Role of abscisic acid in legumes under abiotic stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 150

10.4 aBa mediating whole-legume
responses to abiotic stress

seed set in wheat and pod set in soybean (Dembinska
et al., 1992; Liu et al., 2004).
ABA also has important functions in vegetative
growth under abiotic stresses, as was observed in
Prosopis strombulifera , which showed that endogenous
ABA was affected by salt type and concentration and
relative humidity (Devinar et al., 2013). It was demon-
strated that when three stressing factors are applied
simultaneously they interact bringing about a response
that is different to that triggered by any of them indi-
vidually. At low humidity (30%) ABA accumulation
seemed to be triggered by the conjunction of low
humidity plus salinity, especially in leaves of Na 2 SO 4 -
treated plants from −1 to −2.6 MPa. These high ABA
levels are correlated with increasing toxicity symptoms
caused by this salt, and the maintenance of primary root
elongation and shoot inhibition. Thereby, low humidity
amplified the effects of salinity in plant growth and ABA
response; thus, ABA has a role not only as a stress signal
but also as a growth regulator governing vegetative
growth under different relative humidity and salinity
conditions (Devinar et al., 2013). Similar results were
obtained previously in this species, where the highest
levels of ABA and ABA-GE, and the highest ABA-GE
glucosidase activity were observed in Na 2 SO 4 -treated
plants. Therefore, ABA may act as a triggering signal for
several adaptive mechanisms for survival of plants in
this extremely stressing situation. However, the stress
imposed by the presence of sulphate anion in the
culture medium blocked the activity of ABA, stomata
remained open, and high transpiration values were
recorded (Llanes et al., 2014).
Moreover, it is well known that ABA accumulation
triggers H 2 O 2 production as a signal mediating stress tol-
erance responses (Xing et al., 2008). Several studies
have demonstrated that ABA accumulation occurs prior
to H 2 O 2 appearance and that both molecules are
involved in stomatal closure as well as in stomatal open-
ing inhibition (Lü et al., 2013). In this regard, Furlán
et al. (2012) evaluated the time-course of production of
both compounds, ABA and H 2 O 2 , in peanut plants
exposed to drought stress. In these plants, significant
ABA accumulation occurred by 12 h in polyethylene
glycol (PEG)-treated plants, whereas this increment was
less pronounced at 24 and 48 h compared with the non-
treated plants. On the other hand, a significant increase
in H 2 O 2 content occurred after 24 h. These authors set
sample times at 12 and 24 h to address the question of

Generally, the action of ABA has been related to
processes of inhibition, although there is recent evi-
dence of its presence in developing tissues where it may
have a promoting action (Finkelstein & Rock, 2002;
Sansberro et al., 2004; Peng et al., 2006). It was shown
that endogenous ABA levels were increased in both
flowers and pods of soybean by drought stress, and it
was related to a reduction in pod set. Liu et al. (2003)
suggested that drought induced an increase in ABA
levels causing pod abortion. However, direct cause and
effect could not be shown, and evidence to support this
hypothesis is not clear. Nevertheless, Liu et al. (2004)
demonstrated that different stresses modified endoge-
nous ABA content in pods from plants, and that this
response was correlated with pod set. ABA treatments
were applied simultaneously with both well-watered
and drought stressed conditions to the soybean plants to
test if ABA could alleviate the effect of drought stress on
pod set. Results showed that pod set was correlated neg-
atively with pod ABA content when the latter was
above a threshold value; this conforms with early
reports in wheat (Westgate et al., 1996). In addition,
ABA treatments reduced pod set in plants under well-
watered conditions, results that are in agreement with
data for wheat (Wang et al., 2001) and maize (Mambelli
& Setter, 1998).These findings suggest an ABA role in
controlling pod set in legumes. However, it was also
reported that ABA treatments had a positive effect on
pod set in plants under drought-stressed conditions.
These contrasting effects of ABA treatments on pod set
in soybean plants in both well-watered and drought
stressed conditions are still unclear. However, one
reason could be that ABA affects pod set directly by
altering processes within the ovary (i.e. cell division) or
indirectly by influencing the availability of photosyn-
thates. A similar mechanism had been proposed when
ABA applied in wheat provokede seed abortion (Waters
et al., 1984). Liu et al. (2004) demonstrated that the
effect of ABA treatments on pod set was due partially to
its effect on photosynthesis. Plants in the early stages of
drought stress showed induction of closure of stomata
when ABA was applied. Also observed was a slowing in
development of water deficits that resulted in a high leaf
water potential. Furthermore, several reports, have
shown that a high leaf water potential is essential for