Exogenous application of phytoprotectants in legumes against environmental stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 177

11.4.1.6 Ethylene
Ethylene is a gaseous phytohormone involved in fruit
ripening, but also with roles in senescence, and leaf and
fruit abscission. Its participation in germination, seedling
morphogenesis, root emergence, root hair elongation,
and promotion of flowering is also important. The stress
response and ethylene regulation has attracted the
attention of researchers (Ligero et al., 1987; Lee & LaRue,
1992). Like other hormones, ethylene also has antago-
nistic and synergistic effects.
When M. sativa and Cassia corymbosa plants were exposed
to drought stress the ethylene level significantly increased.
In the case of long-term (8-12 day) severe drought stress
(−0.4 MPa) ethylene production was related to senescence
and abscission (Berumen-padilla et al., 2009). Mung bean
( V. radiata ) and pea ( P. sativum ) pre-exposed to ethylene
were more resistant to O 3 . The resistance of ethylene-
pretreated plants was due to suppression of H 2 O 2 through
stimulation of APX activity (Mehlhorn, 1990). Ethylene
diurea (EDU) treatment led to a significant increase in
biomass and yield of P. vulgaris plants exposed to O 3
(Brunschon-Harti et al., 1995a). The antioxidative roles of
EDU as a free radical scavenger or in reducing oxidative
damage were studied in common bean and snap bean
(Kostca-Rick & Manning, 1993; Brunschon-Harti et al.,
1995b; Lee et al., 1997). Roles for ethylene in mediating
adaptive responses to P deficiency stress in common bean,
soybean ( G. max ) and pea ( P. sativum ) involved morpholog-
ical changes in root systems, gravitropism, aerenchyma
formation and root hair development (Bonser et al.,
1996). Ethylene generally plays a negative role in legume
symbiosis by restricting infection. Ethylene production sig-
nificantly increased in Medicago sativa and P. sativum roots
infected by rhizobia, whereas exogenous ethylene
decreased the number of nodules (Lee & LaRue, 1992).
Exogenous ethylene did not decrease the number of rhizo-
bial infections in root, but most infections were blocked in
the basal epidermal cell or in the outer cortical cells (Lee &
LaRue, 1992). Ethylene and ROS act as signal molecules
in Nod factor-triggered nodule formation. In S. rostrata ,
ethylene and ROS act downstream from the Nod factors
and play roles in the generation of infection pockets and
initiation of nodule primordia (D'Haeze et al., 2003).

regulation in responses to stress (Poonam et al., 2013).
JA has both synergistic and antagonistic effects on plant
developmental processes, including seed germination,
root growth, fertility, fruit ripening, senescence and sto-
matal closure. JA or its methyl esters are omnipresent
in plants and play vital roles under stress conditions
(Wasternack & Hause, 2002; Cheong & Choi, 2003;
Alam et al., 2014a).
Methyl jasmonate (MeJA) (20 and 30 μM) counter-
acted the negative effects of salinity (60 mM NaCl, 2
weeks) stress in G. max seedlings. Improved plant growth,
chl content, leaf photosynthetic rate, leaf transpiration
rate and Pro content were recorded in MeJA-treated
plants. JA also reduced the effects of salt on seed carbo-
hydrates, lipids, proteins, N, P and K (Yoon et al., 2009).
Treating P. sativum with 10 mM JA improved photosyn-
thesis, RWC and protein content in conditions of salt
stress (30 mM NaCl, 4 and 7 days) (Fedina & Tsonev,
1997). Exogenously applied JA (100 μM) increased
drought tolerance in G. max ; JA supplementation
increased the endogenous JA, ABA, ethylene and cyto-
kinin contents in drought-affected plants. Enhancements
of total PA, putresceine (Put), spermidine (Spd) and
spermine (Spm) also correlated with adaptive responses
and safeguard mechanisms of soybean plants coping
with drought stress (Hassanein et al., 2009). Improvement
of drought tolerance in A. hypogaea plants was enabled by
JA's role as a growth substance (El-Meleigy et al., 1999).
The C. cajan plants were subjected to Cu 2+ (5 mM) stress
that resulted in destruction of photosynthetic pigments
and antioxidant systems. Exogenous JA supplementa-
tion with Cu 2+ neutralized the deleterious effects of Cu 2+
on seedling growth, manifest by the accumulation of
osmolyte Pro, improved photosynthetic pigments, and
enhanced antioxidant enzymes SOD and POD activities,
thus contributing to Cu 2+ tolerance (Poonam et al., 2013).
Supplementation by MeJA (100μM) prevented petiole
abscission in AgNO 3 -treated V. radiata plants. In contrast,
AgNO 3 -treated plants were characterized by heavy
petiole abscission and it was suggested that MeJA was
involved in ethylene-independent abscission. In a recent
study, Alam et al. (2014a) showed that exogenous spray-
ing with JA (0.5 mM) might be an effective agent for
eliciting short-term drought tolerance in different Brassica
species although the responses are different. These
species especially appeared to respond differently with
respect to antioxidant and glyoxalase enzyme activities
under short-term drought stress (15% PEG, 48 h). JA

11.4.1.7 Jasmonic acid
Jasmonic acid (JA) is a growth regulator that influences
plant developmental processes. By acting as signal
molecule, it influences physiological and metabolic