Abstract
<title>Abstract</title> <p> Background Drought stress is a critical constraint on the growth and productivity of medicinal plants, including <italic>Myrtus communis</italic> L. This study investigated the potential of manganese oxide (MnO) and copper oxide (CuO) nanoparticles to alleviate drought induced damage in two Iranian myrtle genotypes (Khoraman and Fars). A factorial experiment, arranged in a completely randomized design with three replications, was conducted under three irrigation regimes (100%, 80%, and 60% of field capacity) and six nanoparticle treatments: distilled water (control), MnO (25, 50 mg L⁻¹), CuO (25, 50 mg L⁻¹), and combined MnO + CuO (25 mg L⁻¹ each). Results Drought stress significantly reduced shoot and leaf dry weight and relative water content, while increasing electrolyte leakage, malondialdehyde, and proline accumulation. Antioxidant responses, including enzymatic activities (superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase) and non-enzymatic compounds (total phenolics, flavonoids,nd proline), were elevated under water-deficit conditions. Although drought stress increased essential oil content, EO yield declined due to reduced biomass. Foliar application of CuO and MnO nanoparticle, particularly in combination, significantly alleviated the adverse effects of drought by reducing membrane damage (lower electrolyte leakage and malondialdehyde), enhancing osmolyte accumulation, and upregulating antioxidant defense systems. These changes led to improved biomass production and enhanced essential oil content and yield. Notably, CuO nanoparticle had a stronger effect on growth and essential oil yield in the Khoraman genotype, while MnO nanoparticle was more effective in the Fars genotype. The highest essential oil content was obtained under 50 mg L⁻¹ treatments of either MnO or CuO, and under their combined application. Variations in essential oil composition were also observed in response to treatments, though without a consistent pattern. Conclusions Overall, MnO and CuO nanoparticles act synergistically as stress modulators, enhancing drought tolerance, optimizing physiological and biochemical responses, and ultimately promoting growth and secondary metabolite production in <italic>M. communis</italic> . </p>