Magnesium is essential for photosynthesis and may enhance plant stress tolerance and secondary metabolism, making Mg-based treatments relevant for savory herb production. This study evaluated the effects of foliar application of 5–20 nm MgO nanoparticles (MgO-NPs) at 75, 200, 300, and 600 mg L−1; MgSO4 at 20 g L−1; and aggregated MgO at 300 mg L−1 (Bulk) on sweet basil (Ocimum basilicum L.) plants grown under total controlled environment conditions. Treatments were applied once at the two-true-leaf stage. MgSO4 increased only plant height by 15%, but fresh weight (FW) was not increased. MgO-NPs had no effect on these parameters. However, 600 mg L−1 MgO-NPs and bulk MgO treatments reduced FW by 41% and 39%, respectively. Chlorophyll b content increased in all treatment variants, while anthocyanins increased only in variants with 600 mg L−1 MgO-NPs and MgSO4 treatments. Higher MgO-NP doses induced oxidative stress, reflected by elevated H2O2 and activation of catalase and ascorbate peroxidase. Bulk caused the highest H2O2 accumulation and reduced soluble protein content by 26%. MgO-NPs (600 mg L−1) increased essential oil concentration by 61%, but not oil yield per plant. High-dose MgO-NPs acted as elicitors of essential oil accumulation, offering a potential strategy for industrial essential oil production despite biomass reduction.
Zakharov et al. (Sun,) studied this question.