Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 308

the interaction between plants and groundwater
contaminants based on these pathways is described in this
chapter. Numerous examples and case studies from the lab-
oratory and field are given to provide emphasis. Additional
information on alternative frameworks regarding the inter-
action between plants and contaminated groundwater can be
found in USEPA (2005a, b).

protected. Such studies included those where the chemical,
usually a systemic insecticide, was added to the plant by soil
or trunk (cambium) injection using a Kioritz soil injector or
Wedgle Tip tree injection system, respectively (Gill et al.

1999). These studies examined the interaction between the
chemical, its distribution in the plant, and the predator,
rather than the interaction between the plant and the chemi-
cal, as is the focus of the phytoremediation of contaminated
groundwater. Interestingly, the steady-state concentration of
insecticide was achieved faster by a factor of 3-4 using trunk
injection versus soil injection, probably because of the RCF .
As the number and volume of synthetic herbicides
increased over time, the fate of these compounds was studied
not only to determine the potential for these compounds
to bioaccumulate in the environment but to increase the
efficiency of their mode of action. Many compounds
synthesized in the laboratory used to kill weed plants are
similar to naturally occurring plant growth hormone
compounds. In the laboratory, these compounds are slightly
modified to increase their lethality. The herbicides based on
plant growth hormones essentially act by making the plant
grow itself to death by increasing the rate of respiration; the
plant simply oxidizes more plant photosynthate than can be
produced. The defoliant Agent Orange is a rapidly acting,
growth-hormone-based herbicide. It is a mixture of the n -
butyl esters of di- and tri-chlorophenoxynacetic acid (2,4-D
and 2,4,5-T), which are plant hormones. As we saw in
Chap. 3, plants use hormones for growth, protection, and
reproduction.
Herbicides affect various aspects of plant growth.
Herbicides can be classified in different ways, based on
similarities in chemical structure, causative agent, or appli-
cation schedule relative to the growth cycle of the target
plants. The simplest herbicide is sodium chloride (NaCl),
which works by upsetting osmosis. Some nitrogenous
herbicides work by disrupting the light reactions of photo-
synthesis. Some disrupt respiration reactions through the use
of various halogenated hydrocarbons. Some act as synthetic
growth hormones that mimic the plant growth hormone
auxin, such as carboxylic acids, and prevent cell division
and protein synthesis.
More important to our understanding of the fate of
xenobiotics in groundwater with respect to phytoremedia-
tion, herbicides also can be classified according to their
mechanism of toxicity. The understanding of this interaction
between plant and xenobiotics, such as plant uptake, detoxi-
fication, and fate, provides a fundamental basis to support a
framework that can be applied to examination of the poten-
tial interactions between plants and common groundwater
contaminants. Hsu and Bartha (1979) used hydroponic
experimental methods to investigate the interaction between
two commonly used organophosphate pesticides and the
rhizospheric assemblages of test plants. The tests were

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13.1 Early Evidence of Plant and

Contaminant Interaction: Herbicides
and Pesticides

The production of xenobiotic chemicals specifically used to
remove insect and plant species considered to be pests
increased dramatically after the 1940s following the end of
the Second World War. Plant pests had been around long
before that time, however, and the desire to eliminate them
was not a new phenomenon. Various approaches had been
used to deter faunal and floral pests, particularly for cash and
agricultural crops. As was discussed in Chap. 11, an extract
from tobacco leaves that contained the biochemical toxin
nicotine was sprayed on leaves to render them inhospitable
for ingestion by many insect pests. The use of plant-derived
products to thwart pests also was extended to the planting of
certain “toxic” plants near desirable plants in order to protect
these from attack. Marigolds, for example, often are planted
near cash crops to remove the threat caused by nematode
worms that destroy plants by invading their roots. The
marigolds release the allelopathic chemical pyrethrin that
kills the nematodes.
This interaction between the production of naturally toxic
compounds by plants for protection against plant pests was
introduced in Chap. 11. It provides evidence that challenges
the commonly held perception that the production of nox-
ious chemicals was solely the responsibility of industrial
chemists. In fact, chemists have often looked to plants in
order to come up with ways to deal with plant pests. Even
the idea for the development of widely used systemic
insecticides that kill insects but not the plants they feed on
was an extension of observations of natural plant-pest
interactions seen in the field. For example, some wheat
plants that grew naturally unmolested by aphids were
found to contain selenium, a naturally occurring element.
The wheat acquired the selenium in the form of sodium
selenate from the soil and distributed it throughout the plant.
This natural systemic protection gave rise to the idea of
using artificial chemicals applied to the foliage, roots, or
trunk for systemic protection from predators. Over time,
various chemicals were added to plants to make them
'toxic.' The uptake and distribution of these chemicals was
widely studied, in order to ensure that the whole plant was