Oxidative stress and antioxidant defence systems in response to pesticide stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 106

suggest that algal sensitivity shows considerable diversity
to pesticides. For example, the responses of the unicel-
lular chlorophytes Chlorella vulgaris and Selenastrum
capricornutum , to three herbicides (simetryn, pretilachlor
and thiobencarb) were compared (Kasai & Hatakeyama,
1993) using single species toxicity tests for population
growth. The authors observed that C. vulgaris strains
were more tolerant than S. capricornutum strains for these
three herbicides. Kent and Currie (1995) conducted an
experiment with 12 algal species exposed to fenitrothion
(an organophosphorus insecticide) and observed that
their tolerance may be inversely proportional to cell
surface area, volume ratio, cell lipid content, and the
inherent bioconcentration potential of the cell.
Some herbicides inhibit photosynthetic electron flow
(Trebst et al., 1991) and thus affect algal growth and
development. The substituted phenyl urea herbicides
applied at concentrations of between 1 and 5 mM can
inhibit the Hill reaction of isolated chloroplasts by as
much as 50%, due to the lipophilic nature of pesticides
(Camilleri et al., 1987). Hoffman (1971) found that the
toxic effects of monuron and bromacil on photosynthe-
sizing Chlorella were additive, indicating the probability
of a common mode of action. Kirby and Sheahan (1994)
compared the toxicity of the herbicides atrazine, isopro-
turon and mecoprop on a freshwater green flagellate alga,
Scenedesmus subspicatus , and a macrophyte plant, Lemna
minor . For S. subspicatus , atrazine and isoproturon were
similar and exhibited three orders of magnitude more
toxicity than mecoprop.
Karanth and Vasantharajan (1972) reported the inhi-
bition of taproot elongation and nodulation of sun-hemp
with the fungicide Dexon, which was known to inhibit
the conversion of tryptophan to indole acetic acid (IAA).
The inhibition of root nodulation in bean plants could
be due to the inhibition of rhizobia in the soil or to
metabolic changes in plants. Sen and Kapoor (1974)
have shown that plants treated with carbendzim show
improved growth, in the case of tomatoes and cucurbits,
besides control of wilt ( Fusarium oxysposium ) and powdery
mildew ( Sphaerotheca fulginea ). Benzimidazole fungi-
cides are also known to stimulate plant growth besides
controlling plant pathogens. Majumdar (1974) and
Singh and Kang (1978) reported that Bavistin resulted
in increased fresh or dry weight of groundnut, and the
yield of pods was more than control treatments. There is
considerable variation in the persistence of pesticide res-
idues in soil, water and plant tissues. The half-life of

various pyrethroids in soils varies from 1 day to 4
months. It has been observed that photolabile pyre-
throids usually degrade much more quickly than
photostable pyrethroids, and degradation is usually
much faster in aerobic conditions in comparison to
anaerobic condition (Leahey, 1985).

7.2.1 effect of pesticides on
non-target plants
Plant species that are unintentionally exposed to pesti-
cides are called non-target plants. Any direct or indirect
effects that cause a significant change in the survival,
vegetative growth or reproduction of non-target plant
species are called non-target plant effects. Non-target
plant effects include a range of symptoms, including
vegetative growth changes, plant death or altered repro-
ductive capability; these generally result in reduced
growth of non-target plants and economic losses or eco-
logical disturbances. One of the inadvertent effects of
herbicides is altered susceptibility to disease of both
target and non-target plants (Altman, 1993). It is esti-
mated that for every plant species that becomes extinct,
10--30 other non-plant organisms may also become
extinct (Ware, 1993). Non-target plants growing in
pesticide-contaminated environments may experience
local extinction. It has been observed by various
researchers that desirable species of trees, grasses and
shrubs, particularly those that are highly sensitive to
pesticides, may be eliminated or greatly reduced (Fink,
1972; Tomkins & Grant, 1977; Boutin et al., 2000).
Hamner and Tukey (1946) studied the effects of the
insecticide 2,4-D on various woody plants like juniper
( Juniperis communis L.), elm ( Ulmus americana L.), poplar
( Populus nigra L. var. Italica Du Roi), spruce ( Picea
canadensis Mill.), willow ( Salix nigra Marsh.), plum
( Prunus domestica L.), pear ( Pyrus communis L.) and pine
( Pinus strobus L.). This study concluded that the leaves of
all treated plants except juniper displayed bending, dis-
coloration and wilt within a few days after foliar
application of 2,4-D during warm weather. Powdered
application of 2,4-D at 20% concentration on the cut
stems of peach, poplar, pine, spruce, elm and willow
elicited a typical curvature in new shoot growth within
2 days after treatment, but the degree and type of
response varied in different plants. The upper leaves of
12-m (40-ft) poplar trees exhibited downward bending
of the petioles within 2 days followed by progressive
browning, drying and leaf fall within 1 month of