Oyster mushroom is popular especially because of its delicious taste. Its composition is also very important. Oyster mushroom contains high amounts of proteins and carbohydrates, minerals such as calcium, phosphorus, iron and other, vitamins like thiamine, riboflavin and niacin as well as low fat (Sturion and Oetterer, 1995; Justo et al., 1998; Manzi et al., 1999).

According to Silveira et al. (2006) energy value of P. ostreatus is between 139.36 and 213.05 kcal.100 g^-1 fresh mushrooms. Hyphae are composed of cell wall components such as chitin, other hemicelluloses and β-glucans, which often play a key role in the pharmacological use of mushrooms. For example, in enhancing macrophage function, resisting many bacterial, viral, fungal and parasitic infections, activating non-specific immune stimulation, lowering blood cholesterol levels and blood glucose levels (Cheung, 2009). Although the mushrooms are not plants, they are often included between vegetables in terms of dietary properties. Most species of the genus Pleurotus are known for their healing potential.

Selenium (Se) is an essential element with antioxidant effects. It is an important component of several major metabolic reactions, including synthesis of thyroid hormones, antioxidant defence systems and immune functions (Köhrle and Gärtner, 2009; Gandhi, Nagaraja and Prabhu, 2013). The essentiality of selenium was demonstrated in 1957 (Hegedűs et al., 2006; Hegedűs, Hegedűsová and Šimková, 2007). In 1976, the necessity of selenium for humans was proven, despite the fact that previously were highlighted his negative effects (Hegedűs et al., 2008; Jakabová et al., 2008; Jakabová et al., 2009).

It is therefore clear that mushroom enriched with inorganic forms of selenium can produce functional foods with positive effects on human health, anti-inflammatory and anti-tumour effects (Clark et al., 1996).

Cultivation of saprophytic fungi on substrates rich in selenium may be an effective means of producing food with built-in selenium. The content of selenium in mushrooms is generally higher than most vegetables (Rayman, Infante and Sargent, 2008), but this indication is very variable. In contrast with biologically available selenium concentrations in the soil and the related content of selenium in wild mushrooms, in cultivated mushrooms is the content of the embedded selenium variable depending on the species and their maturity. Its content is also dependent on the type and quality of the substrate (Kalac, 2009). The concentration of selenium in the fruiting bodies of the favourite edible mushroom is in the range from <1 – 20 μg Se.g^-1 dry weight of the fruiting bodies.

The concentration of selenium in the fruiting bodies of mushroom P. djamor produced on high selenium substrates compared to the control substrate without selenium contamination exceeds 800 times the amount, i.e. 141 μg Se.g^-1 weight of dried mushroom. It has been confirmed that as well as other species of genus Pleurotus are able to cumulate selenium (Wang et al., 2005; Estrada et al., 2009; Da Silva et al., 2012).

It is generally known that different species of mushrooms tend to accumulate different substances from the substrate and from the environment, with no exception for risk metals. We evaluated the content of selected risk metals (lead and cadmium) during our experiment. For each cultivation period 40 samples of lyophilized fruiting bodies grown on selenium fortified substrates were analysed. Average values for individual variants are given in Table 2. Dehydrated samples of the control substrate without sodium selenium application were also analysed for the presence of metals (Table 1).

In terms of contamination of the oyster mushroom fruiting bodies by risk metals, currently applicable regulations and decrees set the highest limits only for two risk elements - cadmium and lead. According to Commission Regulation (EU) 2015/1005 of 25 June 2015 amending Regulation (EC) No 1881/2006 as regards maximum levels of lead in certain foodstuffs, the maximum allowed limit for oyster mushroom fruiting bodies intended for consumption is till 0.30 mg.kg^-1 Pb of fresh matter of the fruiting bodies. As regards Commission Regulation (EU) No 488/2014 of 12 May 2014 amending Regulation (EC) No 1881/2006, as regards maximum levels of cadmium in foodstuffs, they were determined at levels of up to 0.20 mg.kg^-1 Cd of fresh fruiting bodies. Since the samples analysed by us were lyophilized, we carried out a conversion to fresh matter of the fruiting bodies. The results are shown in Table 3.

After the calculation we can state, that the critical limits of heavy metals set by the applicable food quality regulations were not exceeded.

Therefore, these fruiting bodies can be considered as a safe food product for the consumer also after application of selenium. In the case of the interaction of selenium and selected heavy metals, it was statistically proven in both cases that the increased content of selenium in the substrate reduces the accumulation of lead and cadmium. Average for both cultivation periods pointed out that in the variant with 2.0 mg.dm^-3 Se was the lead accumulation statistically significant reduced by 64.81%. Reduction in accumulation occurred after application of 1.0 mg.dm^-3 Se (2.47%) too, but it was not statistically significant. In a variant with 0.5 mg.dm^-3 Se, lead accumulation was increased by 19.75%, but it was not statistically significant (p >0.05).

Similar results were also found in the case of cadmium accumulation in fortified oyster mushroom fruiting bodies, on average for both cultivation periods. In variant with 2.0 mg.dm^-3 Se was statistically proven lower cadmium accumulation by 24%, while in a variant with 0.5 mg.dm^-3 Se, its accumulation grew by 4%, but statistically not significant. In variant with 1.0 mg.dm^-3 Se, accumulation of cadmium was statistically not significant decrease by 16%. Based on the above results it can be stated, that the fortification of substrates with selenium not increases the accumulation of selected risk metals statistically significantly in any of the experimental variants. On the other hand, in the variant with 2.0 mg.dm^-3 Se, was statistically proven decreased accumulation of lead and cadmium in the fruiting bodies of edible oyster mushroom. Lepšová (2001) argues that the oyster mushroom cultivated in the wood can accumulate only a small amount of heavy metals from the environment; therefore, there is no need to fear their increased level in the harvested fruiting bodies. The opposite phenomenon can occur in the intensive production of edible mushrooms, where the grain is used as a substrate, which is a by-product of intensive agricultural production.

Stihi et al. (2011) in their study found that wild oyster mushroom in Romania, distant 0.5 km from the source of pollution contained Cr (1.81 mg.kg^-1), Mn (12.4 mg.kg^-1), Fe (387.00 mg.kg^-1), Ni (1.85 mg.kg^-1), Cu (12.5 mg.kg^-1), Zn (41.30 mg.kg^-1), Se (2.64 mg.kg^-1), Cd (0.95 mg.kg^-1) and Pb (0.64 mg.kg^-1) in lyophilized samples. Fruiting bodies in a location 10.5 km distant from the same source of pollution contained only slightly lower concentrations of the monitored elements, namely Cr (1.08 mg.kg^-1), Mn (11.8 mg.kg^-1), Fe (284.00 mg.kg^-1), Ni (1.29 mg.kg^-1), Cu (10.20 mg.kg^-1), Zn (37.90 mg.kg^-1), Se (2.57 mg.kg^-1), Cd (0,87 mg.kg^-1) and Pb (undetectable) in lyophilizates. These results confirm the ability of oyster mushroom to accumulate risk metals. In samples of fruiting bodies fortified with selenium, which were analysed in our work, many lower concentrations of the elements were found. The findings can be explained by a better quality of the production substrate.

Siwulski et al. (2017) on the basis of their research indicate that the lowest levels of Al, Fe, Mn, P and Se were observed in the P. ostreatus 930 strain. The lowest concentrations of Ca, Cr, K, Nd, Te and Zn were determined in P. florida, while in P. pulmonarius, were observed trace amounts of Er, Rh, Sc, Tm and Zr. On the other hand, P. ostreatus strain 930 contained the highest content of Cu and Lu, whereas P. florida of the Rh element. Significant differences were observed in P. ostreatus K 22, P. citrinopileatus and P. eryngii. The highest content of Al, Cr, Er, Fe, Pt, Th, Ti and Tm was determined in P. ostreatus K 22, the elements Mg, Mn, P, Re, Se and U were most represented in P. Citrinopileatus, and P. eryngii accumulated the highest concentration of As, B, Ca, In, Na, Nd and Sr. P. ostreatus strain K 22 contained the lowest concentration of In, P. citrinopileatus lowest concentration of Na and P. eryngii the lowest concentration of Th. From other tested species and strains contained P. ostreatus HK 35 lowest content of As and highest level of Te, P. ostreatus H 195 lowest content of Cd, Pb and U and highest level of Sc. The authors emphasize that the highest concentration of Pb and Cd was determined in fruiting bodies of P. ostreatus 80. The high cumulative potential of the said strain can be specifically used in the soil myco-remediation processes. P. djamor was rated as the most efficient K and Zn accumulator. We confirm the fact that the individual strains of edible oyster mushroom are different from each other in the ability to accumulate selected compounds.

Quarcoo and Adotey (2013) monitored the accumulation of selected heavy metals by oyster mushroom fruiting bodies in Ghana. They found that the fruiting bodies contained on average 0.04 mg.kg^-1 Pb, 0.04 mg.kg^-1 As, 43.77 mg.kg^-1 Fe, 0.35 mg.kg^-1 Cd and 0.04 mg.kg^-1 Hg in a lyophilized mass. Our samples of the fortified fruiting bodies contained lower concentrations of these elements.

Kaya and Bag (2010) analysed 24 species of mushrooms occurring in Turkey. In the case of Pleurotus ostreatus, significant levels of heavy metals were found, namely 20.87 mg.kg^-1 Al, 0.41 mg.kg^-1 B, 2.11 mg.kg^-1 Cd, 0.90 mg.kg^-1 Co, 0.21 mg.kg^-1 Cr, 39.36 mg.kg^-1 Cu, 40.57 mg.kg^-1 Fe, 4.22 mg.kg^-1 Mn, 1.23 mg.kg^-1 Ni, 2.14 mg.kg^-1 Pb and 86.83 mg.kg^-1 Zn in lyophilizate. Again, we conclude that the concentration of selected elements was lower in our experiment.

Demirbas (2001) found in the dry matter fruiting bodies of the edible oyster mushroom Pb (3.24 mg.kg^-1), Cd (1.28 mg.kg^-1), Hg (0.42 mg.kg^-1), Cu (13.6 mg.kg^-1), Mn (6.27 mg.kg^-1), Zn (29.8 mg.kg^-1) and Fe (81.6 mg.kg^-1).

Tuzen, Ozdemir and Demirbas (1998) determined in the dry matter of edible fruiting bodies Pb (0.11 mg.kg^-1), Cd (0.55 mg.kg^-1), Hg (0.31 mg.kg^-1), Fe (48.6 mg.kg^-1), Cu (5.0 mg.kg^-1), Mn (10.3 mg.kg^-1) and Zn (19.3 mg.kg^-1).

Lasota, Florezak and Karmanska (1990) prove Cd (11.2 mg.kg^-1), Hg (1.2 mg.kg^-1), Zn (0.8 mg.kg^-1) and Pb (0.0 mg.kg^-1) in lyophilized fruiting bodies of oyster mushroom.

All of the above-mentioned findings of a large number of authors point to the fact that edible mushrooms are able to accumulate heavy metals from the environment and the from the substrate into the fruiting bodies, which can greatly worsen the quality of the production. Our argument explains the different concentrations of the rated elements compared to the cited authors, as the content of the selected elements in the concrete fruiting bodies is directly related to the content of the analysed element in the growing substrate. For the intensive production of edible mushroom is important to use only high-quality lignocellulosic material analysed for the content of risk metals. From the point of view of the accumulation of selected metals, we recommended to fortify the cultivation substrate of oyster mushroom with the selenium in order to produce a functional and uncontaminated foodstuff.

At work we monitored the accumulation of risk elements – lead and cadmium in the oyster mushroom fruiting bodies. We found that the limit set by the European Commission Regulations, which concern about the content of lead and cadmium in oyster mushroom fruiting bodies has not been exceeded. Fruiting bodies are of satisfactory quality and are suitable for daily consumption.

In the experiment, the reduction of lead and cadmium accumulation was statistically confirmed after application of 2.0 mg.dm^-3 Se. Cadmium content decreased by 22.45% and lead by 64.81% compared to the control variant. From the results it is clear that by fortification of the substrate (the main component of which is straw) by selenium, we produce not only a high-quality food with a high antioxidant potential, but we can also prevent the accumulation of risk elements of lead and cadmium from the substrate.