Environmental Engineering Reference
In-Depth Information
stomata from the formulation deposits, followed by apoplastic, synplastic transport
and phloem translocation, whereas root hairs play the most important part in
absorption of pesticide from soil (Field and Dastgheib 1996). The adjuvants in
formulation including surfactant are considered to affect these processes. When the
extent of pesticide uptake is affected, its metabolism in plants may be modified by
surfactant (Baloch 2000). However, plant metabolism study is usually conducted by
using the solution of a water-miscible organic solvent or the formulation using
14
C-labeled pesticide in accordance with good agricultural practice (OECD 2007),
and hence relevant information on the surfactant effect is very limited.
A RootUptake
The uptake of pesticide by roots is considered to be initiated by its absorption and
translocation through root hairs. Briggs et al. (1982, 1983) introduced the concept
of transpiration stream concentration factor (TSCF), defining the ratio of pesticide
concentration in the transpiration stream and an aqueous phase outside the root,
which is a function of log
P
for nonionic chemicals. The root concentration factor
(RCF) expressing a similar ratio between the root and the aqueous phase was found
to be also proportional to log
P
. Because surfactants can solubilize a chemical by
association with its monomers and micelles, they are considered to affect these
partition processes. Li et al. (2001) examined the effect of Tween 80 on the distribu-
tion of
14
C-phenanthrene applied as an aqueous solution in closed systems includ-
ing wheat seedlings and lava. The surfactant slightly reduced the
14
C-distribution in
the root, probably because of more solubilization of PAH in the aqueous phase
containing lava. Jiang et al. (2005) reported a similar effect by LAS on phenan-
threne in the same system using rushes instead. The distribution of
14
C in shoots
was around 50% of the applied
14
C in wheat but only about 2% for rushes. In both
cases, more mineralization with formation of polar metabolites was observed,
which was caused by the enhanced microbial degradation in the aqueous phase.
Although 10%-40% of the
14
C-labeled dioxin congeners in the 0.05% Tween 80
solution was absorbed by the roots of soybean and oat seedlings, insignificant trans-
location of
14
C to their tops (<1%) was observed (Isensee and Jones 1971).
Enhanced root uptake of PAHs by 6%-35% from aqueous solution of Brij 35 at
concentration less than cmc has been reported for ryegrass seedlings (Gao et al.
2006). The excess amount of the surfactant reduced the uptake by its phytotoxicity
and less bioavailability caused by greater solubilization of PAHs to micelles in the
aqueous phase. In the case of pesticids, information on the surfactant effect on root
uptake is very limited. Lichtenstein et al. (1967b) reported the effect of 0.005%
LAS on the root uptake of the several pesticides by pea seedlings. LAS greatly
reduced the root uptake of parathion (3) but gave insignificant effects for lindane
(34), diazinon (38) and aldrin (52). The translocation of (34) was reduced to one-
seventh of that in the absence of LAS but more (52) with formation of dieldrin (62)
was detected in the green part by addition of LAS. The increased efficacy of
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