Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 78

trunk is a large pipe of a given cross-sectional area that
provides only enough water to support the summed cross-
sectional area of the branches above it.
The same correlation between flow and total cross-
sectional area would have to apply below ground for the
root system as well. Shinozaki et al. (1964) brought this
relation into focus with the unit-pipe analogy; that a group
of leaves on a branch was fed by a section of conducting
tissue fed from underground by a set of roots. The movement
of water from the roots to the leaves, therefore, is not random
but highly specific. This often can be seen in trees struck by
lightning, in which a few roots are killed and the branches
that these roots supply dry up and die, but the main tree
survives.
The flow of water in a well-watered plant has a budget
such that the water uptake equals the water lost. This flow of
water in the plant, Q , is analogous to the flow of an electrical
current in a circuit, such as

the Cohesion-Tension Theory is based on data collected by
Balling and Zimmermann (1990). They used a pressure
probe to assess the water pressure of intact xylem cells.
They found that moderate negative pressures existed, rather
than extreme negative pressures, between 10 MPa,

required by the Cohesion-Tension Theory, which the authors
described as placing water in an extremely unstable, meta-
stable state akin to superheated water. Also, their explana-
tion of flow does not require the xylem vessels to contain a
continuous column of water. As such, Zimmermann et al.
(2004) suggest that water movement in plants occurs in a
series of complicated lifts of various volumes of water.
Finally, no discussion of theories of water movement in
plants can be considered complete without mention of the
possible effect of the moon's gravitational pull on water
bodies on earth, including water in plants. The phenomenon
of tides in oceans and large lakes caused by the moon's
gravitational pull during its phases is well known. Less
well known, but just as verifiable by observation, are
groundwater earth tides in deep, confined aquifers that is
discussed in Chap. 4. The reason that the moon influences
oceans and deep groundwater is because both are large
volumes of water. In plants, water is present in small tubes
that seemingly would not be affected by gravitational
differences. There are many gardeners, however, who
believe in planting based on the moon's phases.

1to

Q ¼ Water potential difference = sum of the resistance : (3.7)

As water moves though the xylem, resistance to flow is
encountered. This happens to the flow of any substance
through a conduit, such as electrons through a wire, where
the resistance is felt as heat. To overcome resistance to flow,
the tension of water permits resistance to occur without the
water becoming vaporized into the gas phase, or for
embolisms to form. The resistance to flow is about 1.5 atm/
32 ft (10 m) in height, so for a 459-ft (140 m) tree, the
resistance is about 35 atm; no trees on earth are higher.
Perhaps the best example of how useful the Cohesion-
Tension Theory is in supplying the water needs of tall plants
is provided by the existence of the coast redwoods ( Sequoia
sempervirens ) in California mentioned previously. These
trees can reach heights of more than 350 ft (106 m). Is this
the limit that water can be transported along a negative water
potential gradient? Would even more negative potentials be
possible, since plant cells need some water pressure to main-
tain turgor? If such negative water potentials induce stomatal
closure in most plants, how do the upper leaves of the
Sequoia photosynthesize? To help answer these and other
questions, Koch et al. (2004) took water-potential
measurements, both predawn and midday, of the leaves of
Sequoia at different heights and found them to correlate
directly, from about

3.5.3 Stomatal Resistance

Carbon dioxide must enter the leaves for chlorophyll to help
make plant food. Entry is gained through stomata, pores in
the leaves (from the Greek, stoma , meaning mouth) that
regulate this gas exchange—they control CO 2 uptake as
well as water vapor loss. The stomata typically account for
less than 1% of the total upper and lower surface area of
leaves and are small (about 15 m), so that even on a small

leaf there are between 13,000 and 100,000 stomata/cm 2 ;an
average-sized leaf can contain millions to 10s of millions of
stomata. Stomata present on one surface of the leaf are called
hypostomatous plants, and stomata present on both leaf
surfaces are called amphistomatous. Many plants, such as
Populus used for phytoremediation, are amphistomatous,
and these plants tend to be characterized by higher rates of
growth and transpiration. Different types of stomata can be
found on plants and classified based on the structure of the
guard cells and adjacent cells. The different types of stomata
are the anomocytic, anisocytic, paracytic, and diacytic.
The movement of water through a leaf is regulated by
stomata, for they act like a valve that balances the input of
water from the root hairs to the demands of water by the
atmosphere by evapotranspiration. Stomata can partially or
fully close, based on the lower water potentials of adjacent

m

0.7 to 1.3 MPa (predawn) and

from 1.84 MPa (midday) at 131 ft (40 m) and

354 ft (108 m), respectively. Turgor was measured also with
respect to height and found to decrease with height from
0.93 MPa at 164 ft (50 m) to 0.48 MPa at 360 ft (110 m).
Pressures are low but still remain positive, even though the
leaf water potentials are

1.2 to

1.84 MPa at 354 ft (108 m).
An alternative explanation of the ascent of water in plants
that does not rely on the large negative pressures required by