Environmental Engineering Reference
In-Depth Information
themselves from the allelopathic attacks from other plants or
organisms. This kind of low-grade chemical warfare may
explain the interaction between plants and the flora present
in the rhizosphere, i.e., a plant can selectively “choose”
which bacteria are present in the plant's rhizosphere based
on exposure to deleterious chemicals, such that these bacte-
ria will possess the necessary genes to code for degradation
enzymes. Therefore, the interaction between plants and alle-
lopathic compounds provides a natural analogy to the inter-
action between plants and xenobiotic chemicals found in
groundwater.
Above ground, as plants respond to transpiration
demands, water that enters the roots moves upward through
the xylem to exit the leaves. During this transport, any
compounds dissolved in the water have the potential to
interact with the plant tissues of the vascular system
according to the physical and chemical properties of the
contaminant discussed earlier in this chapter. Some of
these compounds can have deleterious effects on the living
cells that surround the xylem. Cell culture experiments have
indicated, however, that the cambium cells of plants can
interact with xenobiotic compounds in the xylem and can
transform them into less toxic compounds (Sandermann
et al. 1977; Sandermann 1994). This is similar to how the
blood in mammals must pass
Fig. 12.4
Plant xenobiotic detoxification by Phase I, II, and III
reactions
after uptake
from contaminated groundwater during
phytoremediation.
through the liver
for
detoxification.
In plants, those compounds with a higher lipophilicity
that enter the transpiration stream will more quickly be
absorbed to cells. For such lipophilic compounds to be
eliminated from the plant, they must be transformed into
more water-soluble compounds. In fact, this is the basis of
most animal and plant detoxification mechanisms—increas-
ing the water solubility of initially water insoluble xenobi-
otic compounds for removal from the organism. Much of
this transformation is handled by enzymes. Obviously, these
detoxification processes were not invented by plants to deal
with groundwater priority pollutants; they instead evolved as
part of a plant's evolutionary response to selection pressures
derived from ex-situ and in-situ chemical bombardment.
A variety of processes in plants can act upon a wide range
of potentially harmful compounds in order to render them
into simpler, and less harmful, forms. Some processes are
abiotic, whereas others involve biological processes where
energy is extracted from the reaction. As these processes in
plants are similar to those observed in the mammalian liver,
the nomenclature to describe plant processes of contaminant
degradation is taken from mammalian studies (for a review,
see Burken 2003).
These detoxification processes include what are consid-
ered Phase I, II, and III reactions (Fig.
12.4
). Phase I
reactions, such as chemical activation, transformation, or
functionalization reactions, involve oxidation and reduction
reactions similar to those previously described, as well as
oxidative metabolism, and hydrolysis reactions. Phase II
reactions include detoxification reactions, such as conjuga-
tion reactions that irreversibly bind contaminants to plant
tissues. Phase III reactions include compartmentalization or
elimination reactions, where products of Phase I and II
reactions are handled within the plant cellular organelles.
Additional detoxification reactions include hydroxylation,
dehalogenation, decarboxylation, and dealkylation.
These plant-facilitated, chemical detoxification reactions
are described in detail below. An excellent review also can
be found in various chapters in McCutcheon and Schnoor
(2003). Knowledge of these processes is crucial to the appli-
cation of phytoremediation at sites characterized by
contaminated groundwater, because in order for the
contaminants to be rendered harmless, detoxification by
ex-situ
mineralization in the root zone or detoxification in
the plant after uptake must be demonstrated. For these
reactions to be beneficial to solving groundwater contamina-
tion problems, the initial uptake into a plant has to occur.
This does not mean, however, that all compounds taken up
by plants can be detoxified, because some can move conser-
vatively through the plant and be transpired unaffected
through the leaves. Moreover, some reactions that lead to
detoxification occur in the root zone without the plant taking
the compound into the transpiration stream.
Much of the chemical detoxification information
presented here was developed during the early investigation
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