Inonotus obliquus is a parasite on the birch and other trees and is also a well-known medicinal mushroom. Its sterile conk is highly sought for its bioactive compounds such as phenols, polysaccharides, triterpenoids, and steroids. It was traditionally used to treat various gastrointestinal diseases, viral and parasitic infections, to counteract the progression of cancers, and to stimulate the immune system. We used acute gamma irradiation, followed by short-term submerged cultivation, as an oxidative stress inducer to enhance the synthesis of mycelial metabolites. The 300 Gy and 400 Gy doses showed the best results across the whole experimental design. Each assayed criterion had a different corresponding optimal stimulation dose. In one experiment, sublethal doses of irradiation triggered the dry weight of the cultured mycelium to increase by 19.764%. The free radical scavenging potential of the mycelium extracts increased by 79.83%. The total phenolic content of mycelium extracts and culture broth increased by 55.7% and 62.987%, respectively. The total flavonoid and sinapinic acid content of the broth increased by 934.678% and 590.395%, respectively. As such, gamma irradiation pre-treatment of the mycelial inoculum proved an interesting, economically and environmentally effective tool for stimulating secondary metabolite synthesis in submerged mycelium cultures.
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