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).