Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 336

TNT and some of its intermediate breakdown products were
observed to be taken up into plants and accumulate in the
roots, whereas RDX and HMX were found primarily in the
leaves of test plants (Groom et al. 2002).The fate of these
compounds in leaves after they fell was studied by Yoon
et al. (2006). Regulators are typically concerned about the
potential risk exposure to contaminants by exposure to
leaves that might contain contaminants taken up by the
plant. The researchers added radiolabeled 14 C-TNT, 14 C-
RDX, and 14 C-HMX to flasks that contained a solution of
half-strength Hoagland solution, TNT mixtures, and a
prerooted hybrid poplar cutting ( Populus deltoides

Van Aken et al. (2004b) isolated a bacterium believed to
be a symbiotic endophyte often associated with plants, includ-
ing the poplar trees used in the study, and could in pure culture
degrade TNT, RDX, and HMX to reductive derivatives. The
TNT could not be oxidized by the bacteria, although RDX
and HMX were, and was identified to be a species of the
genus Methylobacterium . TNT reduction produces ADNTs,
which can conjugate with hemicellulose in the roots of
willow trees (Shoenmuth and Pestemer 2004a, b).
Tanaka et al. (2007) have presented novel evidence that
documents the importance of plant-detoxification processes
on the fate of explosives after uptake by poplar trees
( Populus deltoides

nigra
DN-34). Over 2 weeks, the removal of TNT from the solu-
tion was greater than the removal of RDX, with little
removal of HMX. After uptake, the distribution of these
compounds within the plant was depicted. Almost half of
the 14 C-TNT taken up was detected in the roots after
30 days, whereas between one-fifth and one-half of 14 C-
HMX and 14 C-RDX, respectively, were found in the leaves
(Yoon et al. 2006). Due to the detection in leaves, dried
leaves were exposed to deionized water to simulate exposure
to precipitation after leaf drop and were resampled. Very
little TNT was found in the leachate, but one-fourth and one-
half of RDX and HMX was found in the water. Moreover,
when dried roots were exposed to the deionized water, very
little of any compound was detected in the leachate.
RDX exposure to poplar tree tissue cultures and leaf
extracts resulted in the partial reduction of RDX to MNX
and DNX and subsequent mineralization (Van Aken et al.
2004a). The authors present a model for the phytoremediation
of RDX, such that RDX is taken up and translocated to the
leaves where it is reduced to MNX and DNX during Phase I
reactions, possibly by plant reductive enzymes of the “green
liver,” such as P-450. The heterocyclic rings are then broken
and the metabolites mineralized to CO 2 . It is possible that this
CO 2 can then be used by the plant.
Thompson et al. (1998b) confirmed results of others that
the interaction of plants with TNT-contaminated soil and
water results in up to 75% of the TNT being bound to roots,
with little translocation to the leaves. Of the fraction that
made it into the plant tissues, it was transformed into the
Phase II conjugate amino derivatives, such as 4-amino
dinitrotoluene (4-ADNT) and 2-amino dinitrotoluene
(2-ADNT), that remained bound in plant tissues, as well as
other unidentified byproducts that were more polar than
TNT. As with the other studies performed that looked at
the fate of TNT in plants, little mineralization to CO 2 was
detected. Thompson et al. (1998) calculated an experimental
RCF and TSCF for TNT and compared it to previously
calculated values. Their RCF was 49.0 and the TSCF was
0.46; the RCF is much higher than previously calculated by
Briggs et al. (1982) or Burken (2003), whereas the TSCF was
lower.

nigra , DN-34). In the laboratory, pop-
lar trees were exposed to 50 mg/L of RDX under hydroponic
conditions for 1 day. Following exposure, they observed
amplification of genes related to the Phase I to Phase III
detoxification processes, such as GST, cytochrome P-450,
reductases, and peroxidases. This amplification was
observed primarily in poplar leaf tissue, not so much in
root tissue, although the majority of the added RDX
remained in the roots following rapid uptake. Additional
evidence as to the effect of this increased gene expression
on the final fate of the RDX taken into the plant from
hydroponic solution will be needed, however, to link these
expressed genes to contaminant fate.
Tognetti et al. (2007) examined the influence of trans-
genic plants that express a bacterial flavodoxin to
phytoremediate 2,4-dinitrotoluene (2,4-DNT). Flavodoxin
is not naturally found in plant cells and is believed to shuttle
electrons to the nitro group of 2,4-DNT for subsequent
reduction. These transgenic plants could be installed at
sites where levels of 2,4-DNT may be toxic.
Another nitroaromatic compound, although not an explo-
sive, is nitrobenzene. Fletcher et al. (1990) grew soybean
( Glycine max ) in the presence of a range of concentrations of
14 C-UL-nitrobenzene, from 0.02 to 100

g/mL as a mixture
of labeled and unlabeled nitrobenzene. At the end of a 3-day
incubation, the plants were analyzed for 14 C-nitrobenzene
distribution. It is important to note that the authors examined
the potential for contaminant effects on transpiration and
photosynthesis from the exposure to nitrobenzene, and
none was found. However, they did report that there was
some visual evidence that the highest concentration used,
100

m

g/mL of nitrobenzene, did indicate root growth reduc-
tion. In any case, up to 80% of the 14 C label remained in the
roots, and 20% in the shoots.
Seyfferth and Parker (2007) conducted an experiment to
determine the fate of dissolved perchlorate in the water
transpired by lettuce ( Lactuca sativa L.) grown under hydro-
ponic conditions. Plants exposed to higher concentrations of
perchlorate contained higher concentrations of perchlorate
in leafy tissues, as microgram per kilogram (

m

g/kg) fresh
weight. An increase in transpiration led to an increase in

m