Environmental Engineering Reference
In-Depth Information
of Co and Cr, while the precipitation and EDTA
treatments reduced and alleviated the inhibitory
effects of the high concentrations of Co and Cr,
and returned all the measured parameters to be-
come around the control values. So the applica-
tion of these treatments can be recommended,
considering that it is cheap, simple, and easy to
apply and also safe on the plant, soil, and envi-
ronment; this will in turn help maintain soil fertil-
ity, then plant, animal, and human health.
attempt to analyze the effect of Si application on
growth, photosynthesis and ultrastructure of bar-
ley under Cr stress, Ali et al. (
2013
) carried out a
hydroponic experiment. The treatments consisted
of three Si (0, 1, and 2 mM) and two Cr (0 and
100 μM) levels. The study revealed that Si appli-
cation at both levels enhanced plant growth rela-
tive to the control, and alleviated Cr toxicity by
significant increase in growth and photosynthetic
parameters and also alleviated the ultrastructural
disorders both in roots and leaves, with 2 mM Si
having greater effect than 1 mM Si. Exogenous
Si, apparently behaved antagonistically to Cr,
suggesting Si as a candidate for Cr detoxification
in crops under Cr-contaminated soil. In another
study using rice plants, Zeng et al. (
2011
) inves-
tigated the alleviatory effect of Si on Cr toxic-
ity using a hydroponic experiment with two Cr
levels (0 and 100 μmol L
− 1
), three Si levels (0,
1.25, and 2.5 mmol L
− 1
), and two rice genotypes,
differing in grain Cr accumulation. The results
showed that toxic effects of 100 μmol L
− 1
Cr
treatments on antioxidant enzymes (SOD and
APX in leaves; CAT and APX in roots) and other
parameters were greatly alleviated due to Si ad-
dition to the culture solution. Compared with the
plants treated with Cr alone, Si addition mark-
edly reduced Cr uptake and translocation in
rice plants. No significant differences were ob-
served between the two Si treatments (1.25 and
2.5 mmol L
− 1
) in this case. It was concluded that
Si alleviated Cr toxicity mainly through inhibit-
ing the uptake and translocation of Cr and en-
hancing the capacity of defense against oxidative
stress induced by Cr toxicity (Zeng et al.
2011
).
Tripathi et al. (
2012
) have also observed the role
of exogenous Si addition in increasing Cr(VI)
tolerance in rice seedlings, where Si addition al-
leviated Cr toxicity and promoted growth of rice
by decreasing Cr accumulation, root-to-shoot Cr
transport, and MDA level. It is significant in re-
duction of Cr content in edible parts as Si addi-
tion in Cr-contaminated soils can help to reduce
Cr contamination of grains by inhibiting Cr ac-
cumulation, and therefore, its transport into the
edible parts.
6.2.5
Silicon Application
for Amelioration of Chromium
Phytotoxicity
Silicon (Si) is the second most abundant element
both on the earth's surface and in the soil (Gong
et al.
2006
). Majority of Si in soil is insoluble
and combined with other elements to form ox-
ides or silicates, therefore, not available for
plants (Richmond and Sussman
2003
). Si con-
centrations vary greatly with plant species and
tissues ranging from 0.1 to 10 % of dry weight
(Liang et al.
2007
). Rice plants are typical Si ac-
cumulators and are able to accumulate Si up to
10 % of dry weight (Ma et al.
2006
). Although
Si has not been evidenced as an essential ele-
ment for higher plants, it is generally considered
as a beneficial element for higher plants, espe-
cially for those grown under abiotic stressed en-
vironments, in particular for gramineous plants,
including rice (Richmond and Sussman
2003
;
Liang et al.
2007
). Many evidences demonstrated
that Si can reduce the toxicity of heavy metals
to plants such as Cd (Liang et al.
2005
; Shi et al.
2010
), Mn (Shi
2005
), and Zn (Kaya et al.
2009
).
Previous studies suggested that Si-mediated in-
crease in metal tolerance is based on several pos-
sibilities such as decrease in metal accumulation
and transportation, improved mineral elements
status, decreased oxidative stress, increased an-
tioxidant capacity, and maintained ultrastruc-
ture. Therefore, a better understanding of these
mechanisms associated with exogenous Si ad-
dition in plants could shed light on mechanisms
related to Cr tolerance (Tripathi et al.
2012
). In an
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