Plant Interactions with Biogeochemical Environments - Introduction to Phytoremediation of Contaminated Groundwater - page 245

11

Plant Interactions with Biogeochemical
Environments

Evolution has shown that at any given moment out of all con-
ceivable constructions a single one has always proved itself
absolutely superior to the rest.

such as the chlorinated insecticide dichlorodiphenyltri-
chloroethane (DDT), or as a consequence of other processes
such as the production of chloroform during water purifica-
tion, and often have no natural source. Many of these xeno-
biotic compounds interact with plants because the physical
and chemical properties of the compounds impart solubility
in water and, therefore, the compounds move readily
through the hydrologic cycle—for example, chloroform is
the most frequently detected volatile organic compound in
ambient groundwater (Zogorski et al. 2006).
To ensure that the interaction between plants and xenobi-
otic compounds can be applied to the phytoremediation of
contaminated groundwater, scientifically defensible evi-
dence needs to exist to document that plants can take up
such contaminants dissolved in groundwater, and detoxify,
immobilize, or volatilize these contaminant compounds into
less harmful forms. Questions related to phytoremediation
projects that commonly arise and are addressed in Part III
include the following:

Albert Einstein

Plants are essentially chemical factories that naturally pro-
duce sugar by using the raw materials of CO 2 , light energy,
and hydrogen from the splitting of water. A waste product of
this production of sugar, oxygen, and its release to the
environment led to the oxidation of previously reduced
metals, such as iron, that currently are used by man. This
oxygen release resulted in the demise of many predominant
forms of anaerobic life early in the earth's history or forced
them into seclusion through burial in sediments. It also
stimulated the development of aerobic respiration as a way
to deal with the toxic oxygen gas—this scenario set the stage
for all other aerobic forms of life, including us, to develop.
Plants carried out these processes while constantly
responding to changes in their environment from natural
threats, such as fires, volcanic eruptions, radiation emitted
from cooling rocks, methane releases, advancing glaciers,
herbivory, and toxic concentrations of metal deposits. The
plants that survived had selective advantages relative to
those that could not cope with these threats.
Plants were not only capable of responding to these natural
threats, but could themselves manufacture a wide range of
secondary chemicals, or metabolites, that could be used for
defensive or offensive purposes to ensure survival and repro-
duction. Defensive chemicals include those made to protect
plants against threats from other plants or from insects or
animals. Offensive chemicals include those made by plants
to sequester limited resources or to inhibit the acquisition of
these resources by other plants. The fact that these complex
organic compounds are synthesized by plants from the simple
reactants of water, oxygen, and CO 2 is the envy of many
organic chemists backed by modern laboratory facilities.
Since the beginning of the industrialization of many
societies, plants also have been exposed to a variety of
compounds called xenobiotics, from the Greek xenos , mean-
ing stranger. Such chemicals are derived synthetically,

￿ What happens to dissolved groundwater contaminants
that enter a plant?
￿ Do contaminants remain in the leaves after seasonal leaf
drop?
￿ Do contaminants enter fruits or nuts?
￿ Should evergreens be planted at sites to address regu-
latory concerns about the possible lack of groundwater
uptake and contaminant processing during periods of
dormancy for more commonly used deciduous trees?
￿ Does groundwater uptake and translocation occur in
deciduous plants during dormancy?
To answer these questions, fundamental information
about the chemical properties and interactions of common
groundwater contaminants is reviewed. This information
provides a basis for the subsequent process of groundwater
uptake and contaminant detoxification by plants, which must
follow these fundamental physical laws and the results be
reproducible for phytoremediation to be scientifically
defensible.