3. Results

3.1. Soil chemical properties and enzyme activities

The soil salinity (Figure 1a) and soluble ions (Na+, Mg2+, K+, Ca2+, and Cl-) (Figure 1c-g) in rhizosphere soil were significantly lower than those in bulk soil under medium- and high- salinity. The salinity of rhizosphere soil decreased by 57% and 62% compared with that of bulk soil under medium and high salinity conditions, respectively.
Regardless of soil salinity, the rhizosphere soil had higher organic matter content than the bulk soil (Figure 1h). The same changes were found in the activities of S-AP and S-UE (Figure 1k, 1l). Under medium salinity, the contents of available phosphorus and available potassium in the rhizosphere soil were 2.13- and 1.09-fold greater than in the bulk soil, respectively (Figure 1j-k).

3.2. Metabolomic analysis of M. azedarach under salt stress

3.2.1. Venn diagram and volcanic plot analysis of DEMs

There were 382 (ESI+) (Figure 2a) and 277 (ESI-) DEMs (Figure 2b). Venn diagram analysis of DEMs showed there were more common DEMs between RH vs. RL and RM vs. RL than in other combinations. M. azedarachhad most of unique DEMs in RH vs. RL, including 72 DEMs in ESI+ and 90 DEMs in ESI-.
The analysis of the volcano plots showed there were more up-regulated than down-regulated DEMs under salt stress. The DEMs significantly up-regulated in RM vs. RL were homoveratric acid, 2’-deoxyuridine and 7,8-dihydroneopterin (Figure 3a-b). The significantly up-regulated DEMs in RH vs. RL were 13-S-hydroxyoctadecadienoic acid, 20-hydroxyleukotriene B4 and 7,8-dihydroneopterin (Figure 3c-d). The significantly up-regulated DEMs in RH vs. RM were anhwiedelphinine, hyperoside and histidinyl-lysine (Figure 3e-f).

3.2.2. Sugars, amino acids, and flavonoids

Nine sugars (D-mannose, maltotetraose, maltohexaose, stachyose, 1-kestose, raffinose, 3-alpha-mannobiose, GDP-glucose, and D-(-)-lyxose) were identified in DEMs, most sugars were up-regulated in RM vs. RL and RH vs. RL (Table 1).
Six amino acids were detected in DEMs, including L-theanine, L-proline, L-lysine, L-isoleucine, DL-leucine and aspartic acid. Five out of six amino acids in DEMs (except L-Theanine) were up-regulated in RM vs. RL (Table 2).
Flavonoids have antioxidant activity. Twenty-seven types of flavonoids were detected in DEMs. Among them, 19 were up-regulated in RM vs. RL and 23 were up-regulated in RH vs. RL (Table 3).