Agriculture Reference
In-Depth Information
of opening of the sodium channels. Normal opening and
closing should occur in less than a millisecond when an
impulse passes. But pyrethroid poisoning prolongs the
opening and delays the closing leading to sodium leak-
ing out when the channel should be closed. For the type
2 pyrethroids the tail current is much more distinct
and may last for minutes. The resting potential is not
restored and the impulse does not pass distinctly, but
becomes a train of action potentials because a lower
potential rise is necessary to reach the threshold for the
action potential (Stenerson, 2004). Sodium channels
are pesticide specific; not only pesticides like DDT and its
analogues but also poisons from plants, scorpions, sea
anemones and amphibians act by binding to one of the
active sites. It has also been observed that cross-resis-
tance often occurs between DDT analogues and the
pyrethroids. This type of resistance is called knockdown
resistance (Kdr). Various insecticides, either naturally
occurring pyrethrins or the synthetic pyrethroids, are
nerve poisons.
7.4.1 Specific biochemical properties
of rOS in plants
Reactive oxygen species, being derivatives of oxygen, are
the most important group of free radicals in living organ-
isms. Each one has specific properties; for example, H
2
O
2
can easily diffuse through membranes and react at distant
sites by comparing to charge
.
O
2
-
. Therefore, O
2
-
is
significant in initiating oxidative damage in distant cel-
lular components. On the other hand, the protonated
form of O
2
, the hydroperoxyl radical (HO
2
-
), can cross
biological membranes as a neutral molecule and by
extracting protons from polyunsaturated fatty acids
(PUFA) can easily propagate lipid oxidation (Gechev
et al.,
2006). Superoxide dismutase (SOD) is an enzyme that
converts O
2
-
into H
2
O
2
. This conversion usually occurs
spontaneously by dismutation of O
2
-
, but the velocity of
the reaction in the presence of SOD in most living organ-
isms is greatly increased. H
2
O
2
is a less reactive oxidizing
agent present in micro- to millimolar ranges in plant tis-
sues (Cheeseman, 2006; Halliwell & Gutteridge, 1999,
Khan & Kour, 2007), and O
2
-
and H
2
O
2
by themselves are
relatively harmless and removed from biological systems
in the absence of any metal ions. However, their dam-
aging effects become more explicit when they interact to
form highly reactive hydroxyl radicals (OH•) in the
presence of metal ions (Peixoto
et al.,
2006; Moller
et al.,
2007). For a historical view, towards the end of the 19th
century, Fenton defined the oxidizing potential of H
2
O
2
with ferrous salts (Fe
2+
) (Fenton, 1894, 1899):
7.4 Oxidative stress and rOS
production in plants
Molecular oxygen serves as a two-edged sword due to
reactive oxygen species (ROSs) in aerobic organisms
(Halliwell & Gutteridge, 1989). Thus oxidative stress can
simply be defined as imbalance between production and
removal of ROS, leading to several physiological chal-
lenges in any cellular compartment. The hypothetical
fine-tuned balance could be disturbed because of an excess
accumulation of ROS or decline of antioxidants. The redox
equilibrium changes when ROS levels are increased or the
plant's antioxidant defence system (ADS) is depleted.
Generally, molecular oxygen (O
2
) is an inert molecule
in the absence of any catalysts. Higher organisms use it
in the pathway of four-electron reduction, leading to
production of water (H
2
O). However, sometimes one-,
two- and three-electron reduction of O
2
or excitation of
triplet oxygen (3O
2
) occur and cause the formation of
superoxide radical (O
2
-
) or hydroperoxyl radical (HO
2
•),
hydrogen peroxide (H
2
O
2
),hydroxylradical(OH•)and
singlet oxygen (
1
O
2
), respectively (Apel & Hirt, 2004).
Compared to molecular oxygen, all these species are
more toxic and unstable, ready to participate in unlim-
ited chemical oxidation reactions, hence they are
termed reactive oxygen species (ROS) (Bartosz, 1997).
2
+
+
+ →++
−
3
Fe
HO
Fe
OH
OH
(7.1)
22
The above reaction is limited by the bioavailability of
ferrous ion, but with the help of reducing agents such as
O
2
-
, ferric ion (Fe
3+
) can be recycled to the reduced
ferrous state.
O e
−
+ →+
3
+
FeO
2
+
(7.2)
2
2
Fortyyearslater,HaberandWeiss(1934)identifiedOH•
as the oxidizing species. The sum of reactions 7.1 and
7.2givestheformationofOH•fromO
2
-
and H
2
O
2
cata-
lysedbytransitionmetalions.Today,OH•radicalsare
known to be generated by the Haber-Weiss reaction
(Equation 7.3) and other transition metal ions, like Cu
+
and Cu
2+
can replace Fe
2+
and Fe
3+
in these reactions
(Bartosz, 1997).
(
)
HO
+ →+
•
O HOHOHaber
−
•
−
+
−
Weiss reaction
22
2
2
(7.3)
Search WWH ::
Custom Search