Environmental Engineering Reference
In-Depth Information
3
Fundamentals of Plant Anatomy and Physiology
Related to Water Use
Water comprises between 75% and 90% of the mass of
terrestrial and aquatic plants. Such high water content
reflects an aquatic stage during evolution. Moreover, it
indicates that the successful transition of plants from aquatic
to terrestrial environments required the ability to retain and
maintain a high water content. It should not be surprising,
therefore, that water is one of two reactants in the decep-
tively simple equation of photosynthesis, perhaps the most
important chemical reaction on earth:
organometallic pigment called chlorophyll. This compound
singularly harnesses the radiant kinetic energy of the sun
into potential chemical energy to be used by plants and, in
turn, all forms of heterotrophic life. For chlorophyll to work,
however, water is required; therefore, it follows that various
sources of water,
including groundwater, can affect
photosynthesis.
3.1.1 History of Unraveling Photosynthesis
CO
2
þ
H
2
O
!
plant chlorophyll
ð
Þ;
light
!
CH
2
O
þ
O
2
:
(3.1)
The story of photosynthesis is revealed in the paths taken by
carbon, hydrogen, oxygen, and light. Much as it took
hundreds of years and the efforts of many individuals to
understand the hydrologic cycle, understanding photosyn-
thesis also took time and effort. In 1754 Charles Bonnet
(1720-1793) observed gas bubbles on leaves of submerged
aquatic plants. In the 1770s, Joseph Priestley (1733-1804)
performed a series of simple experiments that were the first
steps toward unraveling the source of the gas bubbles
observed by Bonnet. When Priestley placed a burning candle
or a live mouse in a closed airtight jar the candle flame
extinguished and the mouse died long before the candle
wax ran out. If, however, he placed a plant, such as a sprig
of mint used in his experiment, in the jar with a candle or
mouse, the candle remained lit and the mouse survived.
(Priestley's experiment led to the beginning of the still
popular tradition of giving flowers to hospital patients in
the hopes that the plants would help cure the patient by
purifying the air.)
In any case, the role of the plant in keeping the flame lit
and the mouse alive remained a mystery and was the source
of much dispute. Priestley stated in 1774 that
All green plants regardless of size, location, or species
use this reaction to synthesize food. Every day, plants react
about 400 million tons of carbon as CO
2
, with 70 million
tons of hydrogen from H
2
O and liberate 1.1 billion tons of
oxygen (O
2
). This reaction is so important that all CO
2
in the
atmosphere eventually passes through the leaves of plants
about every 300 years.
The photosynthetic reaction indicates the importance
of water as a reactant, and provides an excellent starting
point to describe the important role that water, including
groundwater, plays in the life of plants, and ultimately,
how this role affects the phytoremediation of con-
taminated groundwater. The objective of this chapter is
to provide information on plant and water relations that
canbeusedduringthephytoremediationofcontaminated
groundwater.
3.1
Plant Cell Structure, Photosynthesis,
Respiration, Growth, and Dormancy
The putrefaction of such masses of both vegetable and
animal matter, is in part at least repaired by the vegetable
creation. And not withstanding the prodigious mass of air
that is corrupted daily by the aforementioned causes, yet, if
we consider the immense profusion of vegetables upon the face
of the earth, it can hardly be thought but that it may be a
Equation
3.1
reveals that plants are autotrophs and synthe-
size their own food by turning non-living, inorganic
materials, such as CO
2
and water, into living, organic
matter. The entire conversion rests on a speck of green,
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