Mentha piperita L., which is an abundant source of essential oils (EO) and phenolic acids, is well known for its medicinal significance. The present research aimed to evaluate the impact of various concentrations of methyl jasmonate (MeJA; 0, 0.1, and 0.5 mM), titanium dioxide nanoparticles (TiO2 NPs; 0 and 150 mg L−1), and salicylic acid (SA; 0, 0.1, and 1 mM) on growth, EOs, and phenolic compounds of M. piperita L. The results demonstrated that the simultaneous application of SA (0.1 mM) and TiO2 NPs (150 mg L−1) enhanced shoot dry weight, the shoot length, and membrane stability index of peppermint by 56.17, 19.52, and 36%, respectively, compared to control. Moreover, phenolic content (76%), caffeic acid content (78%), rosmarinic acid content (87%), 2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging ability (78%), and catalase (155%), ascorbate peroxidase activities (95%) were further improved by simultaneously applying MeJA (0.1 mM) and TiO2 NPs (150 mg L−1) compared to control. The highest menthol production (44.51%) was obtained with exogenous application of MeJA (0.1 mM) with 150 mg L−1 TiO2 NPs. The findings of the current study presented an ideal combination of TiO2 NPs with plant growth regulators for promoting antioxidant activities and increasing major components of EO in peppermint plants.

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