moderate MWCNT concentrations can be a useful tool for laboratory-scale production of phenolic metabolites in M. officinalis.

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