Plant Control on the Fate of Common Groundwater Contaminants - Introduction to Phytoremediation of Contaminated Groundwater - page 337

perchlorate concentrations in lettuce tissue, with accumula-
tion in the leaves. However, these concentrations were less
than would be predicted if the tissue concentration were only
a function of the transpiration rate, suggesting exclusion,
as stated by Seyfferth and Parker (2007). Alternative
explanations include sorption of perchlorate onto root or
xylem tissues, or Phase I to III degradation mechanism
in planta .
The fate of perchlorate in aquatic plants also was
investigated by Susarla et al. (2000). Perchlorate was taken
up and chlorate, chloride, and chlorite were detected in
the plants examined, such as sweet gum ( Liquidambar
styraciflua ) and black willow ( Salix nigra ), with the most
mass present in the leaves rather than roots. The fate of
perchlorate in poplar trees also was investigated by Van
Aken and Schnoor (2002). They exposed poplar trees ( Populus
deltoides

considering that perchlorate also has been shown to be taken
up and partially reduced in plants (Van Aken and Schnoor
2002). The key to efficiency of this remedial process will be
to get maximum interaction between plant roots and the
perchlorate-contaminated media. This ex planta reduction
of perchlorate by soil bacteria once again illustrates the need
to be clear about the difference between phytoremediation,
bioremediation, and rhizospheric processes, all of which
interact to some extent.
The study by Aitchison et al. (2000) of the fate of 1,4-
dioxane exposed to poplar cuttings indicated that the major-
ity of dioxane taken up was transpired. For example, 50% of
the 1,4-dioxane removed from the hydroponic solution and
of this percentage, between 76% and 85% was transpired.
The balance of dioxane not transpired was detected in the
stem of the cuttings. The cuttings did not exhibit gross toxic
effects to dioxane even at concentrations of 23 mg/L. The
fate of the plant-volatilized dioxane was not examined.
Tritium that enters roots will be translocated to the leaves
where near 100% of the tritiated water taken up will be
transpired. While it is possible that some tritiated hydrogen
may react to reduce CO 2 during photosynthesis, the amount
that remains in plant carbohydrate is probably low because
99% of water taken up by plants is transpired.

nigra )to 36 Cl labeled ClO 4 (25 mg/L), and
measured the concentration of perchlorate in the hydroponic
solution over time. After 30 days, the original perchlorate
concentration had decreased by 50%, with no apparent toxic
effect on the plants. The perchlorate was taken up,
translocated, and entered the leaves. There, perchlorate (as
ClO 4 ) was found in abundance (26%), but it was
transformed into various reduced species, such as chlorate
(ClO 3 ; 4.8%), chlorite (ClO 2 ; 2.4%), and chloride (Cl ;
1.6%). This is novel because the reduction of perchlorate had
previously only been observed by anaerobic bacteria, in
which perchlorate was used as a terminal electron acceptor.
The plant-induced reductions that occurred in the poplar leaf
tissues were related to the action of reductases and
dismutases that are still present even though oxygen is avail-
able. This may explain the low yield of chloride from plant-
perchlorate reduction.
Because perchlorate is highly oxidized, it can act as an
electron acceptor under conditions of limited oxygen, such
as anaerobic conditions, in groundwater. This is similar to
the case with PCE and TCE. There needs to be an electron
donor, however, to drive the reduction. This can be an
abiotic or biotic process. At sites limited in electron donors,
various organic compounds have been added, such as molas-
ses or vegetable oils. Plants also add organic matter to the
rhizosphere, both as a byproduct of living cells or as the
shedding of dead cells. Shrout et al. (2006) investigated
the effect of root organic matter as a source of electron
donor to drive the microbial reduction of perchlorate in
soil and water samples. They showed in microcosm studies
that perchlorate-reducing bacteria used plant-root exudates
as the sole source of carbon to drive the reduction. The
exudates were prepared from hybrid poplar tree cuttings
(DN-34) by taking the roots and homogenizing them with a
blender device. The fact that root exudates can facilitate the
bacterial reduction of perchlorate is encouraging, especially

13.7 Concerns About Plant and Contaminant

Interactions

The exposure of plants to groundwater contaminants, by
definition and historical precedent, should raise concerns
for the ultimate fate of the contaminant. Questions include:

• Are the concentrations of the groundwater contaminant(s)
toxic to plants?
• Can the groundwater contaminants enter the plants?
• Can these contaminants be degraded in planta ?
• What is the contaminant half-life if exposed to plants?
• Where does the contaminant go once in the plant?

Many existing approaches can determine the effect of
contaminant stresses on plants. They can be classified as
either plant-level or molecular-level methods. Plant-level
methods include gross observation of wilting in the presence
of adequate water potential, change in leaf color, etc. Molec-
ular-level methods include measurement of enzymes such as
peroxidase, a compound found in most plant cells, and in
largest amounts in the cell wall. In general, exposure of a
plant to a variety of chemicals results in an increase in
peroxidase (Byl and Klaine 1991). Other indicators used to
monitor plant stress include chlorophyll a and dehydroge-
nase levels, as well as photosynthesis and respiration rates.