Effect of pesticides on leguminous plants: An overview - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 94

6.3.2 effect on yield
Pesticides have inhibitory effects on crop yield, and on
the soil flora and fauna (Fox et al., 2007; Ahemad &
Khan, 2012). Fungicides and insecticides possess a quick
and effective action and are reliable, and many chemi-
cals have been reported to be effective against insect
pests and various crop diseases (Allen, 1982; Cook,
1986; Adejumo, 2005). Experiments have shown
significant differences between damaged (pod borer)
parts in plants treated with pesticides, and untreated
plants. Damaged parts may fail to produce seeds or may
produce low quality or non-viable seeds. Thus pesticides
protect the pods from pod borer and enhance seed yield
(Mugo, 1989). They also bring about effective control of
foliar fungal diseases and insect pests resulting in higher
biomass yields, which are desirable for efficient use of
nutrients and water. As a result, there is accumulation of
photoassimilates, which are used for kernel development
(Okeyo-Owuor & Khamala, 1980; Olupot et al., 2004).
Abulo et al. (2005) observed that high biomass yield con-
tributes to weed suppression and light utilization. All
these factors collectively increase the grain yield. Okeyo-
Owuor (1978) reported that pesticides like dimethoate
reduced yield losses. Experiments on cowpea (a grain
legume) have shown that pesticides effectively impact
the legume-insect pest complex and help in increasing
the yield (Kyamanywa 1996). The pesticide contami-
nates the soil and influences soil microbe population,
which further affects the fertility of soil. However, Dubey
et al. (2012) studied the effect of different pesticides such
as carbosulfan, phorate, thiomethoxam and carbofuran
on the soil microflora of leguminous crops and found
that no significant change occurred in the viability of
bacteria. They further concluded that bacteria dwelling
in the soil possess the ability to degrade the pesticides.
Pesticides disrupt the phytoestrogen signalling system
between rhizobia and legume that regulates their sym-
biotic relationship. They also disrupt the initial binding
of rhizobia to lectins present on the root hairs (Musarrat &
Haseeb, 2000; Fox et al., 2004). As a result, pesticides
affect the symbiotic relationship between leguminous
plants and rhizobia due to competition of pesticides for
binding sites of rhizobia on the root hairs of legumes.
Insecticides inhibit nodulation in the legumes by
reducing the number of available sites on the host plant
for the binding of rhizobia by causing reduction in the
supply of carbohydrates to existing nodules. Thus,
insecticides decrease the growth and survival of rhizobia

and also disrupt the signalling required to initiate the
development of nodules in the plants. Legume plants
grown under pyriproxyfen-amended soils showed a
decrease in the chlorophyll content. The most drastic
effect of this insecticide was observed in chickpea plants,
where chlorophyll content decreased by 14% compared
to control plants. In lentil the reduction in chlorophyll
was found to be 9% compared to control plants.
Chlorophyll content showed a marginal decrease in pea
and green gram. Under the application of pyriproxyfen,
the seed protein of green gram and chickpea was sig-
nificantly decreased, by 4% and 5% respectively, as
compared to control plants, while a marginal reduction
was observed in the seed protein of lentil and pea, by
3% and 1%, as compared to the control (Anderson et al.,
2004). The maximum reduction in seed yield was
observed in lentil while pea showed the least reduction
in seed yield under pyriproxyfen treatment. Various
agrochemicals including insecticides induce the symbi-
otic phenotype by disrupting the attachment of rhizobia
to the roots of host plants, reducing the production of
root nodules and nitrogenase activity, which in turn
decreases the nitrogen content and reduces the plant
yield (Fox et al., 2007). The decrease in phosphorus
content and seed yield under pesticide application is
mainly due to the reduction of the proteins and enzymes
in metabolic pathways that are involved in synthesis of
proteins and uptake of phosphorus (Boldt & Jacobsen,
1998; Nare et al., 2010; Ahemad & Khan, 2011a).

6.3.3 effect on photosynthetic activity
Any reduction in leaf area lessens photosynthetic activity,
which results in decline of yield. Metabolic enzymes
are adversely affected by pesticides; therefore it appears
that the functioning of various enzymes and cofactors
involved in the photosynthetic carbon reduction cycle,
such as NADP, Rubisco, 3-phosphoglyceric acid (3-PGA),
aldolase and NAD-glyceraldehyde-3-P-dehydrogenase,
is inhibited by insecticides (Mapose & Cossa, 2005). In
leguminous plants, the phosphorus and nitrogen con-
tents are among the most important factors for their
growth. A reduction in the nitrogen content of legumes
follows from any adverse impact of the plants' symbiotic
relationship with rhizobia bacteria (Boldt & Jacobsen,
1998). The chlorophyll content of mung bean decreased
with increased concentrations of herbicides applied.
Moreover, it further resulted in stimulation of the
oxidation process (Kaushik & Inderjit, 2006). Increased