Salt stress and leguminous crops: Present status and prospects - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 38

soybean displayed higher tolerance than chickpea
towards saline conditions and the index for tolerance
appeared to be related to the increased levels of sugars,
amino acids, proteins and nucleic acids as compared to
chickpea (sensitive). Malencic et al. (2003) investigated
salt stress tolerance parameters in various genotypes of
soybean and reported high SOD activity and low lipid
peroxidation in the tolerant genotypes. Luo et al. (2005)
studied differential sensitivity to chloride and sodium
ions in seedlings of G. max and G. soja under NaCl stress
and reported that the control plants had higher osmotic
potential than plants treated with Cl, Na and NaCl. The
studies by Luo et al. (2005) revealed that Cl − was more
toxic than Na + to seedlings of G. max than to G. soja ; the
injury was positively correlated with the content of Cl −
in the leaves, and negatively with that in the roots. A
study was conducted by Lee et al. (2009) to determine
inheritance of salt tolerance in wild soybean ( G. soja
Sieb. and Zucc.). The results showed that G. soja line
PI483463 had a single dominant gene for salt tolerance
that was different than the gene in G. max line S-100.
The symbol Ncl2 was designated for this new salt toler-
ance allele (Lee et al., 2009).
The effects of salinity become exacerbated when the
soil is deficient in oxygen. Therefore oxygenation has
been used to reduce the impact of salinity on plants
under oxygen-limiting soil environments. Oxygenation
increased above-ground biomass yield and water use
efficiency (WUE) by 13% and 22%, respectively, com-
pared with controls in soybean. Higher yield with
oxygenation was accompanied by greater plant height
and stem diameter and reduced specific leaf area and
leaf Na + and Cl − concentrations. Oxygenation also led to
a greater rate of photosynthesis, higher relative water
content in the leaf, reduced crop water stress index and
lower leaf water potential. Oxygenation enhanced yield
and may make an important impact on irrigated agricul-
ture where saline soils pose restraints to crop production
(Bhattarai & Midmore, 2009).
A study conducted by Hakeem et al. (2012b) evalu-
ated growth, lipid peroxidation and antioxidant enzyme
activities in response to salt stress in 10 genotypes of
soybean. The results suggested that Pusa-24 and Pusa-
37 are salt-sensitive and salt-tolerant genotypes of
soybean, respectively, and that the antioxidant defence
system is associated with conferring the sensitiveness
and tolerance in these genotypes (Hakeem et al., 2012b).
The influence of genetic background on salt tolerance

was investigated using 10 soybean genotypes, namely
Pusa-20, Pusa-40, Pusa-37, Pusa-16, Pusa-24, Pusa-22,
BRAGG, PK-416, PK-1042 and DS-9712. Plant growth
in all genotypes was decreased by salt stress as com-
pared to controls. The differences in osmotic adjustment
between all the genotypes were associated with the con-
centrations of ions such as Na + and the leaf proline
concentration (Khan et al., 2013).
Nitric oxide (NO) is a gaseous signalling molecule
involved with the regulation of diverse processes in
plants. Certain studies have demonstrated a role for
exogenous application of NO in mediating responses
to abiotic stress. The role of exogenously applied
2,2′-(hydroxynitrosohydrazono) bis-ethanimine (DETA/
NO) in upgrading long-term salinity stress on soybean
has been studied by Egbichi et al. (2014). Long-term
salinity stress drastically affected the plants as indicated
by decreased biomass of shoots, roots and nodules of
soybean plants. In contrast, supplementation with
10 μM DETA/NO improved growth of soybean plants
under NaCl as evidenced by increased shoot, root and
nodule weights and nodule number. Further analysis
showed that long-term salinity stress led to increased
cellular hydrogen peroxide (H 2 O 2 ) content and high
levels of cell death in the soybean. Treatments with NO,
either as DETA/NO alone or in combination with NaCl,
resulted in reversal of H 2 O 2 to basal levels. It was also
observed that application of DETA/NO resulted in
increased enzymatic activity of ascorbate peroxidase
(APX). The role of NO in increasing tolerance to salinity
stress in soybean may result from either its antioxidant
capacity by direct scavenging of H 2 O 2 or its role in acti-
vating APX activity, which is crucial in scavenging H 2 O 2
(Egbichi et al., 2014).

2.5 Omics technologies for
understanding salt stress responses
in legumes

Plant cells may alter their gene expression in order to
tolerate salt stress, resulting in an upregulation, down-
regulation,, induction or total suppression of some
stress-responsive genes and proteins. Previously, a one-
gene approach was applied to explain abiotic stresses,
including salt stress responses, in a 'cause and effect'
manner. But as salt stress involves polygenic and com-
plex mechanisms, a range of molecular tools are required