## 3 Results
### Combined Transcriptome and Metabolome Analysis of A. mongholicus Leaves  
To comprehensively investigate the mechanisms underlying salt tolerance regulation in _A. mongholicus_, we analyzed DEGs and DAMs to examine the dynamic patterns of differential metabolites, differentially expressed genes, and key metabolic pathways in the leaves of _A. mongholicus_ under salt stress. For the differential genes and metabolites in the leaves of 'LQ' (Figure 12B) and 'DT' (Figure 12A) under salt stress, joint analysis revealed that 'DT' (DT-Treat_vs_DT-CK) was enriched in 16 pathways (\(p<0.05\)), whereas 'LQ' (LQ-Treat_vs_LQ-CK) was enriched in 22 pathways (\(p<0.05\)). Three of these pathways were actively expressed under salt stress (\(p<0.05\)): flavonoid biosynthesis (ko00941), isoquinoline alkaloid biosynthesis (ko00950), and phenylalanine metabolism (ko00360). In the flavonoid biosynthesis pathway (Figure 13), apigenin and neohesperidin significantly accumulated in 'LQ' leaves but were substantially reduced in 'DT' leaves. Gene expression analysis revealed that both _CYP5A_ and _F3H_ genes were upregulated in 'LQ', and seven members of the _CHS_ gene were significantly upregulated, whereas the _HCT_ gene exhibited complex expression patterns, with three genes upregulated and ten downregulated. In 'DT', both _CYP5A_ and _F3H_ genes were downregulated. The _CHS_ gene exhibited a tendency to be mostly downregulated (one upregulated and seven downregulated), while ten members of the _HCT_ gene were upregulated and three were downregulated. This difference in gene expression patterns corresponded to the trend of metabolite accumulation, suggesting that 'LQ' enhanced its resistance to salt stress by activating the flavonoid metabolic pathway, particularly through the antioxidant and signaling functions of key metabolites, such as apigenin and neohesperidin, which effectively alleviated oxidative  
Figure 11: KEGG classification map of differential metabolites. Note: (**A**) DT-Treat_vs_DT-CK and (**B**) LQ-Treat_vs_LQ-CK. damage induced by salt stress. In contrast, the metabolite and gene responses in 'DT' leaves were more restricted, potentially contributing to its weaker salt tolerance. In the phenylalanine metabolic pathway (Figure 14), L-phenylalanine and succinate significantly accumulated in 'LQ' leaves under salt stress, while their accumulation was absent in 'DT' leaves. Gene expression analysis revealed that the _HPD_, _AOC3_, _GOT1_, _TAT_, and _MIF_ genes were significantly upregulated in 'LQ' leaves. Five members of the _PAL_ gene were upregulated, while three were downregulated. The _DDC/TDC_ and _amiE_ genes exhibited a complex pattern of both up-regulation and down-regulation. In contrast, these genes were predominantly downregulated in 'DT' leaves. Figure 12: Differential metabolite and differential gene co-enrichment map. Note: (**A**) DT-Treat_vs_DT-CK and (**B**) LQ-Treat_vs_LQ-CK. These findings suggest that 'LQ' activated a stronger defense response under salt stress by enhancing the phenylalanine metabolic pathway. Notably, the significant accumulation of L-phenylalanine, a key initiator of phenylpropanol metabolism, may have enhanced stress tolerance by modulating the synthesis of antioxidants and cell wall components (e.g., lignin). Furthermore, the increased content of succinate may have provided additional energy support for the defense response. In contrast, the limited metabolite and gene responses in 'DT' indicated weaker salt tolerance. Figure 14: Phenylalanine metabolism. Figure 13: Flavonoid biosynthetic pathway. Note: the boxes in the pathway diagram represent Log2 FC values, where LQ stands for LQ-CK versus LQ-Treat and DT stands for DT-CK versus DT-Treat. Same as below. In the isoquinoline alkaloid synthesis pathway (Figure 15), dopamine and 3-(4-hydroxyphenyl)pyruvate significantly accumulated in 'LQ' leaves under salt stress, whereas 4-hydroxycinnamic acid and narcotoline were downregulated.