## 4 Discussion
The limited number of published research results on MD in lettuce grown in different drought treatments indicates the need for further research. Furthermore, some research results are contradictory to ours. For example, [PERSON] et al. [80] state that the highest MD was recorded in treatments with a larger amount of irrigation water. Also, the results of previous studies have shown that MT has a positive impact on plant-water relationships, with a beneficial effect in growing of drought exposed tomato [80], sesame [81], basil [82], orange [51], wheat [83], and lettuce [84]. Regardless of tested plant species, authors have shown that MT alleviates the negative effects of induced drought, i.e., higher proline accumulation in the drought-stressed plants, suggesting that proline accumulation is of great importance in plant responses to drought stress [80; 85; 86; 87]. When comparing our results to those studies, it must be pointed out that the results are genotypedependent. Those differences are related to the genetic potential of lettuce cultivars to stress tolerance; that is the enhanced proline accumulation in the drought-sensitive genotype (IG). As far as butterhead lettuce is concerned, the results are in accordance with the aforementioned studies, i.e., the accumulation of proline in drought treatments (MT-I\({}_{80}\) and MT-I\({}_{60}\)) is lower in MT compared to NM treatment. In this study, moderate drought (I\({}_{80}\)) in iceberg lettuce triggered the overaccumulation of proline compared to butterhead, indicating that MT promotes the activation of antioxidative routes in the iceberg (Figure 3b). [PERSON] et al. [88] stated that proline accumulation in response to drought stress in oil palm was genotype-dependent and drought-preconditioning enhanced proline accumulation in the sensitive genotype, but not in the tolerant genotype. The statement is confirmed by research on peanuts, in which the author stated that the stress tolerance in peanuts is genotype-dependent [89]. Similarly, to the findings of [PERSON] et al. [90] who reported that MDA content was irrigation, i.e., drought level and harvest time-dependent, we found significant differences in irrigation treatment in both growing cycles, yet the results are not uniform. In the first growing cycle, the MDA reduction was noted on moderate stress treatment (MT-I\({}_{80}\)), while MDA incensement was noted on severe drought treatment (MT-I\({}_{60}\)). In the second growing season, only MDA incensement was noted on mild water stress (G\({}_{\text{IL}}\)-MT-I\({}_{80}\)). The study conducted on cucumber with moderate (80-85% Field Capacity, FC) and severe drought stress (60-65% FC), shows the increased MDA content in both drought treatments [91]. Another study conducted on lettuce confirmed MDA increasing in response to increasing drought stress levels [92], although it should be emphasized that more intensive drought treatments (50% and 20% of FC) were applied in the research compared to our study. As it seems, the ability of mycorrhizal fungi to promote drought resistance appears to be limited to plant genotype, that is, genotypic specificity in terms of drought tolerance level, but also other abiotic factors. The differences in this study obtained in the first and second growing cycles are probably caused by extremely high air temperatures during the second growing cycle. Iceberg lettuce variety (Great Lakes) clearly showed better adaptability and tolerance to extremely high air temperatures, i.e., heat stress that occurred at the beginning of May (lettuce maturation phase). This statement is based on the results of present research, which clearly indicate that a higher RW is recorded for IG in heat stress conditions regardless of the MT or I treatment (Figure 2e). This indicates that RW can be considered as a main root growth attribute that increases a plant's ability to tolerate high air temperatures.