Plant Interactions with Biogeochemical Environments - Introduction to Phytoremediation of Contaminated Groundwater - page 265

1,000 mg/L ferrocyanide, with toxic effects noticed only at
2,500 mg/L (Trapp and Christiansen 2003).
Since the early 1990s, interest in using vascular plants to
phytoremediate contaminated groundwater is the result of a
major shift in thinking concerning the interaction between
plants and contaminants. The uptake and translocation
of herbicides, pesticides, petroleum hydrocarbons, and
chlorinated solvents by plants has primarily been viewed in
terms of plant uptake being a vector for increased risk to
wildlife and human populations. Research into the interac-
tion between plants and herbicides increased but primarily to
determine the effectiveness of the mode of action of the
herbicide rather than on risk exposure, and led to the pro-
mulgation in 1982 by the USEPA of the Federal Insecticide,
Fungicide, and Rodenticide Act (FIFRA).
A shift in the view that plants were vectors for contami-
nant exposure to a view that plants could be used to decrease
environmental risk followed only after multiple laboratory
and field studies were conducted to determine the fate and
transport of contaminant compounds in plants. For example,
observations made in the 1980s that pesticides persisted
longer in unplanted areas compared to planted areas was
attributed to the presence of pesticide-degrading microbes
in the root zone and, therefore, were not present in unplanted
areas (see Walton and Anderson 1992). During the 1990s,
native plants at sites contaminated with polychlorinated
biphenyls (PCBs), polyaromatic hydrocarbons (PAHs),
halogenated benzenes, and munitions (Schnoor et al. 1995)
were known to take up these contaminants, and metabolize
them into less harmful byproducts (McFarlane et al. 1990).

new chemicals. The USFDA, established in 1906 during the
Woodrow Wilson administration, has regulatory authority
over the testing and approval of chemicals used in the
manufacture and production of food additives, food
processing, and the chemicals used in processing products
from animals. A substance is considered toxic by the
USFDA if the maximum environmental concentration
exceeds the concentration of that chemical that is found to
cause adverse effects in test species, or exceeds 1% of the
LC 50 (Harrass et al. 1991). The LC 50 is an acute lethality
test, in which the magnitude of dead test organisms is the
toxic amount and the concentration that causes lethality in
half (50%) of the organisms. For the most part, the effect of
chemicals is observed as changes in seed germination, root
elongation, and seedling growth, although other factors
could be tested, such as an enzyme assay, tissue-culture
growth, and life-cycle changes (Fletcher 1991).
The bioassay tests approved by the USEPA are the Seed
Germination/Root Elongation Toxicity Test (EG-12) and the
Early Seedling Growth Test (EG-13). Life-cycle changes
include changes during the manufacture, use, and disposal
of a regulated product. These tests of the effects of chemicals
on plants typically are referred to as bioassays. The USEPA
has used bioassays that involve the exposure of algae to
chemicals under laboratory conditions.
The potential toxicity of a chemical to a plant can be
related to the ability of a chemical to enter a plant's vascular
system. In general, the properties that render a chemical
more hazardous to the environment also increase the poten-
tial for uptake by plants. For example, chemicals that have
greater solubility in water tend to be more of a risk to water
quality and also are more apt to be taken up by plants. More
information on this relation between the physical properties
of a chemical and its interaction with plants is discussed in
Chap. 12. The use of vascular plants to test the toxicity of
chemicals historically has been limited to aquatic rather than
terrestrial plants. This is based on the assumption that the
chemicals are more apt to be released to aquatic
environments and will be more mobile in water. For exam-
ple, recent research has shown that even treated municipal
wastewater can contain a variety of chemicals used by man
that are not degraded during the wastewater-treatment pro-
cess and, therefore, enter the aquatic environment after
release to streams (Kolpin et al. 2002).
The vascular plant most often used in such tests is the
common duckweed plant ( Lemna ), although Elodea also has
been used. For algae-based toxicity tests, the effect of a
particular compound is measured as the difference between
the number of cells at the beginning of exposure relative to
the number of dead cells at the end of exposure, whereas
multicellular plants are evaluated with respect to acute tox-
icity, root elongation, and seedling growth. Other factors
that are evaluated for a negative response to plant exposure

11.2.3 Plants and Toxicity Assessment

Plants have been used since at least the 1970s to help not
only indicate the presence of a particular compound but to
assess the hazard level posed by certain chemicals on
ecosystems, including man. Plants often are used in tests to
determine phytotoxicity and act as sentinels to guard against
wider ecosystem contamination, as discussed previously. In
phytotoxicity tests, plants are exposed to chemicals, and
various factors thought to be affected by the chemicals,
such as plant growth and health, are observed. Sentinel
tests are similar, in that the plants are used as early-warning
systems to detect degradation of ambient environmental
conditions. In many studies, freshwater algae are used to
assess the potential toxicity of a chemical, because although
algae are structurally simple, they are easy to handle in the
laboratory, have high turnover rates, are ubiquitous, and are
similar to more complex vascular plants in terms of
photosynthesis.
According to USEPA and Food and Drug Administration
(USFDA) regulations, phytotoxicity testing is required of all