Effect of pesticides on leguminous plants: An overview - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 92

To control the pests of leguminous plants various types
of pesticides are sprayed on them such as carbofuran,
chlorfenvinphos, chlordane, benomyl, carbaryl, dieldrin,
aldrin, carbendazim and chlorpyrifos (Lama, 2008).
Uptake of pesticides in plants can occur through roots to
any of the aerial parts such as leaves and fruits. Uptake of
pesticides depends on various factors including the dis-
sociation constant and lipophilicity, the physicochemical
properties of the pesticides and soil, and environmental
conditions such as temperature, precipitation, sunlight
and relative humidity (Finlayson & MacCarthy, 1973;
Schroder & Collins, 2002). Once taken up by plants, the
pesticides translocate to the shoots either through xylem
or phloem, or in roots via the phloem alone. Pesticides'
mobility depends on their water solubility; they can be
stored adjacent to the absorption site, in storage cells
adjoining the translocation pathway or in the areas that
are very actively metabolizing. As the conditions in plants
change, pesticides may be released and transported to
other plant parts.
Metabolism can alter pesticide structure and the pesti-
cide may be detoxified or activated. As previously
mentioned, three phases of plant metabolism can be rec-
ognized (Karthikeyan et al., 2003). Phase I reactions are
the initial reactions of metabolism and are catalysed by
cytochrome P450 enzymes (Mougin et al., 2001). These
reactions transform pesticides by addition of functional
groups. Various types of transformation reactions of pes-
ticides occur in plants, including hydrolysis, oxidation,
epoxidation, O-dealkylation, N-dealkylation and desul-
phuration (Casida & Lykken, 1969). The functional
groups generated in phase I reactions such as hydroxyl
groups act as 'handles' for further conjugation and
storage reactions catalysed by phase II and phase III
enzymes. In pesticide hydrolysis reactions, plant hydro-
lytic enzymes such as esterases, proteases and lipases
detoxify and degrade the pesticide (Singh, 2012).
Oxidation reactions are the reactions in which an elec-
tron is lost by an element or there is an increase in the
proportion of oxygen in a molecule. These are the most
important reactions of phase I metabolism. Oxidation of
pesticides can be divided into two groups: microsomal
oxidation and extramicrosomal oxidation. In micro-
somal oxidation reactions, an atom of an oxygen
molecule is added to the substrate while the second
atom of the oxygen molecule is reduced to water.
NADPH acts as the source of the electron, which is used
to reduce FAD or cytochrome P450. Epoxidation is an

important microsomal reaction catalysed by the enzyme
epoxidase. It leads to the formation of epoxides, which
are stable and persistent in the environment. It also leads
to the production of arene oxides, which are epoxide
intermediates in aromatic compound hydroxylation. In
N-dealkylation reactions alkyl groups in carbamates,
amides or amines or the alkyl groups attached to the
electronegative nitrogen atoms are oxidatively removed
by converting them to the corresponding aldehydes. The
reaction leads to the detoxification of the compound.
There is spontaneous rearrangement of this α-hydroxy
intermediate to form the aldehyde. Ether and ester
structures of organophosphorus insecticides undergo
O-dealkylation and are detoxified. Desulphuration
reactions are also called phosphorothioate oxidations.
These reactions increase the insecticidal activity and
mammalian toxicity of organophosphorus compounds
such as phosphorodithioate and phosphorothionates.
Microsomal mono-oxygenase oxidatively desulphurates
the P = S groups of these compounds to their respective
P = O analogues. The P = O analogues have more affinity
to acetylcholinesterase and thus inhibit acetylchlolines-
terase more potently (Singh, 2012). In phase II reactions
soluble conjugates are formed (mainly with amino
acids and sugars). Glutathione, amino acids, glucose and
malonic acid are the plant products involved in conjuga-
tion. Soluble conjugates formed during phase II reactions
are stored in vacuoles (Sandermann, 1992; Korte et al.,
2000). In phase III reactions bound or non-extractable
residues are formed by conjugation to insoluble struc-
tures in the plant. One such reaction is co-polymerization
with lignin. By turning the toxic pesticides into bound
residues, their water solubility is reduced and so their
reactivity and toxicity are also reduced.
Many of the pesticides have a long half-life and persist
for a long time in plants. This persistence is harmful to
the plants as well as to the animals and humans that
ingest them as food. Neetu (2013) detected pesticide
residues of DDT in four types of pulses, namely red len-
tils, pigeon pea, green gram and black gram. The
pesticide pirimiphos-methyl was detected in toasted
chickpea flour samples collected from the markets of
Tenerife in the Canary Islands (Gonzalez-Curbelo et al.,
2012). Organophosphorus pesticides such as ometho-
ate, isocarbophos and triazophos were detected in
cowpea and kidney pea samples collected from farmers'
markets (Ping-Sheng & Qi, 2010). Various techniques
can be used to determine the types and quantity