## 4 Discussion
The ascorbate-glutathione (AsA-GSH) pathway comprises these two antioxidants and four enzymes--APX, MDHR, DHAR, and GR--that play a key role in ROS detoxification and interact with other plant defense systems to protect plants from oxidative stress-induced damage [47; 48]. Ascorbate and glutathione are among the most important hydrophilic antioxidants in all living tissues. In this study, the effect of AMF/PGPR biofertilizer on the concentration of ascorbate, glutathione, and the glutathione precursor L-cysteine varied depending on the cultivar, tissue type, and growing season. A strong inoculation effect in relation to the concentration of glutathione, L-cysteine, and ascorbate was found in roots in 2012, but in 2013 this relationship depended on the cultivar and concerned only thiol compounds. Similarly, in _Citrus tangerine_ roots, the GSH content was positively correlated with AMF colonization, while a significantly higher content of ascorbate in mycorrhizal plants only occurred in conditions of water deficiency [16]. An enhanced antioxidant system and glutathione-ascorbate cycle activation were observedin colonized barley root [49]. The leaves of inoculated apple plants consistently expressed an increasing content of glutathione in both years, while this was only true for L-cysteine in the second year of the study. Leaf ascorbate was highly year and cultivar dependent, but in both the examined years 'Chopin' exhibited a large increase in leaf ascorbate in inoculated plants. There was no AMF/PGPR effect on the ascorbate and thiol compound contents in apple peel as an indicator of nutritional value in apple. No effect has been found on ascorbic acid and glutathione in mycorrhizal tomato [50], with ascorbate concentration decreasing [13]. More examples, mainly concerning vegetables, can be found in reviews by [PERSON] et al. [2] and [PERSON] et al. [30]. However, the results for ascorbate and less frequently tested glutathione contents depended on the species, cultivar or inoculum composition and varied considerably; an increase, no effect or decrease were noted. No research could be found on the glutathione-ascorbate cycle in AMF or co-inoculated by AMF+PGPR apple trees. Another group of secondary metabolites with diverse biological activities in plants and benefits for human health as protective dietary agents are phenolic compounds. Potential functions, mechanisms of action, and applications of polyphen in relation to both plant protection and human health have recently been updated [51]. The content and distribution of phenolic compounds in apple in relation to several factors including stresses have been the subject of many studies [52; 53; 54]. According to several reports, AMF/PGPR colonization can induce both a local and systemic increase in phenolic accumulation in roots [19; 55], rhizomes [25], leaves [56; 57; 58], and flower heads [56]. Salicylic acid and H\({}_{2}\)O\({}_{2}\) are proposed as signaling molecules involved in these phenomena [19; 46]. In contrast, [PERSON] et al. [59] state that the concentration of anthocyanins in _Fragaria ananassa_ fruit was generally higher in treatments that were not inoculated with AMF. It should be added that all the above studies were conducted in controlled greenhouse conditions. In a three-year field experiment conducted with inoculated 'Topaz' trees, the authors found that the inoculation effect on phenolic content depended on the inoculum type. Of three inocula used, only one increased the phenolic concentrations in apples [14]. Mycorrhizal inoculum resulted in an increase in total flavanols, total anthocyanins, and total polyphenols in berry skin (_Vitis vinifera_ L) in two experimental years [28]. In this field study, global phenolic contents were not greatly influenced by the applied inoculum in the apple cultivars tested. A significant increase in phenolics of treated plants was found in leaves, but only in 2013, and no effect was found in roots or apple peel. According to [PERSON] et al.