class of pesticides typically die in the moulting
position unable to discard their exuviae. Inorganic
substances such as silica gels can also act on the
waxy cuticle of pests to dehydrate them and induce
death.
• Respiratory inhibitors
This includes the class amidino hydrazones, which
inhibit the mitochondrion from carrying out cellular
respiration. The result of this is that cells are unable
to carry out the biochemical functions needed for
survival and the pest dies.
4.4 Herbicides
Till now, we have discussed animal pests, however, plants
also can be pests, in which case, they are termed as weeds.
The term weed is of no botanical significance. Any plant,
which is of economic significance in one circumstance may
be a weed in another, if it is growing in the cropland is
unwanted. Weeds damage crops by directly competing for
resources such as sunlight, water and nutrients. The earliest
methods of weed control probably involved manually
removing them from the fields. However, with the advent of
chemical fertilizers and pesticides, which are input costs in
agriculture, there was a greater need to remove weeds, which
decreased profitability of farmlands. Herbicides have a
specific target, mostly enzyme in the plant cells, which they
inhibit. Herbicides are able to selectively affect weeds over
crop plants based on fundamental biochemical differences,
e.g. some herbicides only affect broad leaves, whereas others
depend on the fact that some crop plants are able to
metabolize and detoxify the herbicide quicker than the weed
(Hall et al. 1999).
Much like pesticides, herbicides can also be classified
based on several categories; chemical class, mode of
application, mode of action, spectrum of use, however, we
will focus on the mode of action only, as the basis of
classification. Based on the mode of action or mode of
toxicity, herbicides can be classified under the following
categories:
A. Lipid synthesis inhibitors
Lipids or fats form an integral part of any cell, animal or
plant during the formation of the lipid bilayer membrane. Herbicides from the chemical classes like,
aryloxy-phenoxy-propionate, cyclohexanedione and
phenylpyrazolin, act by blocking the enzyme acetyl
coenzyme A carboxylase, which catalyses the first step
in fatty acid synthesis and phospholipid production
(Sherwani et al. 2015).
B. Protein synthesis inhibitors
These herbicides inhibit the action of the enzyme acetohydroxy acid synthase, which catalyses the first step
in the synthesis of branch chained amino acids (Sherwani et al. 2015).
C. Nucleic acid synthesis inhibitors
This class of herbicides is also called as synthetic
auxins, as they mimic the activity of indole acetic acid
and disrupt nucleic acid synthesis in the cell (Sherwani
et al. 2015).
D. Photosynthetic pigment inhibitors
These are also called as carotenoid biosynthesis inhibitors, as these herbicides inhibit the synthesis of photosynthetic pigments and lead to bleaching, wilting and
eventual death of pigments. They bind to the Q protein
and stop the electron transport chain (ETC) and inhibit
carbon dioxide fixation (Das and Mondal 2014).
E. Reactive Oxygen Species (ROS) formation
These herbicides include the Photosystem I (PS I)
inhibitor family, which are represented by
the bipyridilium family. They accept electrons from
PS I and generate herbicide radicals, which in the
presence of superoxide dismutase from hydrogen peroxide and hydroxyl radicals generates ROS (Sherwani
et al. 2015). ROS thus generated disrupt the cell
membrane, finally leading to lysis.
F. Proto-porphyrinogen oxidase (PPO) inhibitor
Proto-porphyrinogen oxidase (PPO) enzyme plays a key
role in chlorophyll biosynthesis. Certain herbicides inhibit
the action of PPO, leading to its accumulation in the cell.
The PPO is then converted to proto-porphyrin, which is
toxic and leads to rupture of membranes (Das 2013).
G. Nano-herbicides
Much like pesticides, herbicides also suffer from the
same drawbacks of non-selectivity, instability, volatility, etc. Therefore, much like the nano-pesticides, nanotechnology is being used to design and synthesize
better nano-herbicides also. Research efforts are being
put into polymeric carriers for controlled release of the
herbicide as well as nano-coatings to specifically target
weed root receptors (Manjunatha et al. 2016). In recent
times, some inorganic nanoparticles such as silica
nanoparticles (SiNPs) are also being tested as potential
herbicides due to selective absorption (Abigail and
Chidambaram 2017).
4.5 Limitations of Engineered
Nanoparticles-Based Nano-Pesticides
Nano-pesticides offer a promise of augmenting or even
replacing the current conventional pesticide regime, due to
Nano-fertilizers and Nano-pesticides as Promoters of Plant …
161
position unable to discard their exuviae. Inorganic
substances such as silica gels can also act on the
waxy cuticle of pests to dehydrate them and induce
death.
• Respiratory inhibitors
This includes the class amidino hydrazones, which
inhibit the mitochondrion from carrying out cellular
respiration. The result of this is that cells are unable
to carry out the biochemical functions needed for
survival and the pest dies.
4.4 Herbicides
Till now, we have discussed animal pests, however, plants
also can be pests, in which case, they are termed as weeds.
The term weed is of no botanical significance. Any plant,
which is of economic significance in one circumstance may
be a weed in another, if it is growing in the cropland is
unwanted. Weeds damage crops by directly competing for
resources such as sunlight, water and nutrients. The earliest
methods of weed control probably involved manually
removing them from the fields. However, with the advent of
chemical fertilizers and pesticides, which are input costs in
agriculture, there was a greater need to remove weeds, which
decreased profitability of farmlands. Herbicides have a
specific target, mostly enzyme in the plant cells, which they
inhibit. Herbicides are able to selectively affect weeds over
crop plants based on fundamental biochemical differences,
e.g. some herbicides only affect broad leaves, whereas others
depend on the fact that some crop plants are able to
metabolize and detoxify the herbicide quicker than the weed
(Hall et al. 1999).
Much like pesticides, herbicides can also be classified
based on several categories; chemical class, mode of
application, mode of action, spectrum of use, however, we
will focus on the mode of action only, as the basis of
classification. Based on the mode of action or mode of
toxicity, herbicides can be classified under the following
categories:
A. Lipid synthesis inhibitors
Lipids or fats form an integral part of any cell, animal or
plant during the formation of the lipid bilayer membrane. Herbicides from the chemical classes like,
aryloxy-phenoxy-propionate, cyclohexanedione and
phenylpyrazolin, act by blocking the enzyme acetyl
coenzyme A carboxylase, which catalyses the first step
in fatty acid synthesis and phospholipid production
(Sherwani et al. 2015).
B. Protein synthesis inhibitors
These herbicides inhibit the action of the enzyme acetohydroxy acid synthase, which catalyses the first step
in the synthesis of branch chained amino acids (Sherwani et al. 2015).
C. Nucleic acid synthesis inhibitors
This class of herbicides is also called as synthetic
auxins, as they mimic the activity of indole acetic acid
and disrupt nucleic acid synthesis in the cell (Sherwani
et al. 2015).
D. Photosynthetic pigment inhibitors
These are also called as carotenoid biosynthesis inhibitors, as these herbicides inhibit the synthesis of photosynthetic pigments and lead to bleaching, wilting and
eventual death of pigments. They bind to the Q protein
and stop the electron transport chain (ETC) and inhibit
carbon dioxide fixation (Das and Mondal 2014).
E. Reactive Oxygen Species (ROS) formation
These herbicides include the Photosystem I (PS I)
inhibitor family, which are represented by
the bipyridilium family. They accept electrons from
PS I and generate herbicide radicals, which in the
presence of superoxide dismutase from hydrogen peroxide and hydroxyl radicals generates ROS (Sherwani
et al. 2015). ROS thus generated disrupt the cell
membrane, finally leading to lysis.
F. Proto-porphyrinogen oxidase (PPO) inhibitor
Proto-porphyrinogen oxidase (PPO) enzyme plays a key
role in chlorophyll biosynthesis. Certain herbicides inhibit
the action of PPO, leading to its accumulation in the cell.
The PPO is then converted to proto-porphyrin, which is
toxic and leads to rupture of membranes (Das 2013).
G. Nano-herbicides
Much like pesticides, herbicides also suffer from the
same drawbacks of non-selectivity, instability, volatility, etc. Therefore, much like the nano-pesticides, nanotechnology is being used to design and synthesize
better nano-herbicides also. Research efforts are being
put into polymeric carriers for controlled release of the
herbicide as well as nano-coatings to specifically target
weed root receptors (Manjunatha et al. 2016). In recent
times, some inorganic nanoparticles such as silica
nanoparticles (SiNPs) are also being tested as potential
herbicides due to selective absorption (Abigail and
Chidambaram 2017).
4.5 Limitations of Engineered
Nanoparticles-Based Nano-Pesticides
Nano-pesticides offer a promise of augmenting or even
replacing the current conventional pesticide regime, due to
Nano-fertilizers and Nano-pesticides as Promoters of Plant …
161