Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 64

is not limiting. In aquatic or subsurface systems that contain
either natural or contaminant inorganic and organic matter
that can interact with oxygen, oxygen can become limiting.
Such oxygen-depleted, anoxic conditions also can be an
advantage to some plants. This is because under anoxic
conditions certain essential and micronutrients, such as
iron and phosphorus, can be passively taken up by plant
roots with little energy expended.
As we saw in the discussion of photosynthesis, the
splitting of water produced hydrogen and is used to reduce
CO 2 ; essentially a transfer of hydrogen atoms or electrons.
The reduced carbohydrate produced, such as glucose, can be
oxidized, such that electrons are removed, either by the
plant, an herbivore, or omnivore, through respiration to
derive energy in the form of ATP. The most common oxi-
dant used during respiration is molecular oxygen. The cou-
pling of sequential reductions and oxidations involves
changes in energy, too, because the electrons flow downhill
as the energy to drive life is released.
Although for survival most terrestrial plants need to
maintain constant contact with water, either in the form of
precipitation, surface water, soil moisture, or groundwater,
too much water adversely affects root growth and overall
plant health because it restricts the availability of oxygen.
Ready access to the water table by phreatophytes suggests
robust plant growth, as water would not be limiting for these
plants. However, even phreatophytes will suffer from oxy-
gen limitations in the presence of adequate water supply.
Water physically displaces air from soil pores, and oxygen
has a low solubility in water (8 mg/L at 25 C). Plants have
adapted to low oxygen levels by forming interconnected gas
passageways, the cortex or aerenchyma discussed previ-
ously, that permits the diffusive transport of air to the
roots. The diffusion of atmospheric oxygen from the leaves
to the roots was first investigated in willows, a tree often
planted at phytoremediation sites.
Even though plants produce oxygen during photosynthe-
sis, as described at the beginning of this chapter, this process
occurs primarily in the above-ground portion of the plant.
Conversely, because roots do not photosynthesize, roots
consume oxygen during growth through respiration—if
oxygen is used at a faster rate than it is replaced, root cells
will die from asphyxiation. This interaction between plant
growth, water availability, and oxygen levels is particularly
evident in recently flooded areas before the water recedes
or in areas where the water-table level has increased into
the root zone. On the other hand, the frequent vertical fluc-
tuation of the water-table level in response to precipitation
events and(or) changes in plant use of groundwater may only
affect part of the total vertical extent of root mass. A falling
water table may actually increase the amount of oxygen in
the subsurface—this fact will turn out to facilitate the oxida-
tion of organic xenobiotics released to groundwater. A notable

exception to these extremes is the use of aquaculture
methods to grow some terrestrial plants for commercial or
hobby purposes. In this case, the seemingly stagnant water is
kept fully saturated with dissolved oxygen by using
containers that have a large surface area, shallow depth,
and mechanical aeration.
One of the most extreme examples of how trees respond
to large changes in redox conditions caused by changes in
water levels is the rain forests of the Amazon. There, trees
can be submerged under 45 ft. (13 m) of water for as long as
6 months each year (Kubitzki 1989); the forest essentially
becomes a lake. Fernandez et al. (1999) investigated the
influence of prolonged submergence on water potential,
photosynthetic rate, and leaf conductance in the submerged
trees. Some trees retained their leaves after submergence,
and others lost their leaves, which regrew after the water
level subsided. Water potentials increased in the trees as the
water level rose, as water was not limiting. Submergence
decreased photosynthesis by 50% from levels measured
when the leaves were not submerged, indicating that the
leaves could still photosynthesize during submergence.
The presence of trees in anoxic aquatic environments has
given rise to many hypotheses about how terrestrial plants
can oxygenate these anoxic systems. Cypress trees
( Taxodium distichum L.), for example, have outgrowths on
their lateral surface roots, called knees, that grow upward
and above the mean high-water line. The knees are absent on
cypress that grow in water that remains at a constant level,
and appear only where the surface-water level fluctuates.
The location of the knees suggests that they do not arise
from the terminal meristem of the root tips. Rather, cypress
knees are a localized growth of the cambium on the upper
surface facing the water that produces multiple layers of
xylem in a small, focused location. Although cypress knees
are widely believed to be used by the trees to increase root
aeration, this is contradicted by experimental evidence
(Kramer et al. 1952). The oxygen that enters the roots does
so by diffusion through the fluted part of the trunk,
comprised of an increased volume of aerenchymal tissue
between the bark and the phloem (Fig. 3.9 ).

3.3 Roots, Rhizosphere, Bacteria,

and Mycorrhizae

In fact, man lived in a sea of bacteria. They were everywhere—
on his skin, in his ears and mouth, down his lungs, in his
stomach. Everything he owned, anything he touched, every
breath he breathed, was drenched in bacteria.
The Andromeda Strain, Michael Crichton (1969)

Just as mammals, such as ourselves, carry other life forms on
and in our bodies, plants also harbor other life forms. For
instance, the roots of terrestrial plants increase the organic