Environmental Engineering Reference
In-Depth Information
with disulfide (Broekaert et al. 1997). In plants thionins inhibit
bacterial and fungal growth. These antimicrobial compounds
tend to be found in the cells of the plant periphery.
The presence of antimicrobial compounds in the root
zone would seem to contraindicate the data of increased
microbial populations in the rhizosphere. However, there
must be a selective screening process that removes the
harmful organisms and keeps the beneficial ones. For exam-
ple, the nitrogen-fixing bacteria that the plant needs to
acquire nitrogen must not be adversely affected by plant
antimicrobial compounds. The key may be the properties
of the mycorrhizal bacteria themselves and the establish-
ment of a positive feedback loop. These bacteria produce
growth hormones for the plant as well as antimicrobial
compounds to suppress plant pathogens.
The leaves usually are the primary site of defensive
processes because the leaves are the site of food production
for the plant, and loss of leaves by grazing or insect damage
can be trouble for a plant. Physical modifications of leaf
veins into spines occur at margins of leaves, such as in
hollies. Chemical modifications of leaves, such as the pro-
duction of compounds that impart a bitter taste; crystals of
calcium oxalate, present in the common houseplant
Dieffen-
bachia
, which damage the mouthparts of insects, and; poi-
son, such as in the poison hemlock,
Conium maculatum
.In
the latter case, the poisonous compound is the alkaloid
conine, and was the last thing that many early condemned
prisoners, including Socrates, tasted. The plant chemicals
found in leaves can be used for cooking and for medicinal
purposes. The essential oils are used for flavoring, whereas
the alkaloids are used for recuperative purposes.
Assessments of the shallow groundwater resources of the
United States by the USGS as part of the National Water-
Quality Assessment Program (NAWQA) have revealed that
the most commonly detected volatile organic compound in
shallow, ambient groundwater is chloroform, also known as
trichloromethane (Zogorski et al. 2006). Chloroform detec-
tion in groundwater is attributed to the recharge of previ-
ously chlorinated drinking or waste water. There also are
natural sources of chloroform to groundwater. These natural
sources are derived from plants, either green plants or fungi
in terrestrial ecosystems, or from the upper layers of the
ocean from phytoplankton. Laturnus et al. (2002) reported
that in forested areas where shallow groundwater did not
contain chloroform, concentrations of chloroform increased
in shallow soils. Moreover, the concentration of chloroform
in the upper soil layers exhibited a seasonal trend with lower
concentrations in the winter and higher concentrations in the
summer. These compounds can persist in aerobic soil layers
because of the stability of oxidized organic material under
these conditions. The plant production of chloroform may be
a selective advantage as a defense mechanism against infec-
tion, similar to alkaloids.
Other plant allelopathic compounds include glycosides,
such as the juglone from walnut trees described previously.
When in contact with water, glycosides yields sapogenins,
compounds that essentially dissolve lipids, such as those
found in cell walls. Cyanogenic glycosides can produce
hydrogen cyanide. Glycoside concentrations in plants
increase in those exposed to xenobiotics, and glycosides
play an important role in the detoxification of xenobiotics
in groundwater after uptake; this is discussed in Chap. 12.
Some plant defense chemicals are not synthesized by the
plant but by other organisms associated with the plant. Some
fungal endophytes of grasses impart protection against her-
bivory and damage from insects by producing alkaloids
that are retained in the grass vacuoles. The alkaloid lysergic
acid diethylamide (LSD) is produced by such grass fungal
endophytes. The beans of the castor-oil plant (
Ricinus
communis
) contain the protein ricin, which is 10,000 times
more toxic than rattlesnake venom. Ricin is a lectin that has
two polypeptide chains that are connected by a disulfide
bridge. When ingested, ricin enters the cell cytoplasm and
inhibits protein synthesis.
Allelopathy has many connections to the use of plants to
remediate contaminated groundwater. Disease susceptibility
can be a limiting factor in choosing clones over native
plants. This is because under natural conditions of predomi-
nant sexual reproduction, there is a selective battle between
plant health and insect infestation, and the strongest survive
to reproduce. With clonal selection, this battle is not fought
as effectively, because each clone is an exact genetic copy of
the parent. Such a scenario promotes increased pest infesta-
tion. Many examples of the negative results of this can be
found, but perhaps the best example is the potato blight in
Ireland in the 1840s.
There often are concerns raised by stakeholders
who oppose the implementation of phytoremediation at
contaminated sites because of the potential for translocation
of subsurface contamination to the above-ground parts of
leaves. It is instructive to note by comparison, however, that
many common plants that surround us are, in effect, toxic
(Westbrooks and Preacher 1986). Table
11.1
provides an
incomplete list of commonly recognized popular ornamental
plants sold in garden centers or planted in gardens for human
consumption. Also, many of these plants are specifically
planted by state or municipalities interested in preserving
the aesthetics of a particular area. Anecdotally, most of these
plants seem to thrive with little or no damage by plant pests,
a testament to their evolutionary patience—it may also
explain why these plants are so widely sold as houseplants.
The notion that plants are not simply at the mercy of the
environment and have, in fact, demonstrated the ability to
produce chemicals not only to sustain their growth but to
protect them from predators and to diminish the effect of
resource competition is in direct contrast to the notions
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