Integrative Insights Into UV-B-Induced Growth Regulation, Physiological Responses, Antioxidants, and Essential Oil Remodeling in Ocimum americanum L.

Ocimum americanum L. (American basil or lime basil), an aromatic annual herb of the Lamiaceae family, is widely recognized for its medicinal properties. Traditionally, it has been used in treating cough, respiratory disorders, rheumatism, and renal complications. In the context of global climate change and stratospheric ozone depletion, the increase in ultraviolet B (UV-B) radiation poses a significant threat to plant health and productivity. Understanding how medicinal plants respond to elevated UV-B (eUV-B) is therefore crucial. This study examines the physiological and biochemical responses of O. americanum L. to eUV-B (ambient+7.2 kJ m-2 d-1) at 30, 60, and 90 days after transplantation. Results showed significant reductions in growth, photosynthetic pigments, and overall physiological performance under eUV-B. In contrast, the total phenolic content remarkably increased, indicating activation of secondary metabolism as a protective strategy. eUV-B induced the accumulation of reactive oxygen species (ROS), including superoxide radicals and hydrogen peroxide, resulting in membrane damage and increased electrolyte leakage. Histochemical staining confirmed the localization of ROS in eUV-B-treated leaves. In response to eUV-B, antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) showed significantly enhanced activity across all the developmental stages, suggesting an active defense mechanism. GC-MS analysis further revealed increased levels of bioactive compounds, such as β-Caryophyllene and β-Ocimene, key terpenoids with therapeutic and ecological properties. Overall, O. americanum L. demonstrates adaptive resilience to UV-B stress by strengthening antioxidant defense and enhancing the production of protective specialized metabolites, despite reductions in growth and physiological efficiency.