Plant Interactions with Biogeochemical Environments - Introduction to Phytoremediation of Contaminated Groundwater - page 267

coastline of the United States have what would at first seem
to be the most inhospitable conditions for plant growth,
such as organic-rich, anoxic sediments that are high in
concentrations of the toxic gas hydrogen sulfide; high
concentrations of sodium chloride in the sediment pore
water; and a daily cycle between inundation by saline
water and exposure to dry conditions in areas affected by
tides. For most plants the presence of salt acts to upset
the entry of water into cells. In areas with high salt
concentrations or high osmotic pressure and low water con-
centration, plants can have higher water concentrations and
actually lose water to the surrounding area; as a result, the
plants die from a lack of water (Gough et al. 1979).
Such conditions are so harsh to most plants that the
eastern coastline is often dominated by only Spartina
alterniflora and Spartina patens . Sodium is a trace element
in plants, but an essential element for most animals. These
plants deal with the deleterious effects of salt by increasing
the content of sodium chloride in their root tissues above that
of the surrounding saltwater. This lowers the water concen-
tration in the plant cells below that of the seawater, thereby
allowing the osmotic entry of water into the plant (Flowers
et al. 1977). Spartina is acclimated to high saline conditions,
but growth decreases as salinities increase above 40 g/dm 3
(Bradley and Morris 1991).
As the Spartina plants transpire, salt-free water exits the
stomata because the excess NaCl has been excluded from
entry into the plant beyond the root membranes. This ion
exclusion by the roots was reported to be 91% of the theoreti-
cal concentration that would have been taken up based
on transpiration rates (Bradley and Morris 1991). The
accumulated salt near the roots in the rhizosphere is washed
away by each tidal event. Any incidental salt that enters the
transpiration stream is excreted along the plant leaves by
special ducts or glands, after which the salt crystals are
washed away by the tides. For example, Bradley and Morris
(1991) reported that about 50% of the ions that entered the
transpiration stream were removed from the plant tissue by
leaf excretion. Moreover, even though ions were excluded,
concentrations in the roots of the plants remained high enough
to reverse the osmotic gradient so that water entered the roots.
In such marsh conditions, salinity is not the only environ-
mental factor that results in the selection of a monoculture.
The concentration of oxygen in the pore water of the
sediments in these saline marshes and estuaries is low. This
is because the oxygen that enters the upper layers of sediment
after each tide is rapidly consumed by aerobic organisms
present in a thin layer of sediment and satisfies the abiological
oxygen demand exerted by the presence of reduced mineral
species. Plant roots are living tissue and require oxygen,
however, in order to respire. For Spartina , the need for oxy-
gen in an oxygen-poor system is accomplished by diffusion.
Oxygen levels in the air near 20% diffuses from the stomata

through the plant toward lower oxygen concentrations in the
roots. Moreover, as a result of root respiration, CO 2 levels in
the root zone are higher than the CO 2 levels in the atmo-
sphere, and the higher partial pressure of CO 2 in the root zone
establishes a diffusion gradient from soil to air. To ensure that
the diffusion rate of these gases is at a maximum, these
vessels remain dry in Spartina and are not filled with water,
which would slow the rate of O 2 and CO 2 transport and result
in plant death. Leakage of excess O 2 into the immediate area
around the root zone also is responsible for the rhizosphere
and its effect on plant health and contaminant remediation, as
discussed in Chaps. 12 and 13.
Saline conditions also can be tolerated by woody plants.
For example, mangroves trees are found along the southern-
most coast of Florida. Like Spartina , they have been able to
cope with high salinity by excluding it after uptake of the
highly saline water into their xylem. As with Spartina ,
uptake and exclusion leaves behind a more concentrated
solution in the root-zone pore water. Measurements of
pore-water salinities near red mangrove ( Rhizophora man-
gle ) roots in Tampa Bay, FL, indicated that in the 0.9-2.2 ft
(0.3-0.7 m) deep root zone, sediment salinity concentrations
were two to three times those in the surface water (Green-
wood et al. 2006; Fig. 11.11 ).
Fass et al. (2007) reported that the ability of mangrove
trees to create zones of high-chloride pore water also has led
to increased salinity in groundwater in areas of Australia.
Although such enrichment of the salinity of shallow ground-
water near mangroves would be expected, Fass et al. (2007)
reported the occurrence of saline shallow groundwater many
miles inland. These pockets of high-salinity groundwater
represent locations of when sea levels were higher some
4,000-6,000 years ago than during recent times. The high-
salinity groundwater that formed at

that time sank into

deeper aquifer units over time.
Mangroves have adapted to the low-oxygen content of the
sediment pore water in which they grow. In this case, form
follows function. The characteristic structural features of the
mangrove Rhizophora are the stilt-like roots that exit the
trunk above the mean high-water line and below the lowest
leaves. These roots are always exposed to the atmosphere.
These plants probably are derived from those that can be
found today along the west coast of Africa. Their aquatic
habitat, like many riparian species, consists of a seed that can
float and be transported great distances on open water and
prevailing currents, much as with palms and coconut seeds.
Although mangroves are always found near water, they are
derived from predominantly land plants (spermatophytes)
that produce seed.
In the previous examples, surface water is the source of
the salinity excluded by the plants. Groundwater also can
have high salinity values and effect trees. The effect of high
salt concentrations on tree species used for phytoremediation