## 4 Discussion  
One challenge facing contemporary horticulture is to ensure a sufficient amount of high-quality food for the world's growing population without aggravating existing environmental threats [44]. Health-promoting and protecting attributes of phytochemicals in edible horticultural crops have also led to an intensive search for different ways of increasing their contents. One promising approach in sustainable intensification seems to be the use of microorganism-based products called biofertilizers/biostimulants [7; 10; 13; 28]. AMF or the co-inoculation of PGPR and AMF of plant roots intensifies the oxidant scavenging system in host plants, and thus minimizes damage at a cellular level in stressful conditions [45]. Thus, inoculated plants exposed to biotic or abiotic stresses display lower hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) and superoxide (O\({}_{2}\)) contents and lower lipid peroxidation with a simultaneous rise in enzymatic and/or non-enzymatic antioxidants compared with control plants [16; 19; 22; 24; 46]. An increase in the content of antioxidants in inoculated plants is also reported as a possible way to enhance the biological value of crop plants. Papers summarizing research on the role of AMF and PGPR in shaping health-promoting properties of plant products have recently been published [2; 30]. Investigations on phytochemical contents in food crops as affected by bacterial/fungal symbiosis have been limited to a few plant species and cultivars. These were mainly pot experiments in specific production systems, such as greenhouse conditions, and with vegetables as the test plants [2; 44]. Due to differences in the experimental designs, including differences in the microorganisms used, different effects have been observed. Therefore, understanding of the mechanism of action of these microorganisms and their impact on the quality of horticulture products is still lacking and contradictory. Field research containing all the complexity of the soil environment as well as variable weather conditions is even more of a challenge. In addition to the above-mentioned factors, another difficulty when testing perennial plants, especially trees, is plant age, which influences tree physiology. The present study attempted to evaluate selected elements of the antioxidant machinery following application of a bacterial-fungal inoculum for organically grown _Malus domestica_ in field conditions. Roots, leaves, and fruits were evaluated to establish a possible site effect due to the introduction of inoculum to the rhizosphere. Different cultivars and growing seasons were considered as coexisting factors. In this study, GR and CAT were selected as enzymatic markers of the possible interaction between apple trees and applied inoculum, and of a possible occurrence of oxidative stress. Among other things, these enzymes are involved in the removal of H\({}_{2}\)O\({}_{2}\), one of the first metabolites in gradual oxygen reduction [24]. In the case of GR, removal takes place via the glutathione-ascorbate cycle (also examined in this study), while CAT is directly involved in H\({}_{2}\)O\({}_{2}\) removal. Previous studies on a very large group of apple cultivars reveal GR to be an enzyme that is highly sensitive to environmental conditions [31]. The activity of GR in inoculated trees was significantly greater in both growing seasons in root tissues, and in leaves in the first growing season, while GR was not affected in apple peel. On average, the roots of inoculated trees exhibited double the GR activity. However, the level of increase in GR activity in tested cultivars was strictly related to the growing season, with large differences between the control and treated plants in GR activity noticed in the first year of the study, which lessened in the subsequent season. Similarly, the poor GR response to the inoculum in the first year was stronger in the subsequent growing season.