Evaluation of MgO Nanoparticle Foliar Treatment on the Early Stages of Sweet Basil (Ocimum basilicum L.) Vegetation

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.