Salt stress and leguminous crops: Present status and prospects - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 27

physiological and biochemical mechanisms of salt toler-
ance. This part of the chapter covers physiological and
molecular mechanisms that have been extensively
studied in leguminous crops especially beans, broad
beans, peas, chickpea, cowpea, pigeon pea, lentil, lupins,
groundnut and soybean.

>340 mM) rhizobial strains on common bean. The
results showed that RP119 was the most effective osmo-
tolerant strain, forming a high number of nodules with
greater nodule dry weight. In another study, Bouhmouch
et al. (2005) also reported that nodule differentiation of
P. vulgaris plants was affected by salt stress; white nod-
ules emerged that lost their pink colour (leghaemoglobin
(Lb) content).
Jebara et al. (2001) assessed the effects of NaCl on
interactions between native rhizobia from Tunisian soil
and common bean lines. This study reported that the
nodular dry weight was usually more affected than
shoot dry weight by the salt stress. The NAG310 line, in
contrast with Flamingo and DOR 364, in symbiosis with
12a3 was further affected by the salt for nodular
efficiency, in spite of its high tolerance in regard of
nodule growth. In another study, Jebara et al. (2005)
evaluated antioxidant enzyme expression in response to
salt stress (50 mM NaCl) in P. vulgaris genotype BAT477
inoculated with strain CIAT899. The results revealed
that superoxide dismutase (SOD) and catalase (CAT)
nodular isozymes had bacterial and root origins. The
same Cu-, Zn-, Fe- and Mn-SOD isoforms were expressed
in nodules and roots, whereas in free rhizobia they
found only one Fe- and Mn-SOD. Ascorbate peroxidase
(APX) and peroxidase (POX) nodule and root profiles
had only root origins, as no rhizobial band was identi-
fied. Thus, NaCl salinity caused a differential regulation
of distinct SOD and POX isozymes, and the enzymes
seemed to protect the symbiosis from stress.
The addition of proline to NaCl-treated seedlings of
P. vulgaris cv. Kizilhac effected enhanced growth in
comparison to NaCl-treated seedlings (Demir &
Kocakalikan, 2002). Jungklang et al. (2003) also
observed an increase in the amino acid and proline con-
tents in P. vulgaris under salt stress. Ashraf and Bashir
(2003) have observed significant reductions in fresh and
dry weights of shoots and roots, and shoot length and
leaf area of P. vulgaris plants under salt stress. Higher
amounts of Na + and Cl − accumulated in the nodules,
whereas higher amounts of Cl − accumulated in the
leaves. The contents of free amino acids, proline and
glycine betaine increased whereas soluble proteins
declined in all plant parts of common bean under salt
stress. However, significant reductions in nodule fresh
mass and nodule number were also observed, but nodule
dry weight did not decrease significantly (Ashraf &
Bashir, 2003).

2.4.1 Beans
The dry bean category in FAO includes all species of
Phaseolus and Vigna . The genus Phaseolus comprises
about 70 species, all native to the Americas, primarily
Mexico. At least four species of this genus have been
domesticated since pre-Columbian times for their beans.
Most prominent among these is the common bean,
P. vulgaris , which today is cultivated worldwide in
tropical, semitropical and temperate climates. Previous
classifications placed a number of other well-known
species in this genus, but these have now been removed
to the genus Vigna , sometimes necessitating a change of
species name (Table 2.1).
Salt stress inhibited growth, nodule dry weight and
acetylene reduction activity (ARA) in P. vulgaris (Ferri
et al., 2000). Serraj et al. (2001) examined the short-term
effects of 0.1 M NaCl on ARA and growth of four geno-
types (W, B, R and C) of common bean. After 5 days'
exposure to stress, the shoot dry weight of two geno-
types, W and B, was 83% of that of control plants not
subjected to NaCl; there was no major effect of the salt
treatment on the growth of other two genotypes, R and
C. For all genotypes, the ARA was reduced dramatically
during the first hour following NaCl application, which
was followed by a slower decrease during the next 5 h
during which genotypical differences in response to
salinity were found.
Saadallah et al. (2001) studied the genotypic vari-
ability for tolerance to salinity of common bean lines.
They found variability among common bean lines in the
response to salt stress. Lines ABA16 and BRB17 were
found to be the most salt-sensitive lines whereas
BAT477 and Flamingo were relatively tolerant. Some
lines that displayed vigorous growth under the control
treatment were seriously affected by salt treatment.
Further, the comparative tolerance of some lines to salt
appeared to depend on the capacity to maintain a
sufficient leaf area and to develop a profuse and well-
organized nodular system. Bouhmouch et al. (2001)
investigated the infectiveness and effectiveness of 19
most osmotolerant (growing at NaCl concentrations