Exogenous application of phytoprotectants in legumes against environmental stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 183

11.4.4.3 Tocopherols
Tocopherols are exclusively located in green tissues
such as algae and some cyanobacteria. Their presence
in plastid or thylakoid membranes protects against
photosynthesis-derived ROS (Munné-Bosch, 2005;
Hasanuzzaman et al., 2014e). The tocopherols in cooper-
ation with other antioxidants take part in controlling
ROS, and maintaining the redox state (Munné-Bosch
2005, 2007). Tocopherol levels within plants change
according to stages of growth and development or in
response to environmental stress. Tocopherol synthesis
is linked to levels of different stress hormones like JA,
SA and ABA, and thus tocopherol is supposed to have
roles in stress signalling (Falk et al., 2002; Sandorf &
Holländer-Czytko, 2002).
Pisum sativum plants subjected to drought (leaf water
potential of −1.3 MPa) showed remarkable elevation
(67%) of α-tocopherol (Moran et al., 1994). Chilling
stress (10 °C) increased lipid peroxidation or MDA and
H 2 O 2 accumulation in M. sativa leaves. However, the
redox properties of α-tocopherol played roles in seques-
tering free radicals. α-Tocopherol also gave antioxidant
protection as higher α-tocopherol levels were correlated
with increased activities of SOD, CAT, APX and GR
(Bafeel & Ibrahim, 2008). Studies with different lines of
G. max showed that in comparison with normal growth
temperatures, HT stress resulted in a several-fold increase
in α-tocopherol content, whereas the same temperature
had no significant effect on total tocopherols. When
those same soybean cultivars were exposed to drought
stress, a similar increase in α-tocopherol but no change
in total tocopherol was observed (Britz et al., 2008).
Different levels of salt tress (25, 50, 100, 200 mM)
increased the anthocyanin, ascorbic acid and α-tocopherol
contents in soybean seedlings, but the increases in these
metabolites were shown to be beneficial only in 25 mM
NaCl stress (Mohammed et al., 2012). Exogenous
application of the α-tocopherol analogue 6-hydroxy-
2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox),
showed effective results in protecting thylakoids from
oxidative damage caused by paraquat (Tambussi et al.,
2004). Bean ( P. vulgaris ) seedlings were exposed to
different heavy metals (PbCl 2 , CuCl 2 , CdCl 2 and HgCl 2 )
for 10 days. All heavy metal treatments increased
α-tocopherol, AsA and Pro contents. The extent of the
increase of these metabolites, from highest to lowest,
was: Hg > Cd > Cu > Pb. The increase in α-tocopherol
together with other antioxidants and metabolites is

thought to be due to enhancement of the plants' self-
defence mechanism under heavy metal stress conditions
(Zengin & Munzuroglu, 2005).

11.4.5 Nitric oxide
Recently NO has emerged as an important signalling
molecule and antioxidant. NO triggers expression of
many kinds of redox-regulated (defence-related) genes,
directly or indirectly, to establish plant stress tolerance.
Several recent reports indicate that the application of
exogenous NO donors confers tolerance to various abi-
otic stresses (Hasanuzzaman et al., 2010a, 2011a, 2012c,
2013e; Hasanuzzaman & Fujita, 2013; Gill et al., 2013b).
Nitric oxide has a protective function against oxidative
stress mediated by (i) reaction with lipid radicals, which
stops the propagation of lipid oxidation; (ii) scavenging
O 2 • − and formation of peroxynitrite (ONOO − ), which
can be neutralized by other cellular processes; (iii)
activation of antioxidant enzymes (SOD, CAT, APX,
GPX, GR, POD, etc.); and (iv) functioning as a signalling
molecule in the cascade of events leading to changes in
gene expression. These mechanisms together confer
enhanced antioxidant protection against oxidative
stress (Hasanuzzaman et al., 2013e) (Figure 11.4). Many
experiments have been conducted to investigate the
effect of NO on plant stress tolerance using sodium
nitroprusside (SNP) as NO donor.
In C. arietinum plants, Sheokand et al. (2008) showed
the antioxidant effects of NO (0.2 mM SNP) under salt
stress (100 mM NaCl, 48 h). In their study, salt stress
caused a decrease in RWC and increases in electrolyte
leakage and lipid peroxidation. Interestingly, an exoge-
nous NO donor (SNP) could completely ameliorate the
toxic effects of salt stress on electrolyte leakage and lipid
peroxidation and partially prevent a decline in RWC.
Salt stress also activated the antioxidant defence system
by increasing the activities of POD, CAT, SOD and APX
without altering the activities of GR and DHAR.
However, NO donors further increased the activities of
these enzymes under salt stress. In their experiment,
Moussa and Mohamed (2011) investigated the effects of
seed pretreatment with an NO donor (10 μM SNP) on
drought tolerance in P. sativum seedlings. Drought stress
(PEG-6000, −0.5 MPa) caused a decrease in the activ-
ities of APX, GPX and CAT enzymes and overproduction
of O 2 • − in pea leaves, which in turn caused exacerbation
of lipid peroxidation and depression of photosynthesis.
On the other hand, NO-pretreated plants increased the