Drought is a key abiotic stress that limits the growth and productivity of medicinal and aromatic plants; however, strategies to improve tolerance under water-limited conditions remain scarce. Essential micronutrients play a vital role in protecting plant physiological and metabolic functions under stress, and nanoparticles (NPs) have emerged as a promising approach to enhance nutrient delivery and stress resilience. This study investigated the effects of zinc oxide (ZnO) and manganese oxide (MnO), applied in bulk or NPs forms, on the morpho-physiological and biochemical responses of Origanum majorana L. under open-arid field conditions over two consecutive growing seasons (2023–2024). All plants were grown under naturally arid field conditions, experiencing moderate drought at approximately 50% of field capacity throughout the experimental period. A uniform drip irrigation regime was applied every six days to eliminate differences in water availability and to assess the effects of nanoparticles under consistent drought stress. Drought conditions moderately reduced plant growth and induced oxidative stress, as indicated by elevated MDA and H₂O₂ levels and increased CAT and SOD activities. Foliar applications of ZnO and MnO NPs alleviated oxidative damage. These treatments promoted growth, resulting in a reduction in excessive CAT (–32.4%) and SOD (–33.1%) activities compared to the control, indicating a lower oxidative burden rather than a weakened antioxidant defense. The combined NP treatment (ZnO + MnO) exhibited the most pronounced effects, markedly reducing H₂O₂ and MDA accumulation and restoring redox homeostasis. Moreover, it rebalanced hormonal dynamics, elevating indole-3-acetic acid (IAA) by (+47.9%) and markedly suppressing abscisic acid (ABA) by (-79.7%), thereby coordinating redox and hormonal networks to improve drought resilience. Moreover, this treatment enhanced photosynthetic rate by 56%, stomatal conductance by 19%, and water-use efficiency by 158% compared with the control, while reducing transpiration rate by 53%. It further increased chlorophyll content by 18%, total carbohydrates by 46%, proteins by 14%, phenolics by 51%, and flavonoids by 54%, alongside improving nutrient uptake and essential oil yield and composition, particularly through higher terpinen-4-ol and linalool contents. These results highlight the synergistic potential of ZnO and MnO nanoparticles (NPs) as sustainable nanofertilizers that enhance drought tolerance and secondary metabolite biosynthesis in O. majorana, providing a potential strategy to improve the productivity and quality of medicinal and aromatic plants under water-limited conditions.
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