## 3 Results
### PCA Analysis  
PCA analysis for lettuce varieties was done separately for both lettuce varieties and both growing cycles (Figures 4 and 5). The total variation characterized by PC1 and PC2 explains 73.47% of the variation for the iceberg and 81.37% for the butterhead, respectively. In the iceberg, six clusters were notable (Figure 4). According to the loading cases, mycorrhizal treatment (MT) promoted dry weight (DW1, DW2), stem diameter (SD1), and plant diameter (PD1) (Cluster 1) while the untreated group (NM) showed improved SD2, HW2, and RW2 values at the second growing cycle (Cluster 2) (Supplement Table S3). Moderate drought level (I80) caused the separation of cluster 3 (80) and cluster 4 (G-I\({}_{80}\)) based on morphological and biochemical parameters. Mycorrhizal treatment promoted root length (RL), MDA, and proline accumulation in the second growing cycle. Unlike cluster 4, group 3 showed higher values of head weight (HW2), plant height (PH1), plant diameter (PD2), leaf number (LN1, LN2), and root weight (RW1). The most intense drought (I\({}_{80}\)) split two treatment categories on cluster 5 (60) and cluster 6 (G-I\({}_{60}\)) according to different biochemical responses (MDA and proline content) in the first and second growing cycles. In butterhead, the score plot also distinguished six groups (Figure 5). The first cluster denotes control (C) which is separated from the second cluster (G) based on lower stem diameter (SD1, SD2), root diameter (RD1), head weight (HW2), and root length (RL1), respectively. Mycorrhizal treated plants grown under medium drought (G-I\({}_{80}\)) were clustered (cluster 4) according to higher head weight (HW2), leaf number (LN1), root weight (RW1, RW2), and root length (RL2) compared to non-mycorrhizal group (80) (cluster 3) (Supplement Table S4). Finally, mycorrhizal application at the highest drought level (I\({}_{60}\), G-I\({}_{60}\)) reduced proline accumulation in both time points (proline 1, proline 2).  
Figure 4: _Cont._  
Figure 4: Principal component analysis of data sets of morphological and biochemical parameters for iceberg in non-mycorrhizal and mycorrhizal (MT) treatments 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. Factor loadings (A) and scores (B) of the first two factors. The number beside each variable represents the day of treatment. Black letters present non-mycorrhizal treatment and red marks mycorrhizal treatment.  
Figure 5: Principal component analysis of data sets of morphological and biochemical parameters for butterhead in non-mycorrhizal and mycorrhizal (MT) treatments 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. Factor loadings (A) and scores (B) of the first two factors. The number beside each variable represents the day of treatment. Black letters present non-mycorrhizal treatment and red marks mycorrhizal treatment.