Exogenous application of phytoprotectants in legumes against environmental stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 167

different legume crops behave differently under drought
stress, whereas only a few can tolerate moderate levels
of salt stress. Numerous research findings have indi-
cated a strong negative correlation between water
shortage in soil and legume growth and yield. Among
the growth stages, the reproductive stage is considered
the most sensitive to water stress (Katerji et al., 2011). It
is noteworthy that drought stress also considerably
impairs the symbiotic N fixation in leguminous crops
(Katerji et al., 2011).
Zare et al. (2012) reported that grain yield of V. radiata
was decreased by 23% after imposition of drought com-
pared with non-stress conditions; this was attributed to
reductions in pod weight, number of pods per plant and
plant dry weight under drought stress. While studying
lentil and grass pea, Talukdar (2013) observed that plant
growth traits and seed yield components were reduced
significantly in both crops with increasing duration of
water stress. However, lentil was more sensitive to
drought compared to grass pea. Drought stress caused
decreases in leaf RWC, K + /Na + ratio, chl a , chl a / b ratio,
stomatal conductance and net photosynthetic rate.
Nodulation of both crops was drastically reduced due to
drought stress. Seed yield differences among genotypes
under drought stress conditions have been reported in
P. vulgaris (Terán & Singh, 2002a; Beebe et al., 2008)
besides associated decreases in yield attributes such as
number of pods per plant, number of seeds per plant,
number of seeds per pod and 100-seed weight (Terán &
Singh, 2002b; Beebe et al., 2008, 2010). Under drought
stress (non-irrigated conditions) P. vulgaris showed
65%, 34%, 29% and 12% reductions in seed yield, seed
number per square metre, pod number per square
metre and 100-seed weight compared to irrigated con-
ditions (Assefa et al., 2013). However, there was great
variation among the genotypes tested for drought stress
response. Importantly, pod harvest index, which was
reported as a selection criterion to improve drought
resistance in white pea bean, was greater in drought-
tolerant genotypes. Under laboratory conditions,
polyethylene glycol (PEG) is widely used to impose
drought stress in plants. In an experiment, Muscolo et al.
(2014) investigated four lentil genotypes (Castelluccio,
Eston, Pantelleria and Ustica) under different levels of
drought (10, 15, 18 and 21% PEG-6000). Drought
stress caused reductions in germination percentage,
root length, tissue water content, α- and β-amylase and
α-glucosidase activities, and osmolyte content in

dose-dependent manners. The drought-induced damage
was variable depending on the genotypes - drought-
tolerant Eston and Castelluccio genotypes exhibited
higher accumulations of Pro and soluble sugars as well as
greater activities of antioxidant enzymes.

11.3.3 toxic metals
The rapid increase in human population together with
speedy industrialization is causing serious environ-
mental problems, including the production and release
of considerable amounts of toxic metals into the envi-
ronment (Sarma, 2011). Excessive uptake of toxic
metals by plants may occur, and these elements may
participate in some physiological and biochemical
reactions that can harm the normal growth of the plant
by disturbing absorption, translocation or synthetic
processes (Hasanuzzaman & Fujita, 2012a,b). Most of
the cultivated legumes are sensitive to toxic metals;
however, many legumes have been found as natural
pioneers in contaminated sites undergoing revegetation,
prompting their use for remediation purposes (Dashti
et al., 2009; Khan et al., 2009). For instance, Lupinus
albus L., Mimosa caesalpiniaefolia , Erythrina speciosa and
Schizolobium parahyba have been proposed as potential
species to remediate sites, leaving aside considerations
of growth or biomass production in soils contaminated
with metals like zinc (Zn) and lead (Pb) (Pastor et al.,
2003; de Souza et al., 2012). Heavy metals also hamper
the legume-bacteria symbiosis. Talano et al. (2013)
observed considerable reduction in germination and
growth of soybean plants at early stages of growth when
they were exposed to 10 μM arsenate or arsenite. In
addition, arsenic (As) contamination also reduced the
number of effective nodules, even after inoculation
with Bradyrhizobium japonicum - a metalloid-tolerant
microorganism. It was also observed that roots accumu-
lated more arsenic (250 times higher) than shoots
(Talano et al., 2013).
Ergün and Öncel (2010) investigated the toxic impacts
on growth of L. esculenta of cadmium (Cd) and Zn over a
concentration gradient of 250 mg/kg. Increased levels of
Cd caused a huge decrease in root and shoot growth as
well as their dry weight. Both Cd and Zn at 250 mg/kg
resulted in increased Pro content, but this decreased
upon exposure to 500 mg/kg of these metals (Ergün &
Öncel, 2010). Cokkizgin and Cokkizgin (2012) also
investigated the effects of Pb stress on germination of
four lentil genotypes. They observed that germination