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Drought impacts on food security, land degradation and rates of biodiversity loss. Here, we aimed to investigate selenium nanoparticles (Se NPs) influenced plant resilience to drought using the morphological, physiological, and essential oil (EO) quantity and quality of basil (Ocimum basilicum L.) as drought proxies. Treatments included irrigation at 100% field capacity (FC100) as no stress, 80% FC as moderate water stress (FC80) and 60% FC as severe water stress (FC60), together with application of Se NPs at either 0 mg L−1 (control), 50 mg L−1, or 100 mg L−1. The highest (257 g m−2) and lowest (185 g m−2) dry matter yields were achieved in nil-stress and severe-water-stress conditions, respectively. Dry matter yields decreased by 15% and 28% under moderate and severe water stress, respectively. Applying Se NPs enhanced the dry matter yields by 14% and 13% for the 50 and 100 mg L−1 treatments, respectively. The greatest EO content (1.0%) and EO yield (1.9 g m−2) were observed under severe water stress. Applying Se NPs of 50 and 100 mg L−1 enhanced the essential oil content by 33% and 36% and the essential oil yield by 52% and 53%, respectively. We identified 21 constituents in the EO, with primary constituents being methyl chavicol (40%–44%), linalool (38–42%), and 1,8-cineole (5–6%). The greatest methyl chavicol and linalool concentrations were obtained in FC80 with 50 mg L−1 Se NPs. The highest proline (17 µg g−1 fresh weight) and soluble sugar content (6 mg g−1 fresh weight) were obtained under severe water stress (FC60) for the 50 mg L−1 Se NP treatment. Our results demonstrate that low-concentration Se NPs increase plant tolerance and improve the EO quantity and quality of basil under drought stress.
