Exogenous application of phytoprotectants in legumes against environmental stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 186

study, Bellaloui and his group (2013) reported that
foliar B application could improve the seed composition,
cell wall B content, and seed δ 15 N and δ 13 C isotopes
in water-stressed soybean plants. The boron-induced
increases in δ 15 N and δ 13 C isotopes under water stress
suggests a role of B in regulation of N fixation, which
is of great importance for legumes.

and net photosynthetic rate, and increased Na + and Cl −
contents. However, Si application (1.5 mM) enhanced
growth of salt-stressed plants and reduced Na + content.
Exogenous Si also improved the photosynthesis gas-
exchange parameters and net photosynthesis (Zuccarini,
2008). While studying soybean, Hamayun et al. (2010b)
observed that both salinity (100 mM NaCl) and drought
stresses (12% PEG) decreased shoot length, and plant
fresh and dry weights. However, these adverse effects of
salinity and drought on plant growth were alleviated
by adding Si (100 and 200 mg/L) to salt- and drought-
stressed treatments. During salt and drought stress,
exogenous Si also improved physio-hormonal attributes
of soybean contributing to the mitigation of the adverse
effects of these stresses (Hamayun et al., 2010b). In
alfalfa ( M. sativa ), Wang et al. (2011) revealed the role of
Si in enhancing antioxidant defence in plants exposed
to salt stress. They grew two alfalfa cultivars (highly
salt-tolerant Zhongmu No. 1 and poorly salt-tolerant
Defor) under saline conditions (120 mM NaCl). After 15
days of salt stress, the plants exhibited alterations of
their antioxidant metabolism in the roots and shoots of
both cultivars. However, applying Si (1 mM) to both cul-
tivars under NaCl stress significantly increased the
antioxidant enzymes' activities in different plant parts,
except for SOD. These results suggested that Si could
alter the activity of antioxidant enzymes of plants to
improve their salt tolerance. Responses of two widely
cultivated pulse crops, V. radiata and V. unguiculuta , were
studied under drought stress (100, 75, 50 and 25% soil
moisture regimes) by Hamid et al. (2012b). Both crops
showed reductions in carbohydrates and protein upon
exposure to drought. However, Si application (20, 40 or
60 mg/kg) significantly increased total carbohydrate and
protein contents in those plants under drought condi-
tions. However, the effect of Si on plant metabolism
depends upon the applied concentration and the plant
species (Hamid et al., 2012b). In their experiment,
Parande et al. (2013) observed that salt stress (1-5 dS/m)
caused significant reductions in number of seeds per
pod, number of fertile pods and dry matter in P. vulgaris .
However, application of Si (0.5, 1 or 2 mM) significantly
increased the 100-seed weight and yield of beans under
saline environments. However, the effects of both salt
and Si greatly depended on the doses of application. It
was evident that application of 1 mM Si could give the
best protection against salinity (Parande et al., 2013).
In salt-sensitive V. faba , high salinity (100 mM NaCl)

11.4.7 trace elements
11.4.7.1 Silicon
Many reports have appeared on silicon (Si) research
in the last few decades, but this element continues to be
an anomaly - an underappreciated element and in most
cases one not considered essential for plant function.
Numerous research reports have provided evidence for
the notion that Si may play a vital role in conferring
tolerance to adverse environmental factors in plants.
Hence, Si is considered as a beneficial element for plants
growing under stressful conditions (Hasanuzzaman
et al., 2014c).
Wang and Han (2007) studied the mechanisms of
Si-mediated salt stress tolerance in alfalfa ( M. sativa ),
reporting that exogenous Si could alter the ion distribu-
tion in different plant parts. They found that salt stress
(NaCl 120 mM) affected not only the macronutrient
distribution but also the micronutrient distribution in
alfalfa plants, which was reflected by a drastic increase
in Na + content, a slight increase in Zn 2+ content and
significant decreases in K + , Ca 2+ , Mg 2+ and Cu 2+ .
Compared with the NaCl treatment, the added Si signif-
icantly decreased Na + content in the roots, but notably
increased K + content in the shoots and leaves of the
highly salt-tolerant Zhongmu No. 1 cultivar. Applying Si
to both sensitive and tolerant cultivars under NaCl stress
did not significantly affect the Fe 3+ , Mg 2+ and Zn 2+ con-
tents in the roots, shoots and leaves of Defor or in the
roots and shoots of Zhongmu No. 1, but increased the
Ca 2+ content in the roots of Zhongmu No. 1 and the
Mn 2+ content in the shoots and leaves of both cultivars,
while it decreased Ca 2+ and Cu 2+ contents of the shoots
and leaves of both cultivars under salt stress. These well-
integrated ion distribution systems in plants due to Si
application conferred better salt tolerance in alfalfa
(Wang & Han, 2007). Zuccarini (2008) reported that
exogenous Si could enhance the photosynthesis, water
relations and nutrient uptake of P. vulgaris under NaCl
stress. This study showed that salt stress (30 and 60 mM
NaCl) significantly reduced growth, stomatal conductance