Optimizing Antioxidant Activity and Phytochemical Properties of Peppermint ( Mentha piperita L.) by Integrative Application of Biofertilizer and Stress-Modulating Nanoparticles under Drought Stress Conditions.

Drought stress (DS) negatively affects plant growth, productivity, and quality in semi-arid and arid regions. Nowadays, application of biofertilizers and stress-modulating nanoparticles (NPs) improves plant performance under stressful conditions. The study evaluated the impacts of arbuscular mycorrhizal fungi (Myco-Root) and TiO 2 NPs on the nutrient uptake, dry yield, essential oil (EO) productivity, and EO quality of peppermint ( Mentha piperita L.) under different irrigation regimes. The treatments included three irrigation regimes containing irrigation after 20% (I 20 , well-watered), 40% (I 40 , mild DS), and 60% (I 60 , severe DS) maximum allowable depletion (MAD) percentage of the soil's available water as well as four fertilizer sources contain no fertilization (control), Myco-Root biofertilizer, TiO 2 NPs, and an integrative application of Myco-Root + TiO 2 NPs. The results demonstrated that the highest (195.72 g m -2 ) and the lowest dry yield (78.76 g m -2 ) of peppermint was obtained in well-watered conditions with integrative application of Myco-Root + TiO 2 NPs and severe drought stress (I 60 ) without fertilization, respectively. The dry yield of peppermint was reduced by 27.7 and 53.4% in mild (I 40 ) and severe drought stress (I 60 ), respectively. The maximum EO content (1.49%) and EO yield (2.30 g m -2 ) was recorded in mild drought stress (I 40 ) treated with Myco-Root + TiO 2 NPs. Based on the GC-MS and GC-FID analysis, 29 constituents were identified in peppermint EO, with the major constituents being menthol (38.99-52%), menthone (12.72-20.13%), 1,8-cineole (6.55-7.84%), and neo -menthol (3.14-4.52%), respectively. The maximum content of menthol, 1,8-cineole, and neo -menthol was obtained under mild drought stress (I 40 ) fertilized with Myco-Root + TiO 2 NPs. The results indicate that the integrative application of Myco-Root + TiO 2 NPs could be used as an alternative method of using chemical fertilizers in sustainable agricultural systems for improving the EO quantity and quality of peppermint grown under drought stress conditions.