Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 52

The sugar is then reacted with phosphate (PO 4 ) using energy
from ATP to create glucose phosphate. Two molecules of
ATP and two molecules of pyruvate are released per mole-
cule of sugar undergoing respiration.
The pyruvic acid is then exposed to oxygen to yield
energy, CO 2 , and water as part of the Kreb's cycle, also
called the citric acid cycle or tricarboxylic acid (TCA)
cycle. The Kreb's cycle occurs in the mitochondria of each
cell and consists of reactions of organic acids. The cycle
begins when one molecule of pyruvate is converted to three
molecules of CO 2 , remembering that pyruvate had three
carbons atoms. Not all three molecules of CO 2 come from
the 3-carbon compound of pyruvate; rather, two molecules
become associated with coenzyme A, or acetyl coenzyme A.
The Kreb's cycle has to complete the cycle two times to
convert each 6-carbon sugar into CO 2 . The cycle has to have
a constant supply of oxidized nicotine adenine diphosphate
(NADP) and flavin adenine diphosphate (FAD). Each of
these reactions is controlled by specific enzymes, and these
enzymes are proteins that are encoded by specific genes. The
bottom line is that the glucose is mineralized to CO 2 and its
energy is converted into ATP.
Where does the oxygen in the Kreb's cycle come from?
For terrestrial plants, it is gaseous oxygen from the atmo-
sphere. Oxygen enters in the stomata and by pores in the
stem and branches called lenticels, which are discussed later
in this chapter. Because the gas-phase concentration of oxy-
gen is much higher in the atmosphere relative to the lower
partial pressures of oxygen in the soil, unsaturated zone,
capillary fringe, and water table, oxygen can diffuse from
leaves to roots along a concentration gradient. The transport
through the plant and to the roots is through an
interconnected series of air spaces between loosely packed
cells that are collectively called the aerenchymal cortex.
Much like how the xylem and phloem act as pipes to conduct
fluid flow, these tissues act as pipes to conduct gas flow. The
oxygen is consumed by respiration and the Kreb's cycle and
the balance diffuses into the rhizosphere.

G oethe,

Privy Councilor of the Duchy of Saxe-Weimar, to Explain
the Metamorphosis of Plants (1790; Durant 1953) (Table
1.3). He later composed a poem with the same central idea.
Today this idea can be observed by the repetition a common
angle between stem, branch, leaf main axis, and leaves of
many plants.
What is plant growth, and why is a basic understanding of
plant growth important to the phytoremediation of
contaminated groundwater? The sugars produced by photo-
synthesis provide the energy needed to support metabolism
and to drive plant growth. As may be expected, the selection
of fast-growing plants often used for phytoremediation is
directly dependent on the growth characteristics and water-
use potential of the plants. Moreover, the different growth
characteristics of various plants have important implications
for the capital costs of phytoremediation and the time for
remediation to occur at contaminated sites.
Growth is a prerequisite of a living entity. In general, for
most woody plants, growth can be defined as active cell
division at rates greater than cell death, and this typically
can be found in both above-ground and below-ground meri-
stematic tissues. The most obvious indication that plants
grow is the transformation over time of a small seed, such
as an acorn, to a mature tree such as a majestic 200-years-old
oak. Cell growth is associated in plants with the production
of food, respiration, and metabolism. Obviously, this type of
cellular or tissue growth cannot occur without first having
started with a fertilized female cell, or zygote, that started
the subsequent chain of cell division; additional discussion
of this topic is beyond the scope of this chapter.
There are two kinds of growth tissue in plants, the pri-
mary and secondary tissues, both of which arise from cells
grouped into meristematic tissues, as introduced earlier in
this chapter. Primary growth occurs in cells that can grow
indefinitely and are found at the tips of shoots and roots,
where growth results in an increase in length rather than
thickness. The growth beneath the phloem and xylem that
arises from the cambial tissues is called secondary growth,
because this growth is lateral rather than vertical with a
corresponding increase in girth. This is why initials carved
in a tree trunk at chest height or a nail driven into a tree trunk
is not displaced higher when revisited many years later. Both
types of tissues are also found below ground. Shoots and
roots elongate and expand in girth in dicotyledon plants,
whereas monocotyledons increase in height only. This dif-
ference is regulated by many factors, both internal and
external to the plant. External factors include the degree of
sunlight, amount of precipitation, porosity of soil, degree of
herbivory, etc. Internal factors include those similar to
animals, such as hormone production and regulation. These
internal factors can be affected, however, by changes in
external variables.

€ oethe published this idea in An attempt by J.W. von G €

3.1.6 Growth and Hormones

The fundamental growth of plants has fascinated resear-
chers, as well as laypersons, over the long history of the
interaction between plants and man. The writer and poet
Johann Wolfgang von Goethe, known today simply as
G oethe, also did experiments in and wrote topics on geology

and optics, as well as plant life. He theorized that all varieties
of plants could be reduced to a simple pattern of growth of
basic parts or segments, and that mature plants were
variations on this structural scheme. Moreover, he concluded
from his observations that all parts of a plant above ground
were modifications of a proto-leaf, except for the main stem.

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