Environmental Engineering Reference
In-Depth Information
O
O
O
2
O
2
O
CH
3
OH
S
S
H
2
H
3
C
O
H
3
C
O
CH
3
CH
3
H
3
C
Ethofumesate
H
3
C
DHDBM
FIGURE 4.4
Chemical structures of ethofumesate and its degradation product DHDBM.
weeds is
S
-ethyl-
N
,
N
-dipropylthiocarbamate (EPTC) sulfoxide, which is the oxidation
degradation product of the herbicide thiocarbamate
S
-ethyl-
N
,
N
-dipropylthiocarbamate
(EPTC) (Somasundaram and Coats 1991a).
Just as herbicides can be selective among plant species, metabolites can differ in their
phytotoxicity pattern. Metabolites can have different mechanisms of action and selec-
tivities than the parent compounds. For instance, bromoxynil is biologically degraded
in soil into 2,6-dibromophenol, which is a potent growth regulator (Frear 1976).
Kawahigashi and coworkers (2002) showed that the phytotoxicity of the deethylated
metabolite of ethofumesate 2,3-dihydro-2-hydroxy-3,3-dimethyl-5-benzofuranyl meth-
anesulfonate (DHDBM) (Figure 4.4) to rice plants was at least four times greater than
that of the parent compound.
Reddy and coworkers (2004) suggested that injury to glyphosate-resistant soybean from
glyphosate is due to its degradation product formed in plants, aminomethylphosphonic
acid (AMPA). The degradation product of irgarol 1051, M1 in the root-elongation inhibition
bioassay, showed phytotoxicity at least 10 times greater than that of irgarol and six other
triazine herbicides (Okamura et al. 2000).
For herbicides, bioassays are important tools to screen herbicide residues and can be
useful to exclude the occurrence of low levels of phytotoxic residues in soil (Hsiao and
Smith 1983; Sandín-España et al. 2003). In this sense, Sandín-España and coworkers stud-
ied the phytotoxicity of alloxydim, and its main metabolite was studied with hydroponic
bioassays on wheat (Sandín-España et al. 2005). The effect of the degradation product of
alloxydim (Figure 4.5) on root growth occurred at 10 mg/L, causing a 32% reduction in
root growth. Root system control presented normal growth (main tap root plus secondary
roots), while those from injured plants were increasingly deformed (main tap root twisted
and lack of secondary roots).
It is also important to highlight that a part of the degradation products formed in
the soil may remain as bound residues (Albers et al. 2008; Bresnahan et al. 2004; Rice
et al. 2002). This nonextractable residue retained by organic matter in soil is bioavail-
able. Therefore, this portion of residue as degradation products and/or metabolites is
CH
2
CH
2
CH
2
CH
2
OH
N
OH
N
(CH
2
)
2
CH
3
(CH
2
)
2
CH
3
H
3
C
H
3
C
O
O
H
3
C
CH
3
H
3
CO
2
C
CI
CO
2
CH
3
Alloxydim
A
1
FIGURE 4.5
Chemical structures of alloxydim and its chlorinated degradation product A1.
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