Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 299

of the effect of herbicide/pesticide application on food crops,
with little a priori concern of its implication for gasoline
compounds and chlorinated solvents found in groundwater.
Additional studies of the interaction between groundwater
contaminants and plant detoxification reactions are
warranted and offer a fruitful area for future research. More-
over, plants had developed these biochemical processes long
before the release of manmade contaminants. Finally, even
though these processes of chemical detoxification occur in
plants, a controlling factor that will determine the success of
this detoxification is how much contaminant source is pres-
ent relative to contaminant sinks in the plant.

diffusion, or to the kidneys and then bladder. Mammalian
livers purify the blood before its circulation through the
body. Cells called hepatocytes act in concert with the rest
of the immune system to destroy invading items, and this is
accomplished through the release of oxidative chemicals
that purposely cause inflammation. These “free radicals”
produced by the body need to be neutralized, or they them-
selves will cause liver problems. How does the liver deal
with this threat? The liver releases the compound glutathi-
one, an antioxidant that can depress inflammation. This
antioxidant also is found in plants, as is described below.
Potentially toxic compounds that enter the body from
external sources are processed by the liver in a stepwise
fashion. For example, the ingestion of alcoholic beverages
involves the ingestion of ethanol, a known toxin. The etha-
nol is rendered harmless in the liver, however, by detoxifica-
tion by oxidization to acetaldehyde, acetate, and then to CO 2
and H 2 O. As you can see, the compound is broken down into
its components that are then excreted through the process of
exhalation and urination. The ingestion of such harmful
chemicals induces the production of enzymes that facilitate
this detoxification.
A major part of the Phase I oxidative detoxification sys-
tem in mammals and plants is the production of microsomal
cytochrome P-450 monooxygenases. These enzymes are
present in most organisms, ranging from the Archaea to
plants to mammalian livers. Cytochrome P-450 is not one
compound, but rather a name that encompasses a large
number, or group, of separate enzymes. Cytochrome P-450
works by adding functional groups such as hydroxyls to
contaminant compounds. The P-450 monooxygenases initi-
ate electron transfer by the NADPH reductase, with the
electron grounding to the P-450 cytochromes. In animals,
the P-450 enzymes reduce molecular oxygen to water and
also render the contaminant compound more polar so that it
can be excreted.
There are low levels of cytochrome P-450 enzymes in
plants, but their role in detoxification is unclear, although its
protective purpose proceeds undoubtedly by oxidative pro-
cesses. Some oxygenases are present in the parts of the plant
cells and in the apoplast as well as in the cell membranes,
and some are present in the cytoplasm. Oxidation of xenobi-
otic compounds such as PAHs by these oxidative enzymes
differs based on the source of the oxidation. For example,
simple microbes such as the prokaryotes degrade PAH in the
presence of oxygen using dioxygenase that contains two
oxygen atoms, whereas eukaryotes use monooxygenases
that have one oxygen atom.
Oxidative dehalogenation in plants by hydrolysis also
can occur. Most dehalogenation reactions, however, are
reductions carried out by reductases. Some evidence
exists that poplar trees exposed to TCE in the transpiration
stream can undergo oxidative dehalogenation to form

12.4.1 Phase I Reactions

Phase I reactions involve the transformation of potentially
harmful compounds taken up by plants into more water-
soluble byproducts or intermediates that undergo further
detoxification by other processes. In general, Phase I
reactions include oxidation, reduction, and hydrolysis, with
the final product potentially being CO 2 (Fig. 12.4 ). Other
Phase I reactions include hydroxylation, decarboxylation,
and dealkylation. In these cases, organic molecule-based
functional groups are either removed or added to the initial
compound, called functionalization. This process renders a
formerly hydrophobic contaminant to become less so after
addition of a hydrophilic functional group, such as
-hydroxyl, -amino, or -carboxyl groups, following enzy-
matic transformation by oxidation, reduction, or hydrolysis
reactions.
Phase I reactions are initiated by enzymes produced in the
endoplasmic reticulum of a cell's cytoplasm. In many cases,
the exposure of plant cells to xenobiotics induces morpho-
logical changes in the cells, such that separate organelles are
brought closer together, in an attempt to facilitate the trans-
fer of electrons during the redox processes; this has been
termed mitochondrial control (Kvesitadze et al. 2006). This
is an interesting phenomenon, because the membranes that
surround the endoplasmic reticulum are compounds primar-
ily of lipids and, hence, act to attract the very lipophilic
compounds that they act upon.

12.4.1.1 Oxidation and the “Green Liver”
Oxidation is the process where electrons are removed from a
compound to form a negatively charged entity. The electron
can be removed from a variety of areas on the compound,
which are referred to using organic chemical nomenclature,
such as the alpha (

) positions.
In mammals, waste is continually produced as a conse-
quence of life and metabolism. The in-situ production of
potentially toxic metabolic byproducts, such as CO 2 or urea,
are excreted to the blood stream and then the lungs by

a ), beta ( b ), and gamma ( g