Environmental Engineering Reference
In-Depth Information
et al.
2003
; Foyer and Noctor
2005
; Shao et al.
2008
). In addition to the above, GSH also acts
as a precursor for the synthesis of PCs. These are
a set of novel heavy metal-binding peptides also
found in higher plants (Gekeler et al.
1989
). PCs
are synthesized inductively by exposure not only
to Cd but also to other heavy metals. Thereafter,
numerous physiological studies have indicated
their role in heavy metal detoxification as well
as in the maintenance of ionic homeostasis (Zenk
1996
; Hirata et al.
2005
). A survey of the plant
kingdom has provided evidence for the occur-
rence of PCs in angiosperms, gymnosperms, and
bryophytes (Gekeler et al.
1989
; Yadav
2010
).
Recently, the role of reduced GSH in maintaining
cellular oxidation balance and protection against
drought, salinity, and heavy metals has been
proven (Chen et al.
2010
; Cai et al.
2011
; Zeng
et al.
2012
). In one such study, a hydroponic ex-
periment was conducted to determine the pos-
sible effect of exogenous GSH in alleviating Cr
stress through examining plant growth, chloro-
phyll contents, antioxidant enzyme activity, and
lipid peroxidation in rice seedlings exposed to Cr
toxicity (Zeng et al.
2012
). The results showed
that plant growth and chlorophyll content were
dramatically reduced when rice plants were ex-
posed to 100 μM Cr. Addition of GSH in the
culture solution alleviated the reduction of plant
growth and chlorophyll content. It also enhanced
antioxidant capacity in Cr-stressed plants as an-
tioxidant enzymes like SOD, CAT, glutathione
reductase, and glutathione peroxidase showed
increased activities under Cr stress in both leaves
and roots. Furthermore, exogenous GSH also
caused significant decrease of Cr uptake and
root-to-shoot transport in the Cr-stressed rice
plants, assuming that GSH was involved in Cr
compartmentalization in root cells (Zeng et al.
2012
). The effects of exogenous reduced GSH
on alleviation of Cr(VI) toxicity to rice seedlings
and its physiological mechanisms were also in-
vestigated in a series of experiments by Qiu et al.
(
2013
). The addition of GSH alleviates negative
effects due to Cr-induced toxicity. It was con-
cluded that the alleviation of Cr(VI) toxicity by
exogenous GSH is directly attributed to its regu-
lation on forms of Cr ions in the rhizosphere and
their distribution at subcellular levels. In addition
to reduced GSH, Cao et al. (
2013
) conducted a
series of experiments to determine the alleviating
effects of GSH, Se, and Zn under combined con-
tamination of Cd and Cr in rice. GSH and GSH
+ Zn application significantly alleviated growth
inhibition induced by combined stress of Cd and
Cr in rice plants. Exogenous GSH and GSH + Zn
effectively decreased Cr accumulation.
6.2.3
Iron Application
for Amelioration of Chromium
Phytotoxicity
Iron (Fe) is a cofactor for approximately 140 en-
zymes that catalyze unique biochemical reactions
(Brittenham
1994
). It is also required at several
steps in the biosynthetic pathways and fills many
essential roles in plant growth and development,
including chlorophyll synthesis, thylakoid syn-
thesis, and chloroplast development (Miller et al.
1995
). Fe is also required by both legume and
root nodule bacteria for many metabolic func-
tions at several key stages in the symbiotic N
2
fixation process and is critical for N
2
fixation due
to its role in the activity of both leghemoglobin
and nitrogenase (Kaiser et al.
2003
). Symbiotic
N
2
fixation was shown to have a high require-
ment for iron in lupine (Tang et al.
2006
) because
Fe is an essential component of nitrogenase,
leghemoglobin, and ferrodoxins (Evans and Ros-
sel
1971
). Several theories have been proposed
to explain the underlying protection mechanism
of Fe to heavy metal toxicity. Supplemental Fe
on roots was suggested to act as: (a) a shield that
protects roots by coprecipitation of other heavy
metals, (b) a nutrient reservoir, and (c) a reser-
voir for active ferrous (Fe
2 +
)-Fe inside cells
that could compete with heavy metals for meta-
bolically sensitive sites inside plants (Sinha et al.
2005
). Improvement in growth characters as a
result of application of micronutrients might also
be due to the enhanced photosynthetic and other
metabolic activity, which leads to an increase
in various plant metabolites responsible for cell
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