## 5 Conclusions  
The tested morphological and biochemical traits of lettuce genotypes considerably varied across mycorrhizal and drought treatments. As expected, drought stress reduced lettuce growth-related parameters regardless of genotype or growing cycles. The differencesin the mycorrhizal treatment efficiency between the first and second growing cycles are because of differences in environmental conditions, where the influence of heat stress in the second growing cycle stands out. The increased cell membrane damage (MDA, \(\mu\)mol g\({}^{-1}\) FW) was noticed for both lettuce genotypes in the first growing cycle; 0.32 (NM-I\({}_{60}\)) to 0.47 (MT-I\({}_{60}\)) for butterhead lettuce and 0.79 (NM-I\({}_{60}\)) to 1.21 (MT-I\({}_{60}\)) for iceberg lettuce. In heat-stressed conditions the MDA content was reduced in both lettuce genotypes; 1.12 (NM-I\({}_{60}\)) to 0.49 (MT-I\({}_{60}\)) for butterhead lettuce and 0.31 (NM-I\({}_{60}\)) to 0.04 (MT-I\({}_{60}\)). The results are consistent with the proline content in the first growing cycle, where a reduced content was recorded in MT for both genotypes; 5492 (NM-I\({}_{60}\)) to 3413 (MT-I\({}_{60}\)) for butterhead lettuce and 12,705 (NM-I\({}_{60}\)) to 9991 (MT-I\({}_{60}\)) for iceberg lettuce. Lettuce genotypes reacted differently to heat stress in the second growing cycle whereby the proline content (\(\mu\)mol g\({}^{-1}\) FW) in butterhead lettuce was reduced in MT; 5492 (NM-I\({}_{60}\)) to 3413 (MT-I\({}_{60}\)) and increased in iceberg lettuce; 4385 (NM-I\({}_{60}\)) to 4963 (MT-I\({}_{60}\)). Further studies are needed to test different heat stress in combination with drought treatments to receive more precise insights into the beneficial effects of mycorrhizae in different environmental conditions and varieties of lettuce genotypes. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app132212117/s1](https://www.mdpi.com/article/10.3390/app132212117/s1)]([https://www.mdpi.com/article/10.3390/app132212117/s1](https://www.mdpi.com/article/10.3390/app132212117/s1)), Table S1: Mean squares and significant test after analysis of variance for morphological and biochemical traits of two lettuce genotypes evaluated across two pretreatments and three water regimes for the first growing cycle; Table S2: Mean squares and significant test after analysis of variance for morphological and biochemical traits of two lettuce genotypes evaluated across two pretreatments and three water regimes for the second growing cycle; Table S3: Factor loadings of morphological and biochemical parameters for iceberg in non-mycorrhizal and mycorrhizal (G) treatments under control (C) and drought (I\({}_{60}\), I\({}_{60}\)) conditions. Head weight (HW), plant height (PH), plant diameter (PD), leaf number (LN), root weight (RW), root length (RL), root diameter (RD), stem diameter (SD), dry weight (DW), malondialdehydedehyde (MDA), and proline. The number beside each variable represents the growing cycle; Table S4: Factor loadings of morphological and biochemical parameters for butterhead in non-mycorrhizal and mycorrhizal treatments (G) under control (C) and drought (60, 80) conditions. Head weight (HW), plant height (PH), plant diameter (PD), leaf number (LN), root weight (RW), root length (RL), root diameter (RD), stem diameter (SD), dry weight (DW), malondialdehydedehyde (MDA), and proline. Number beside each variable represents the time point. Conceptualization, N.V. and M.M. ; methodology, M.M., A.K. and B.J.-P.; formal analysis, T.M., L.L., A.A. and B.J.-P.; investigation, M.M. and B.J.-P.; data curation, T.M. ; writing--original draft preparation, M.M. and T.M. ; writing--review and editing, M.M. and A.K. ; visualization, T.M. and A.A.; supervision, N.V. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.