## 3 Results
### Morphometric and Biochemical Parameters in the First Growing Cycle  
A separate analysis of variance revealed significant effects (\(p\leq 0.05\)) of genotype (G: G\({}_{\rm{IL}}\), G\({}_{\rm{BL}}\)), irrigation treatments (I: I\({}_{100}\), I\({}_{80}\), I\({}_{60}\)), and mycorrhizalae (M: NM and MT) and their interactions for morphometric and biochemical parameters in the first growing cycle (Supplement Table S1). On average across treatments, uniform results were obtained for irrigation (I) and lettuce genotype (G) regardless of the growing cycle. The irrigation rate linearly increased all tested variables in both growing cycles, meaning that the highest values were recorded on the I\({}_{100}\) treatment. The highest HW in both growing cycles was recorded for butterhead lettuce (GBL). Significant interactions were noticed among the main effects of all study factors in most of the tested variables. Exceptions were HW, PH, PD, RW, and MDA (for MT); RD and DW (I \(\times\) MT); HW, PH, SD, and MDA (for MT \(\times\) G); PH and RD (I \(\times\) G) while RD, DW, and proline showed non-significant interaction for G \(\times\) I \(\times\) M. Results revealed the reduction of lettuce growth with the increasing drought intensity in both genotypes within NM treatment. The negative impact was observed for the HW, PH, PD, LN, RW, RL, SD, and RD, respectively (Figure 2a-h). In iceberg lettuce, MT had a beneficial effect on PD while in butterhead PD values were significantly lower than in the non-mycorrhizal group (Figure 2c). In butterhead, mycorrhizal treatment promoted root growth parameters (RL and RD) but showed an inhibitory effect on RL in iceberg suggesting that the success of mycorrhizal application depends on genotype properties (Figure 2f,g). The inhibitory effect of drought on most of the morphometric parameters (HW, PH, PD, LN, RD, and SD) was also noticeable in an MT group (Figure 2a-h). The exception was the butterhead in which there were no differences in SD between control (I\({}_{100}\)) and drought stress groups (I\({}_{80}\) and I\({}_{60}\), Figure 2h). The MT under moderate water stress (I\({}_{80}\)) stimulated HW and LN in both lettuce genotypes (Figure 2a,d). The positive effect of MT at the severe drought treatment (I\({}_{60}\)) was noticed for HW, LN, and RD in both varieties (Figure 2a,d,g). Additionally, MT on I\({}_{60}\) treatment improved PH and PD values in the iceberg and RL values in butterhead compared to nonmycorrhizal treatment (Figure 2a-c). On the other hand, butterhead showed better lateral growth (PD) in the nonmycorrhizal treatment (I\({}_{60}\)) than in the mycorrhizal treated group (Figure 2c). Opposite, in butterhead, the mycorrhizal treatment seemed to be more effective in root growth promotion (RL) (Figure 2f). On the other hand, at the same drought intensity (I\({}_{80}\)) butterhead lettuce was higher (PH) than iceberg plants meaning that mycorrhizal treatment in this variety induces stem elongation (Figure 2b). At the highest drought intensity (I\({}_{60}\)) the differences between varieties in mycorrhizal treatment were not detected. The response of G\({}_{\rm{IL}}\) to mycorrhizal colonization, i.e., mycorrhizal dependence (MD) ranged from 0.76% (I\({}_{80}\)) to 11.89% (I\({}_{60}\)). The MD of G\({}_{\rm{BH}}\) ranged from 7.17% (I\({}_{80}\)) to 10.23% (I\({}_{60}\)). Mycorrhizal treatment also modified biochemical response. Both lettuce varieties responded to water deficit by overproduction of MDA and proline content (Figure 3a,b). Figure 2: The effect of mycorrhizal (NM—nonmycorrhizal, MT—mycorrhizal treatment) and irrigation (I\({}_{100}\)—control treatment (100% water holding capacity (WHC); I\({}_{80}\)—80% WHC and I\({}_{60}\)—60% WHC) treatments on head weight (HW, (a)), plant height (PH, (b)), plant diameter (PD, (c)), leaf number (LN, (d)), root weight (RW, (e)), root length (RL, (f)), root diameter (RD, (g)) and steam diameter (SD, (h)).