## 1 Introduction  
Environmental changes generate free radicals and reactive oxygen species (ROS), resulting in abiotic stress in plants and fungi, also acting as natural regulators. This causes alterations in many metabolic pathways, especially those related to oxidative stress management [1]. Both polyphenols and flavonoids are chemical compounds, synthesized by the body, involved in antioxidant defense. They can block reactive oxygen species that lead to the onset of oxidative stress, which disorganizes the cell membrane and cell organs. Increased oxidative stress can lead to degradation of DNA molecules and oxidation of histones [2,3]. Gamma irradiation is itself considered to be a physically induced stress on living organisms or cells. Radiation treatment can be a much faster way to quantitatively improve the chemical synthesis of antioxidant compounds that may play a role in defending irradiated tissue [4,5]. Exposure to solar ultraviolet radiation and to atmospheric ozone activates both enzymatic and non-enzymatic antioxidant defense systems in response. Some flavonoids and phenolic compounds help in protecting the plants and their photosynthetic tissues from UV-B induced damages [1]. Gamma ionizing radiation generates significant amount of free radicals and ROS [6], thereby simulating natural environmental stressors. Fomes fomentarius_ (tinder fungus) is a medicinal lignivorous tough polypore, producing a white-mottled rot mainly on beeches and birches. This wood-decaying macrofungus is well-known for its potential use in a wide range of biotechnological applications [7]. Its perennial woody fruiting body is shown to have cytotoxic activity against murine sarcoma S180 in vitro and to inhibit in vivo tumor growth activity of the same line cell [8]. Its ethanol extract exerts inhibition of cell growth and motility induction of apoptosis via targeting AKT in human breast cancer MDA-MB-231 cells and also decreases cell viability in six cancer cell lines; the fungus is also known to have anticancer, anti-inflammatory, and anti-diabetes effects [9; 10]. According to [PERSON] et al., (2021) [11], the main chemical constituents of this fungus were suggested to be triterpenoids and ergosterols. The authors isolated new structures: two pentacyclic lupane-type triterpenes, 3-formyloxybetulin and 3-formyloxybetulin acid; two rare carbon-degraded ergosterol derivatives, pyropolincisterols (A and B); along with ten known triterpens and four ergosterols. The most significant phenolics in _F. fomentarius_ methanolic extract are benzoic acid, rutin, quercetin and protocatechuic acid; p-hydroxy benzoic acid, catechchin, syringic acid, p-coumaric acid, benzoic acid, cinnamic acid, p-hydroxy benzoic acid, and vanillic acid were also found [12]. Gamma irradiation is an established sterilization method for medical devices and one which is particularly suited to plastics. A more recent application has been in the sterilization of single-use disposable components [13]. Due to DNA sensitivity, gamma irradiation was also used in plant mutation breeding, to generate crop varieties with improved treats [14; 15] by selecting for useful permanent mutation in the genetic material. This latter application was however gradually abandoned in favor of targeted mutagenesis. Nowadays, gamma irradiation treatment for the improvement of various biological-active properties begins to develop as a result of its accessibility, due to the spread of industrial irradiators. We used a \({}^{60}\)Co source of radiation to experimentally induce oxidative stress in living mycelium of _Fomes fomentarius_ fungus, in order to obtain a late response of oxidative stress tolerance after treatment. We measured the response by mean of free radical scavenging capacity (RSC) on DPPH, total phenols, and total flavonoids in methanolic extract, at 24, 48, and 72 h after the irradiation exposure of living mycelium.