## 4 Discussion
Hence, the year effect in perennial plants may be of greater importance and increase the difficulty of interpreting the results than is the case in annual plants. With reference to the above, the 'Topaz' and 'Odra' cultivars displayed the greatest increase in root GR activity in the first of the seasons tested, while this was the true of 'Chopin' in the second season. The same regularity was recorded in the case of leaf GR activity in 'Topaz' and 'Odra', while 'Chopin' in both the years studied expressed the same differences in leaf GR between inoculated and non-inoculated plants. Cultivar differences in mycorrhizal frequency, absolute mycorrhizal intensity, and relative mycorrhizal intensity were more noticeable in 2012 than in 2013 [15]. In fact, in 2013 there were no cultivar differences in mycorrhizal parameters [15]. Thus, in addition to the inoculation effect, the age of the plants and year effect could be involved in tree antioxidant expression or could be related to differences in the endogenous antioxidant levels of cultivars. The large number of apple cultivars available on the market also means a broad range of variability in quality traits. Lastly, a substantial variation can be expected due to agotechnical approaches, including the effects of biofertilizers [31; 32]. The picture of changes in CAT activity was more inconsistent than those in GR activity. The roots and apple peel of inoculated apple trees expressed significantly higher CAT activity in the first growing season only, irrespective of the cultivar examined. This effect was not found in 2013. Leaf CAT activity was considerably higher in control plants in 2012, while in 2013 the increase in CAT activity due to inoculation was exhibited in 'Chopin' and 'Odra' leaves. Several studies describe the relationship between enzymatic antioxidants such as GR, CAT, SOD, APX, POD and plant AMF/PGPR symbioses [12; 16; 17; 22; 23; 26]. It should be noted that in these studies the microbial inoculation effect on antioxidative enzyme activity was investigated in relation to various abiotic stresses, such as drought [22], salinity [12; 26], water stress [16; 17], and herbicide [23]. [PERSON] et al. [22] emphasize that of several enzymes tested (APX, SOD, CAT, GR), only GR activity was higher in roots and the nodule of mycorrhizal soybean plants than in non-mycorrhizal plants. Mycorrhizal citrus roots in a pot culture demonstrated an increase in GR activity [16]. In turn, AMF colonization was accompanied by an enhancement of activity CAT in the leaves of _Lycopersicon esculent_ L. [12], _Elaagmus angustifolia_ L. [26], and _Olea europaea_[17]. Inconsistent results regarding leaf CAT activity as an effect of AMF in soybean plants and oxidative stress responses generated by paraquat have been reported by [PERSON] et al. [23]. As the increase in enzyme activity in inoculated plants is frequently accompanied by lower levels of malondialdehydedehyde (MDA), H\({}_{2}\)O\({}_{2}\), and superoxide (O\({}_{2}\). ), an assumption was made that AMF mitigates oxidative stress due to the activation of several enzymatic antioxidants in an adverse environment [16; 17]. Assuming that organic production generates several stresses since this system excludes agrochemical applications [36], the presented results also seemed to confirm the above statement. No research on apple trees could be found that was more directly comparable with the results presented in this study. Higher activity of antioxidant enzymes is frequently accompanied by enhanced synthesis of non-enzymatic antioxidants that are important for both plant and human health due to their involvement in oxidative stress regulation and many other regulatory functions. The beneficial effects of mycorrhizal symbiosis on the yield and fruit quality of crops, however, can vary depending on internal (plant genotype) and external factors (overall experiment conditions) as well as fungal strains [2; 29; 30].