Salt stress and leguminous crops: Present status and prospects - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 24

Halophytes complete their life cycle in environments
where the concentrations of NaCl lie in the 200-500 mM
range. Being the natural inhabitants of highly saline
soils, halophytes efficiently exclude salts from their
roots and leaves, and some can endure salts at levels
more than twice the concentration of seawater. The
remarkable levels of salt tolerance shown by the legumes
make them good candidates to identify salt-tolerance
genes. Halophytes represent only 2% of terrestrial plant
species but have a wide taxonomic diversity (Flowers
et al., 2010). At the other extreme non-halophytes or
glycophytes are sensitive and can tolerate very little salt,
suffering irreparable damage at 4 dS/m (≈40 mM NaCl)
or more (Chinnusamy et al., 2005). During adaptive evo-
lution in nature, plants living in oceans (i.e. halophytes)
retained their tolerance to high salt, while most plants
that migrated to land lost their ability and adopted a gly-
cophytic lifestyle. Many agricultural crops, including
legumes, are glycophytes and are sensitive to salinity.
Salt tolerance is a complex trait that is controlled by
multiple genes and involves various biochemical and
physiological mechanisms. The functions of the distinct
sets of genes involved in specific biochemical and
physiological mechanisms must be combined to achieve
substantial increases in salt tolerance. In the last 10 years,
significant progress towards understanding the mecha-
nisms of plant salt tolerance has been made in several
crop plants including legumes. Plants adapt develop-
mental, morphological, physiological, biochemical and
molecular strategies to cope with the detrimental effects
of salt stress (Flowers et al., 2010). Moreover, salt-tolerant
plants (halophytes) have evolved a number of mecha-
nisms for adaptation to stressful saline environments.
Adaptive mechanisms of plants exposed to salinity are
based upon utilization of one or more of the following
major mechanisms (Parida & Das, 2005):
1 phenological avoidance;
2 salt avoidance through low root permeability for
certain salt ions (e.g. Na + );
3 salt avoidance by secretion using salt bladders or
glands;
4 succulence for distribution and dilution of high salt
accumulations;
5 compartmentalization of salts in vacuoles;
6 biochemical tolerance (e.g. synthesis of compatible
osmolytes, induction of antioxidative enzymes and
stimulation of phytohormones);
7 nutritive tolerance.

In summary, salt tolerance mechanisms are of two
kinds: those minimizing the entry of salts into the
plant and those minimizing the concentration of salt in
the cytoplasm (Munns, 2002). Plant responses to
salinity and adaptive mechanisms that confer plant
salinity tolerance have been extensively reviewed
(Hasegawa et al., 2000; Zhu, 2001; Apse & Blumwald,
2002; Wang et al., 2003; Chinnusamy et al., 2005;
Munns, 2005; Parida & Das, 2005; Hussain et al., 2008;
Munns & Tester, 2008; Türkana & Demiral, 2009;
Zhang & Shi, 2013).

2.4 Lessons from studies
of the leguminous crops

The Leguminosae (Fabaceae) represent the second larg-
est and most diverse family of dicotyledonous flowering
plants, with 20,000 species classified into around 700
genera (Raven & Pothill, 1981; Doyle & Luckow, 2003).
Legumes form a symbiotic association in their root
system (sometimes on their shoots) with nitrogen-fixing
microbes (diazotrophs: in association with legumes col-
lectively called rhizobia) in specialized organs called
nodules (Bruning & Rozema 2013).
Legumes as food crops play an important nutritional
role in the diet of millions of people living in the devel-
oping countries, and legumes are sometimes referred to
as the 'poor man's meat'. They provide a significant
part of the diet of vegetarians, being vital sources of
protein, calcium, iron, phosphorus and other minerals.
In addition to those legumes cultivated for human con-
sumption, many yield important fodders, forages and
green manures since they assist in nitrogen fixation.
Green manures have been used since the beginning of
agriculture, but their use has diminished since industri-
ally produced fertilizer became available (Zahran,
1999). Green manure adds nitrogen to the soil and
improves soil quality by increasing organic matter
content of the soil (Sullivan, 2003). Legumes are
known to survive and compete effectively in nitrogen-
poor conditions. Root nodules are generally found in
the Mimosoideae and the Papilionoideae, but rarely
formed in the Caesalpinoideae. Legumes are also used
for a variety of other purposes including timber, medi-
cine, tanins and gums. The International Legume
Database and Information Service (ILDIS) states that
the economic importance of the Leguminosae family is