Salt stress and leguminous crops: Present status and prospects - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 28

Salinity reduced shoot growth more than root growth,
increased the root:shoot ratio, and reduced the number
of leaves, leaf area and relative growth rate (RGR) in
four wild and two cultivated species of Phaseolus . The
leaf water, osmotic and turgor potentials were also
decreased in all the species. Tissue concentrations of Cl −
and Na + ions increased significantly, while K + ion
concentrations were reduced. Further, Ca 2+ concentra-
tions in roots and stem stayed nearly constant, whereas
leaf Ca 2+ concentrations were raised considerably with
salinity. Rates of uptake of K + , Ca 2+ and Mg 2+ ions were
significantly disrupted by increasing NaCl (Jimenez
et al., 2003). Verdoy et al. (2004) reported the greatest
expression of proline-rich proteins (PRPs) and late
embryogenesis abundant (LEA) proteins in nodule
cortex cells and in the cells of vascular bundles in
responses to salt stress and water deficit in P. vulgaris .
Under salinity conditions plant growth was reduced
in salt-tolerant mutants of common bean plants when
inoculated with salt-tolerant Rhizobium tropici wild-type
strain CIAT899. Common bean plants formed a more
active symbiosis with salt-tolerant Rhizobium tropici
wild-type strain CIAT899 than with its decreased salt
toleraant (DST) mutant derivatives. The DST mutants
formed nodules that exhibited lower antioxidant
enzyme activities than wild-type nodules. Levels of
nodule CAT appeared to correlate with symbiotic
nitrogen-fixing efficiency, and SOD and dehydroascor-
bate reductase seem to function in the molecular
mechanisms underlying the tolerance of the nodules to
salinity. Nitrogenase activity declined under salt stress in
P. vulgaris nodules when inoculated with a salt-tolerant
wild-type Rhizobium tropici strain rather than with its
DST mutant derivative (Tejera et al., 2004). In another
study, Tejera et al. (2005) investigated the effect of NaCl
on growth responses in common bean plants. The
adverse effects of salinity were observed on shoot dry
weights and shoot:root ratio in all the treatments. An
increase in the contents of total soluble sugars and free
amino acids was noted. It was suggested that these
metabolites might be related to a nodule osmotic adjust-
ment response under saline conditions, or the increase of
amino acids could be a consequence of protein degrada-
tion. The common bean plants also responded to salinity
by decreasing nitrogen fixation parameters (ARA per plant
and nodule number) (Tejera et al., 2005).
Shamseldin and Werner (2005) observed that at 0.2%
NaCl stress, no significant difference was noticed between

the nodule number formed by the salt-sensitive strain
EBRI 2 without salt and under salt stress. However, at
0.4% NaCl, stressed P. vulgaris plants showed reductions
in nodule number, nodule fresh weight and acetylene
production of 45%, 51% and 53%, respectively. Jebara
et al. (2005) subjected Phaseolus genotype BAT477 to
50 mM NaCl and reported that ARA was considerably
decreased by salt treatments. Cabot et al. (2005) com-
pared the salt tolerance of P. vulgaris cv. Contender
plants, grown in spring and summer seasons at less than
75 mM NaCl and achieved a 2.6-fold greater RGR in
summer than in spring under non-saline conditions.
The effect of abscisic acid (ABA) and 100 mM NaCl on
growth, nitrogenase activity and nodule metabolism
was studied in P. vulgaris cv. Coco. The plant dry weight,
nodule dry weight, nitrogen fixation (acetylene
reduction activity and ureide content), and most
enzymes of ammonium and ureide metabolism were
affected by both ABA and NaCl. The addition of 1 μM
ABA to the nutrient solution before the exposure to salt
stress reduced the negative effect of NaCl. It was sug-
gested that ABA application improves the response of
common bean plants' symbiosis under saline stress con-
ditions, including the nitrogen fixation process and
enzymes of ammonium assimilation and purine cata-
bolism (Khadri et al., 2006). In another study, Khadri
et al. (2007) compared two cultivars - one salt-sensitive
(cv. Coco) and one salt-tolerant (cv. Africa) - of P. vul-
garis in symbiosis with Rhizobium tropici CIAT899 under
controlled conditions versus pretreatment individually
with ABA or salt or both. The ABA pretreatment in salt-
stressed plants improved the growth parameters and
normalized nodule weight of Coco and induced lower
nitrogenase activity. Plants of tolerant cultivars seem to
limit translocation of sodium to shoots to maintain a
high K/Na ratio. It was also observed that ABA and pro-
line were involved in the osmotic adjustment to reduce
the impact of salt stress.
CAT activity during development of the Rhizobium -
legume symbiosis as well as its response in salinized
plants of P. vulgaris was studied. Leaf CAT activity was
decreased throughout the ontogeny, but root nodule
catalase maintained a high, stable value. In contrast,
under salt stress conditions, leaf and nodule catalase
activities were decreased (Garcia et al., 2007). The effect
of salinity was determined on Rhizobium tropici strain
CIAT899, native Rhozobium and growth of a commercial
cultivar (Akman 98) of P. vulgaris . The results indicated