Silybum marianum is an important medicinal plant with high antioxidant properties. Static magnetic field (SMF) is a kind of abiotic stresses which can affect growth and antioxidant capacity of medicinal plants. In this research, the effects of SMF on total phenolic and flavonoid contents, radical scavenging activity, and anatomical alterations were investigated in S. marianum seedlings. Five-day-old seedlings were treated with different intensities of SMF (0, 2, 4 and 6 mT) for one hour, and then were irrigated with 1/2 Hoagland solution for four weeks. Results showed that SMF up to 4 mT increased significantly fresh weight, dry weight, leaf area, relative water content (RWC), root length and then decreased these parameters at 6 mT. At 4 mT, SMF caused an 125, 132.73 and 52.19% increase of dry weight, leaf area and root length as compared to control, respectively. Metaxylem number and size, aerenchyma formation and stele diameter improved under SMF. Lipid peroxidation (MDA) decreased especially at 4 mT, although H₂O₂ level increased in S. marianum shoots. SMF also induced total phenolic (93.89%) and flavonoid contents (36.43%), and decreased IC₅₀ values (43.01%) especially at 4 mT in leaves. It suggests that SMF at proper frequency could improve antioxidant capacity of plant cells by regulation of H₂O₂ level and membrane stability.

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