Environmental Engineering Reference
In-Depth Information
the leaves were found to contain more PAHs than the seeds
and nuts. More interestingly, the concentration of PAHs in
these tissues was not related to the concentration in the
sewage sludge, as might be expected from the
TSCF
for
these contaminants. Finally, because most sewage sludges
are frequently monitored for contaminant concentrations,
most PAH concentrations will be lower, less than 1 mg/kg
and, therefore, would most likely not be taken up by plants.
In fact, some early studies indicate that the exposure of
vegetable plants to the PAH naphthalene resulted in most of
the compound staying in the root (Schwarz and Eisele 1984).
For example, the plants were exposed to a nutrient solution
that contained
14
C-naphthalene, and at the end of the experi-
ment, for the pea plant (
Pisum sativum
), almost 60% of the
14
C radioactivity was detected in the roots, 37% in the stems,
and 3% in leaves—also, less than 0.6% was detected in pea
pods. The same experiment was done for onion (
Allium
cepa
), and almost 95% was detected in the roots, and less
than 3% in the bulb and leaves. For lettuce, more than 91%
was detected in the roots, and no more than 4.5% in the
stem and leaf parts. As was the same for even more recent
studies of the fate of such compounds initially added, it
is not known what
partners within species. Transgenics, however, changes this
limitation barrier, as now genes of insects and animals can
be added to plants, an extension of our desire to improve
plant traits.
As was introduced in Chap. 10, transgenic plants are
being used more frequently in phytoremediation plantings.
Transgenic plants are plants that have been genetically
modified such that desirable traits are induced using recom-
binant DNA technology in plants that did not have these
traits originally, or undesirable traits have been removed. An
example is the inclusion of genes taken from the soil bacte-
rium
Bacillus thuringiensis
(or Bt) into plants. This bacte-
rium has been available for use as a spray for foliar defense.
The plants to which these genes are added did not evolve
these adaptations through the force of natural selection; they
were added in one fell swoop in the laboratory.
For trees used to remediate contaminated groundwater,
such as hybrid poplars, researchers are attempting to add
bacterial genes that encode for the production of enzymes
that will decrease the negative effects of slower conjugation
reactions that involve plant-produced glutathione. Other
bacterial enzymes have been added to plants to help degrade
explosives-related compounds and metabolites. It also is
possible that traits such as metal accumulation or organic
solvent degradation will be developed in plants and used to
reduce risk at contaminated sites.
The development of transgenic plants also raises
concerns previously addressed in Chap. 10, such as the
widespread escape from cultivation with uncontrolled entry
into the ecosystem (like happened with
Tamarix
). Also, the
potential exists that a decrease in genetic variability will
occur with transgenic plants used in phytoremediation
applications.
The largest concern about transgenics centers around the
transfer of genes from the transgenic plants to the neighbor-
ing plant community, particularly food crops or nuisance
plants. For example, what would happen if herbicide resis-
tant corn, modified genetically to be able to handle annual
application of herbicides, was introduced to the weed
population at large, the very plants that the herbicide was
developed to eradicate? However, the perceived risk of
introducing transgenics must be weighed against the benefit
of environmental restoration using transgenics. Only experi-
ence in this arena will help determine the risks. Field trials of
transgenics are regulated by the U.S. Department of Agri-
culture, Animal and Plant Health Inspection Service.
Linacre et al. (2003) acknowledged the need for
continued use of transgenic plants in phytoremediation
applications but stated that this use must come with
the appropriate risk assessments and communication. The
assumed risk is to wildlife and human health, but the conse-
quence of inaction as far as remediation goes must also be
quantified. For example, a dissolved-phase plume of TCE
14
C was in as detected
form the
in the plants—
14
C-naphthalene,
14
C-metabolite, etc. Most
are
probably metabolites
and
other
nonextractable
14
C-containing compounds.
Garden plants such as carrots, spinach, and tomatoes were
exposed to water that contained dissolved radiolabeled
14
C-TCE under laboratory conditions (Schnabel et al.
1997); it is unclear if it was uniformly labeled or not. The
concentrations tested were in the range of that often found in
contaminated groundwater, between 140 and 560
g/L. At
the end of the 106-d study, much of the added
14
C-label was
detected in the headspace of the plant microcosms, and
indicated that the
14
C-TCE had volatilized from the leaves
after uptake by the plants from the contaminated water. A
small amount (1-2%) of the label was detected in the plant
itself, and was higher than that found in the soil, and was
probably bound in the plant as a non-TCE transformation
product(s) after oxidation by cytochrome P-450 or reduction
and conjugation by glutathione, and of lower toxicity than
TCE. Moreover, it was observed that the higher the dose of
TCE, the higher the amount of TCE was in plant tissues,
following the diffusion-based concept of the
TSCF
.
m
16.2
Potential for Transgenic and Mutagen
Activation at Phytoremediation Sites
The natural and artificial mixture of different species to
produce new ones has been going on since the beginning
of the time when single-celled plants arose. The potential for
these crosses is immense, but
is limited to only those
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