Plant Interactions with Biogeochemical Environments - Introduction to Phytoremediation of Contaminated Groundwater - page 266

include measurement of differences in CO 2 uptake (photo-
synthesis), and the commensurate production of O 2 .A
reduction in CO 2 uptake or of O 2 production could be used
to indicate a toxic effect of a chemical, as long as the
production of CO 2 and consumption of O 2 by plant-cellular
respiration are accounted for.
The results of toxicity testing using plants can be used to
determine the need for further actions. If no adverse expo-
sure or toxicity effects are observed with respect to seed
germination, root elongation, and seedling growth, then the
effect of the particular chemical on the environment can be
deemed low. Conversely, if an effect is detected additional
steps can be taken, such as preparing an environmental
impact study (EIS). Finally, the effects of organic chemicals
on terrestrial plants are recorded in the internet-based data-
base PHYTOTOX, which is composed of a bibliography and
dose-response information (Royce et al. 1984).

sodium chloride. Other plants indicate the presence of zinc,
such as yellow violet ( Violet lutea ), or selenium ( Astragalus
pattersoni ). The vanadium bush ( Cowania stansburniana )
found in the southwestern United States is used by
prospectors to indicate the location of vanadium and ura-
nium deposits. These plants absorb the uranium dissolved in
groundwater that contains dissolved oxygen, under which
the oxidized uranium is mobile. All these are examples of
natural selection, in which the locations of plant species are
determined by competitive exclusion.

11.3 Plants and Extreme Natural

Environments as an Analogy
for Groundwater Contamination

Plants exist in environments of many extremes. Plants are
exposed to the electromagnetic energy contained in incident
solar light. They use the light energy to split water to process
sugar from gaseous CO 2 . Plants also need to protect them-
selves, however, from damaging solar energy. Similar to the
use of sunscreen compounds to protect human skin from
ultraviolet A and B, plants produce a compound called
zeaxanthin to protect themselves from solar radiation (Flem-
ing and Niyogi 2005). This process of protection is called
feedback de-excitation. The compound zeaxanthin, a carot-
enoid similar to the non-photosynthetic structures that give
rise to different colors in plant leaves and fruits, apparently
permits overheated chlorophyll to disperse its heat.
Plants are exposed to extremes in a variety of factors even
in the same location. For example, temperatures on this
planet can range from over 100 F to below

11.2.4 Geobotanical Prospecting
and Phytomining

Plants are what they take up, so to speak—they accumulate
and redistribute chemicals, such as iron for example, into the
heartwood to increase structural support. Analysis of plants
can be performed, therefore, to determine the presence of
elements in the soil, such as iron. Cannon (1971) reported
the relation between above-ground plant distribution and
growth and below-ground variables that may be used to
indicate the presence or absence of water, minerals, or geo-
logic processes. This report confirms previous investigations
(Meinzer 1927) that plants can indicate the depth to and
sometimes the quality of groundwater. The presence of
certain geologic strata and the resultant weathered soil has
control on plant distribution, either from the standpoint of
mineralogy or permeability to air and water.
The detection of ores or mineral deposits is connected to
plants in two ways. First, the analysis of plant matter for
particular minerals and the assumption that these minerals
are present in the soil are referred to as biogeochemical
prospecting. On the other hand, the use of the distribution
or appearance of plants and their health are known as geo-
botanical prospecting. The application of geobotanical
prospecting and the relation between plants and local hydro-
geology for remediation purposes also are referred to as
phytomining. The interaction between plants and heavy
metals has long been studied. Plants are protected from
negative effects of the heavy metals by the mycorrhizae
that exclude the entry of the metal into the plant. A review
of this topic can be found in van der Lelie et al. (2001).
In the simplest application, the presence of halophytes,
such as mangrove, and Spartina indicates the presence of

100 F. More-
over, the same location may have a wide fluctuation in
temperature. The length of solar radiation per day changes
over time. Short-term moisture levels can fluctuate rapidly if
precipitation is infrequent. Insects and animals, including
man, ingest various parts of plants for their own sustenance.
Some plants can tolerate high concentrations of metal
deposits that would kill other plants that do not possess
such metal-detoxifying processes.
Below are additional examples in which plants have
thrived in less than ideal geochemical or physical conditions,
which indicates that the interaction between plants and
contaminated environments, including contaminated
groundwater sites, is the rule not the exception.

11.3.1 Spartina and Mangrove Monocultures

The interaction of plants with natural metals provides an
interesting story about how harsh environments can actually
be niches for some plants to the exclusion of other plants.
For example, the saline marshes and estuaries of the eastern