Role of abscisic acid in legumes under abiotic stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 151

whether the increase in H 2 O 2 production was related to
ABA accumulation in peanut leaves exposed to water
stress. After pretreatment with 5 mM tungstate, an ABA
biosynthesis inhibitor, the increase in ABA levels
induced by water stress was inhibited. At the same time,
this pretreatment suppressed the increase in H 2 O 2 gen-
eration in leaves exposed to water stress at 12 h.
Treatment with 100 μM ABA replaced the ABA partially
inhibited by tungstate at 24 h and prevented the
reduction in H 2 O 2 generation. A possible explanation for
this increase of H 2 O 2 content could be that some of the
ROS sources, which include electron transport, many
non-enzymatic and enzymatic reactions (Foyer &
Noctor, 2000; Asada, 1999), photorespiration and the
Mehler reaction (Cruz de Carvalho, 2008), might be
ABA-independent. Similar results were found in the
tropical legume Stylosanthes guianensis (Zhou et al., 2005).
Therefore, ABA accumulation is the first signal that trig-
gers H 2 O 2 generation during the initial phase, but its
subsequent production is partially ABA-independent
(Furlán et al., 2012).
In addition, ABA sprayed on the leaves of soybean
benefits dry matter accumulation favouring vegetative
growth of the plants grown in field conditions (Travaglia
et al., 2009). For this cultivar and at the dose used, no
restraint of shoot growth was observed as had been pre-
viously demonstrated (Sloger & Caldwell, 1970).
Similarly, field-grown wheat plants under water stress
that had been treated with ABA showed higher shoot
biomass accumulation (Travaglia et al., 2010). This ABA
effect has also been seen in investigations carried out
under controlled conditions with different levels of
water deficit, where plants sprayed with ABA showed
greater growth than those with lower endogenous ABA
levels. These studies suggest that ABA is an important
regulator controlling water content of both cells and
whole-plant, likely due to enhanced turgor that allows
optimal expansion of plant cells (Finkelstein & Rock
2002; Sansberro et al., 2004).
According to He and Cramer (1996), ABA application
on Brassica can improve salt and drought tolerance, and
it was proposed that an adequate concentration of ABA
in plants under abiotic stress may lead to growth stimu-
lation. In this regard, Khadri et al. (2007) demonstrated
that in both salt-sensitive and salt-tolerant Phaseolus
vulgaris plants, in symbiosis with Rhizobium tropici
CIAT899 strain, shoot and root growth decreased under
salt treatment. In addition, these authors showed that

nitrogenase activity and ureide levels in nodules and
nitrogen percentage in shoot tissues were also decreased.
However, when these plants were pretreated with ABA
growth parameters increased, nodule weight of salt-
sensitive plants was normalized and nitrogenase activity
was restored or less inhibited in both salt-sensitive and
salt-tolerant cultivars. Besides, ABA treatment in bean
plants seems to limit Na + translocation to shoots, result-
ing in the maintenance of a high K/Na ratio. Therefore,
these results support the role ABA in the mitigation of
salt stress in common bean plants (Khadri et al., 2007).
Some studies support a role for ABA in legume root
development. ABA acts in regulating lateral root
formation in higher plants, but the mechanism of this
response is still not clear. Enhancement of the
number of lateral roots by ABA has been previously
reported (Trewavas & Jones, 1991). Liang and Harris
(2005) have proposed that ABA inhibits lateral root
development in non-leguminous species, but has a
positive effect in leguminous species. The increasing
density of lateral roots provoked by ABA can be a
positive effect to obtain a higher number of root nod-
ules by the plant. Therefore, ABA is a key regulator in
the responses of plants to abiotic stresses such as
drought, osmotic stress, salinity and chilling. Lateral
root formation coordinated by ABA provides one of the
first links between environmental conditions and con-
trol of root architecture (Finkelstein et al., 2002).
Besides, there is a direct correlation between the
response of lateral roots of legumes and non-legumes to
ABA and their ability to accommodate nitrogen-fixing
bacteria in root nodules. Thus, the characterization of
genes involved in the ABA signalling cascade of legume
lateral roots may help to elucidate the changes in
development and physiology of the root necessary to
accommodate a rhizobial endosymbiont.

10.5 aBa regulation of leaf
expansion under abiotic stress

Numerous studies have reported that ABA is involved
in the regulation of leaf expansion under salt and
drought stress in legumes (Hartung et al., 1999; Cramer &
Quarrie, 2002; Thaler & Bostock, 2004). Previous expe-
ri ments with Phaseolus vulgaris demonstrated a direct
relationship between stomatal conductance and leaf
or xylem ABA levels under salinity (Sibole et al.,