4. Discussion
Coastal saline soil contains
excess salinity, with
Na+ and
Cl- being the main base ions (Zhang et al., 2022).
Soil organic matter (SOM) is dominant to global carbon cycle
(Bhattacharyya, 2022), S-AP activity is an indicator of soil phosphorus
cycle (Liberti et al., 2013), and
S-UE plays a crucial role in influencing the content of soil nitrogen
(Albiach et al., 2000). In this
paper, salinity, contents of
Na+ and Cl- in the rhizosphere soils
were significantly lower than bulk soils, whereas SOM and activities of
S-AP and S-UE showed the opposite trend (Figure 1). These results
indicated that M.
azedarach played a critical role in reducing soil salinity and
promoting soil nutrient cycling.
Excess salinity in saline-alkali land can cause osmotic stress, ion
toxicity, oxidative damage, nutrient deficiency and even cell death in
plants (Elkelish et al., 2019; Ghazali, 2020). Plants can cope with salt
stress by regulating biosynthesis of metabolites such as sugars, amino
acids and flavonoids (Batista-Silva et al., 2019).
Sugars can be bound to inorganic solutes (e.g. K+) to
maintain cell expansion and are active in osmotic regulation under salt
stress (Arbelet-Bonnin et al.,
2018). Similar results were reported in other studies, with sugars
acting not only as osmoprotectants and cell membrane stabilizer, but
also as scavengers of reactive oxygen species (Nishizawa et al., 2008).
The contents of
various sugars were up-regulated
under medium and high salinity
(Table 1), indicated that M. azedarach could produce more sugars
to regulate osmotic pressure when exposed to a gradient of salt stress.
Besieds, amino acids are
important osmoprotectants and antioxidants, and their accumulation can
enhance the stability of cell membrane and improve salt tolerance of
plants (Slama et al., 2015; Widodo et al., 2009). Hosseinifard et al.
(2022) discovered that proline could inhibit the accumulation of toxic
ions under salt stress. With the increase in the severity of salt
stress, the amino acid DEMs were up-regulated, and more amino acids were
synthesized under medium compared with low salinity
(Figure 5a-b, Table 2). The
result showed that M. azedarach increased its salt tolerance by
up-regulating amino acid as the severity of salinity progressed from low
to medium.
Flavonoids have antioxidant activity, protecting cell structures from
oxidative damage and enhance the stress resistance of plants (Saeed et
al., 2012; Sumner et al., 2003). More than 70% of the flavonoids were
up-regulated in M. azedarach roots with soil salinity increased
(Table 3). And DEMs involved in flavonoid biosynthesis were largely
up-regulated with salinity increased (Figure 5).
These results indicated thatM. azedarach could enhance
the antioxidant activity and salt tolerance of plants by up-regulating
the expression of flavonoids.
Among M. azedarachmetabolites, traumatic acid was shown to participate in the plant
adaptation and tolerance to salinity by diminishing the negative effects
of sodium and chloride ions and increasing the monosaccharide content
(Pietryczuk et al., 2014). Alkaline phosphatase can release inorganic
phosphorus for plants growing on phosphorus-deficient saline soils (Li
et al., 2021a).
In
the present study, alkaline phosphatase activity was positively
correlated with traumatic acid concentration (Figure 6), indicating that
there should be a synergistic effect between soil alkaline phosphatase
activity and traumatic acid in M. azedarach , facilitatingM. azedarach tolerance to salt stress.