Agriculture Reference
In-Depth Information
radicals. The chlorophyll pigments associated with the electron transport system are
the primary source of
1
O
2
. It may also arise as a byproduct of lipoxygenase activity
and is highly destructive, reacting with most biological molecules at near diffusion-
controlled rates. Superoxides, produced by the transport of electron to oxygen, are
not compatible with metabolism and are required to be eliminated by the antioxida-
tive defense system while recycling of phosphoglycolate to phosphoglycerate (re-
enter the Bassam-Calvin cycle) results in a considerable loss of assimilated carbon.
Cadmium has been found to induce oxidative stress in plants (Liu et al.
2007
;
Djebali et al.
2008
; Gill and Tuteja
2010
; Gill et al.
2011
), but in contrast with
other heavy metals, such as Cu, it does not seem to act directly on the production
of ROS through Fenton type reactions (Salin
1988
). cadmium-exposed plants adopt
the process of avoidance of the production of ROS as the first line of defense against
oxidative stress. Once formed, ROS must be detoxified as efficiently as possible
to minimize eventual damage. Thus, the detoxification mechanisms constitute the
second line of defense against the detrimental effects of ROS (Moller
2001
). In fact,
compounds having the property of quenching the ROS without undergoing conver-
sion to a destructive radical can be described as 'antioxidant'. Antioxidant enzymes
are considered as those that either catalyse such reactions, or are involved in the
direct processing of ROS (Medici et al.
2004
). Hence, antioxidants (enzymatic and
non-enzymatic) function to interrupt the cascades of uncontrolled oxidation (Noctor
and Foyer
1998
). Though the expression for antioxidant enzymes is altered under
stress conditions, their upregulation has a key role in combating the abiotic stress-
induced oxidative stress. However, the level of upregulation is subject to type and
magnitude of the stress. Superoxide dismutase (SOD), catalase (CAT), ascorbate
peroxidase (APOX), glutathione reductase (GR), monodehydroascorbate reductase
(MDHAR), dehydroascorbate reductase (DHAR), guaiacol peroxidase (GOPX)
and glutathione S-transferase (GST) showed great variations in their activities de-
pending on the Cd concentration and the plant species used.
Plants exposed to heavy metal stress exhibited an increase in lipid peroxida-
tion due to the generation of free radicals (Vanaja et al.
2000
). Cadmium notably
increased the accumulation of lipid peroxides in
Pisum sativum
(Metwally et al.
2005
),
Oryza sativa
(Ahsan et al.
2007
),
Helianthus annuus
(Groppa et al.
2001
),
Arabidopsis
seedlings (Cho and Seo
2004
),
Brassica juncea
(Mobin and Khan
2007
),
Glycine max
(Noriega et al.
2007
),
Lycopersicon esculentum
(Ammar et al.
2007
),
Brassica napus
(Filek et al.
2008
),
Vigna mungo
(Singh et al.
2008
) and
Lep-
idium sativum
(Gill et al.
2011
). The accumulation of H
2
O
2
after Cd exposure has
been detected in the leaf of different plant species such as
Pisum sativum
(Romero-
Puertas et al.
2004
),
Arabidopsis thaliana
(Cho and Seo 2005),
Brassica juncea
(Mobin and Khan
2007
) and
Vigna mungo
(Singh et al.
2008
). Balestrasse et al.
(
2006
) also reported that Cd produced increased concentrations and in situ accumu-
lation of H
2
O
2
and O
2
·ˉ in soybean leaves. Guo et al. (
2007
) reported that exposure
to 50 mM Cd significantly increased the H
2
O
2
content in the roots of
Oryza sativa
.
It has also been reported that Cd increased the accumulation of H
2
O
2
in soybean
root tips (Yang et al.
2007
).
