## 4 Discussion  
In line with previous studies [52; 53; 54; 55; 56] drought treatments (I\({}_{80}\) and I\({}_{60}\)) reduced lettuce head weight (HW) as well as growth parameters (PH, PD, RL, RD, RW, and SD), regardless of the lettuce genotype and growing cycles. On average across treatments, the highest stability was shown by the butterhead genotype (G-BL) with the highest HW in both growing cycles. This is an important finding in the understanding of the genetic advance for the most important yield-forming growth parameter, and the beneficial effect of MT on crop growth. The genetic dependence of different lettuce cultivars has also been shown to be significant in many previous studies regardless of the study factors, such as irrigation method [68; 69], fertilizers management [70], and nutrient solution [68]. Similar results are obtained for the MT \(\times\) I \(\times\) G effect, whereby in severe drought treatment (MT-GBL-I\({}_{60}\)), better response to MT showed butterhead lettuce in 1 st growing cycle, i.e., the highest HW, PH, RL, RD, and SD. In the second growing cycle, on severe water stress treatment (I\({}_{60}\)) the highest HW, PH, HD, RW, RL, RD, and SD were recorded for iceberg lettuce (MT-GIL-I\({}_{60}\)). [PERSON] et al. [71] claim that MT can effectively improve plant productivity under various water deficit stresses. In this study, the results showed the beneficial effect of MT on lettuce grown in moderate (I\({}_{80}\)) and severe (I\({}_{60}\)) drought stress, whereby a more pronounced HW increase in iceberg lettuce was recorded in a second growing cycle; i.e., for 41% (MT-I\({}_{60}\)) and 31% higher (MT-I\({}_{60}\)) compared to control treatment (NM-GIL-I\({}_{100}\)). As previously stated by [PERSON] et al. [72] plants with deeper root systems extract water from deeper soil layers and help the plants to avoid drought stress. Therefore, drought stress reduces root length [73; 74; 75] which is also confirmed in this study (Figure 2f) where the beneficial effect of MT on root growth was shown in both drought treatments (I\({}_{80}\), I\({}_{60}\)). On average, the root elongation (RL) ranged from 11.7% (MT-I\({}_{80}\)) to 35.7% (MT-I\({}_{60}\)), while the RD increase ranged from 6.2% (MT-I\({}_{80}\)) to 11.5% (MT-I\({}_{60}\)) compared to NM treatment. Therefore, the more pronounced effect of MT is on severe drought treatments. This result ties well with previous studies wherein the symbiotic relationship with mycorrhizal may improve root size and efficiency, under drought-exposed crops [76; 77]. We showed that MT increases drought resistance by providing lettuce with increased water uptake. The improved drought resistance comes because of symbiotic relations due to which developed mycorrhizal mycelia proliferate deeper and form a network specialized in the acquisition of water [78]. Also, the more pronounced effect of MT on severe drought treatments was for RW, whereby an increase in RW (41%) is visible only in the severe drought treatment (MT-I\({}_{60}\)), compared to the NM treatment (NM-I\({}_{60}\)). In moderate water stress (MT-I\({}_{80}\)) and optimal water treatment (MT-I\({}_{100}\)), mycorrhizalice reduced RW. The same conclusion was reached by [PERSON] et al. [79] who reported that the application of mycorrhizal fungal inoculum was significant on all lettuce traits, except for root fresh weight. The higher root system attributes (RL, RW, and RD); i.e., the larger root system confirms that plants in MT have better adaptation in drought-stressed conditions, while the differences in lettuce genotypes point to better genetic potential in adaptation to abiotic stress; in this case the adaptation to drought, and heat stress for G\({}_{\text{HL}}\). In this study, MD is more pronounced in treatments with increased water stress (I\({}_{60}\)), regardless of a growing cycle or lettuce genotype, aiming that both genotypes benefited from MT. However, in both growing cycles G\({}_{\text{HL}}\) showed higher mycorrhizal response.