Enhancing plant growth, polyphenols accumulation, and bioactivity of Mentha rotundifolia L. by dual application of exogenous indole acetic acid and adenine-like cytokinin

The present work explores the effects of combined treatments with two phytohormones, adenine-type cytokinin (AD) and indole acetic acid (IAA), on the growth, phenolic content, and biological activities of Mentha rotundifolia L. Four combinations of IAA and AD, which consist of 10 mg/L and/or 20 mg/L (T1, T2, T3, and T4), were applied to the plant and parameters including fresh weight, dry weight, polyphenols contents, HPLC profile, and the activity of the extracts were determined and compared to a control (untreated plants). The application of phytohormones significantly improved plant fresh and dry weights. Total phenolic contents were also increased for all treatments (a maximum of 235.7 µg GAE/mL in the aerial parts and 252.69 µg GAE /mL in the roots were recorded with the treatment T4), while flavonoids and flavonols contents varied according to the type of treatment and the plant organ. Treatments T1 and T4 induced the most significant increase in the main phenolic acids of the aerial part (quinic acid, rosmarinic acid and salvialonic acid), while individual flavonoids were mainly promoted by the treatment T4 in both organs. The antioxidant and enzyme inhibitory effects of the extracts were also enhanced for the treated samples. Naringin, rosmarinic acid and salvianolic acid B were suggested as the major phenolics contributing to AChE and α-glucosidase inhibition exhibiting low binding energies (−9.8 and −10.5, − 8.9 and −8.4, − 8.8 and − 9.8 respectively) and their contents were highest in the treatments giving the strongest inhibitory effect. The various discoveries affirm the possible use of IAA/AD in combination to induce the development and accumulation of polyphenols in Mentha rotundifolia L. and therefore the improvement of its pharmacological effects.