Oxidative stress and antioxidant defence systems in response to pesticide stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 105

Formulations that are made up of a single isomer like
deltamethrin are relatively more toxic than those for-
mulations that are made up of two, or four to eight
isomers. For example, the female rat acute oral LD 50 of
permethrin increases from 224 mg of the pyrethroid per
kilogram of body weight (mg/kg) to 6000 mg/kg as the
proportion of the trans isomer increases from 20% to
80% (Bradbury & Coats, 1989).

well as high concentrations (Bandyopadhyay et al.,
1979; Ruiz-Sainz et al., 1985). Abu-Gharbia (1996) in
his experiment observed that the growth of both
free-living and symbiotic rhizobia in four leguminous
species was affected by four pesticide treatments. Gaucho
(an insecticide containing imidacloprid) and Vitavax-300
fungicide (carboxin and captan) when applied in higher
concentrations inhibited the growth of root nodule
bacteria under in vitro conditions (Miettinen & Echegoyen,
1996). Hancock et al. (1998) in an experiment on
fumigated agricultural soil observed that rhizobia can
oxidize methyl bromide, which suggests the importance
of these microbes in decontamination and recycling of
organic compounds and finally suggests their role in
environmental cleanup.
Pyrethroids being highly hydrophilic can adhere to
organic matter and are transported into the lipid bilayer
of plant cells and get strongly adsorbed by soil particles
(Briggs et al., 1983; Anonymous, 1986). Once pyrethroids
are absorbed in the plant body they tend to remain
immobile, and their leaching into the groundwater
via soil or translocation through a plant is uncommon
(Anonymous, 1986). Pyrethroids mostly remain con-
fined to the upper 5-10 cm of soil after field applications
(Reed, 1983; Anonymous, 1986). But it has been observed
that several principal pyrethroid degradation products
like 3-phenoxybenzoic acid and dichlorovinyl acid leach
readily (NRC,1986; IRPTC,1990). Pyrethroids are some-
times removed from the site of their application due to
spray drift, soil erosion or volatilization (evaporation).
Pyrethroids can drift from highly contaminated agricul-
tural fields and pollute nearby surface waters; residues
have been reported several months after their applica-
tion. Pyrethroid runoff from a cotton field after a heavy
rain shower affected various invertebrates in the nearby
pond (NRC, 1986).
Blue-green algae (cyanobacteria) found within moist
soils serve as excellent tools for the study of pesticide
toxicity (McCann & Cullimore, 1979). Raghu and
MacRae (1967) observed that sometimes pesticides in a
poorly soluble state actually stimulate and enhance algal
growth. Algae occasionally display positive responses to
pesticides with little or no effects at low doses, and
growth inhibition only at high doses. For example,
impact on growth parameters like cell volume, cell
number and protein content in Chlorella was low with
10 mg/L methyl parathion, moderate with 20 mg/L and
severe with 30 mg/L (Saroja & Bose, 1982). Studies

7.2 Fate of pesticides in plants,
soil and water

Pyrethroids undergo degradation in the environment
through both biotic and abiotic processes, including
metabolic degradation by plants, animals and micro-
organisms and degradation by light via photolysis.
Degradative photolysis of pyrethroids occurs in three
main ways by the action of light, including: cleavage of
a double bond between a carbon atom and an oxygen
atom (i.e. ester cleavage); removal of halogens (chlo-
rine, fluorine or bromine atoms), i.e. dehalogenation;
and conversion of one isomeric form to another (i.e.
isomerization). The main product of photolysis of
pyrethroids is 3-phenoxybenzoic acid (Davies, 1985).
Pyrethroid degradation in the soil is mostly by chemical
and microbial action, and the rate of degradation
depends on the type of pyrethroid, soil, climatic condi-
tions, and the nature and size of microbial species present
in the soil.
Insecticides are non-target specific; i.e. besides killing
insect pests, they also affect the activity of beneficial
microbial communities in soil and beneficial insects.
Microbes present in the soil play a major role in the
metabolism of both organic and inorganic constituents
in soil. Accumulation of insecticidal residues in soil
may have deleterious effects on the various activities of
soil microbes (Edward, 1972). Excessive concentration
of insecticides in the environment may shift microbial
populations to temporarily favoured group(s) of micro-
organisms, which may overpopulate the soil (Tu &
Mile, 1976).
Rhizobia exhibited varied growth patterns following
pesticide treatments. Some pesticides do not affect
the growth of rhizobia when applied at field rates
(Mongollon et al., 1986; Torralba-Redonde et al., 1986;
Martensson, 1992), whereas some pesticides are found
to be highly toxic to rhizobia when applied in low as