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Ultraviolet‐B (UV‐B) radiation is one of the abiotic stresses that can significantly affect the secondary metabolite accumulation in in vitro tissue cultures of medicinal plants. The present study investigated the effects of UV‐B radiation on the secondary metabolites and antioxidant activities of Scutellaria baicalensis in vitro shoots grown at different concentrations of 6‐benzyl aminopurine (6‐BA), which is the cytokinin most widely used in plant tissue culture. The UV‐B radiation caused significant increases in lipid peroxidation, total phenolic, and flavonoid contents, and antioxidant activities in the in vitro shoots grown at lower 6‐BA concentrations (0 and 1 mg L−1), while it did not cause any significant changes in those grown at higher 6‐BA concentrations (2 and 3 mg L−1). However, the UV‐B radiation significantly altered the contents of main individual flavonoids at both lower and higher 6‐BA concentrations. Upon UV‐B radiation, aglycones (including baicalein, wogonin, and scutellarein) increased, while glucuronides such as baicalin and wogonoside decreased; this was more evident at higher 6‐BA concentrations. This study demonstrated that the effects of UV‐B radiation on the secondary metabolites of S. baicalensis in vitro shoots highly depended on the 6‐BA concentration in the culture medium.

Melatonin is a stress‐related hormone that plays a critical role in triggering the plant defence system and regulating secondary metabolism when plants are exposed to stress. To explore the potential roles of melatonin in response to Ultraviolet‐B (UV‐B) radiation, we examined the effects of exogenous melatonin on rosemary in vitro shoots under UV‐B stress. The application of melatonin (50 μM) alleviated the adverse effects of UV‐B stress on the biomass, photosynthetic pigment contents, and membrane lipids of the rosemary in vitro shoots. Melatonin significantly increased superoxide dismutase (1.15.1.1, SOD), peroxidase (1.11.1.7, POD), and catalase (1.11.1.6, CAT) activities by 62%, 99%, and 53%, respectively. The contents of total phenols, rosmarinic acid, and carnosic acid increased under UV‐B stress, and they further increased by the melatonin treatment by 41%, 68%, and 67%, respectively, compared with the control group. Under UV‐B stress, the increased total phenol content in melatonin‐pretreated plants could be attributed to the activation of phenylalanine ammonia‐lyase (4.3.1.5, PAL) and tyrosine aminotransferase (2.6.1.5, TAT). In addition, melatonin enhanced the antioxidant and antibacterial activities of the rosemary in vitro shoots under UV‐B stress. These results suggest that melatonin can alleviate the damage caused by UV‐B stress and also enhance the secondary metabolism and bioactivity of rosemary in vitro shoots.

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