Plant Interactions with Biogeochemical Environments - Introduction to Phytoremediation of Contaminated Groundwater - page 262

Fig. 11.10 A variety of tree
genera and species release natural
volatile organic compounds, such
as camphene and d3 -limonene.
(Cwd; cottonwood.)

Another example of plant self-defense can be found in
oak trees, which produce tannins to inhibit predation by
herbivores. As oak leaves age, the concentration of tannins
increases inside each cell, and tannins are stored in cell
vacuoles. These tannins bind rapidly with proteins and,
therefore, can interfere with enzymes that control cell
metabolism. Tannins are useful for plant tissues that contain
openings and are, therefore, exposed to potential invasion by
viruses, bacteria, and fungal spores, such as the roots (root
hairs), leaves (stomata), bark (lenticels and cork), heartwood
and many seeds. During herbivory, the vacuoles are broken,
and the released tannin combines with plant proteins and
renders the plants nutritionally unavailable to the herbivore.
However, because younger leaves have less tannin, they can
be consumed without adverse effect on the population of
predators, and some leaf-eating beetles can devour a leaf by
avoiding the vacuoles altogether, which is the reason for the
appearance of “laced” leaves.
Heartwood contains a higher concentration of such
tannins and is, therefore, naturally protected from invasion,
which explains the desirability of heartwood as a source of
building supplies. Tannins from bark are used to render
animal skins stable against microbial degradation; after
soaking in tannic-acid solution, the skin becomes protected
from microbial attack. Oaks that grow in infertile soils tend
to contain higher concentrations of tannins. As winter
approaches, the green, red, orange, and yellow pigments
fade and give way to brown, the color of tannic acid,
which remains because it is less degradable than the other
colors.

Rivers that drain the broad, sandy land of the coastal plain
along the eastern seaboard of the United States typically are
dark colored even though they contain very little suspended
sediment. The dark color, called “blackwater,” is derived
from the tannic acids that leach from the trees that grow
along the banks, and was noted as early as the eighteenth
century by William Bartram, a naturalist who traveled
throughout the southeastern colonies between 1773 and
1778 (Van Doren 1955). These are the same tannins that
are used to protect the various parts of the tree from fungal
attack. Because of the antimicrobial property of tannins,
black water actually is suitable as a source of drinking
water. In fact, the crews of early sailing ships along coastal
North America would fill barrels with blackwater because
the tannins in the water and its lower pH would act as a
biocide to inhibit microbial growth during extended sea
travel, especially after supplies of beer ran out. Such black-
water was an important source of relatively clean drinking
water on land in the time before refrigeration and was even
believed to possess medicinal properties.
Many plants contain antimicrobial compounds for protec-
tion (Broekaert et al. 1997). Plants and animals are continu-
ously exposed to potentially pathogenic organisms above
ground and in the soil. Whereas animals have developed
immune systems that are immunoglobulin-based, plants
have developed chemicals to ward off or remove infestation.
Of the many chemicals plants produce, peptides are one of the
most common. Peptides consist of a number of amino acids.
Plant peptides contain cysteine as a building block. For exam-
ple, thionins are peptides that contain cysteine interconnected