Environmental Engineering Reference
In-Depth Information
3.5.2 Cohesion-Tension Theory and Water
Movement
(Scholander et al. 1965). He stated that water potentials of
cut shoots could provide an estimate of the tension of the
water in the transpiration stream prior to the cut. The tension
that a thin column of water can withstand is immense,
between about 3,000 and 150,000 lbs/in.
2
, and is much
stronger than steel.
Because the transpiration stream is under considerable
tension, or low water potentials, in the Cohesion-Tension
Theory, there is a point when the tension is overcome and the
water column breaks, which can lead to the cessation of
water flow. The water in the sap can turn into vapor to
block water flow through cavitation. As stated previously,
cavitation can be the result of the lower pressures of water
under tension causing gases to come out of solution as the
pressure is lowered. It reduces the hydraulic conductivity of
the xylem. Cavitation in tall redwoods was reported to occur
once leaf water potentials exceeded
Even though the flow of water in plants has been pondered
since the days of Aristotle, the cause or causes behind the
movement of water against gravity from the roots to leaves
remains a topic of considerable debate (Zimmermann et al.
2004; Brooks et al. 2004). As was discussed in Chap. 2, a
molecule of water consists of two hydrogen atoms bonded to
an oxygen atom. There is a bond between each hydrogen
atom and the oxygen atom of an adjacent water molecule.
Hence, a series of adjacent water molecules is linked by this
cohesive force. This intermolecular force is what drives the
tension found in water. Therefore, a molecule of liquid water
that exits a leaf after being turned into water vapor is linked
to a molecule of liquid water that is still in the leaf, and these
water molecules all are linked to water in the plant down to
the root hair. This can be envisioned as being similar to the
bucket brigades used to fight fires in the seventeenth and
eighteenth centuries. As such, water molecules that exit the
plant pull up additional water molecules through the plant,
which causes additional water molecules in the root hairs to
rise after entry from the soil by osmosis.
Near the turn of the twentieth century, this cohesive force
between adjacent water molecules was offered as the reason
for fluid flow in trees against gravity; this came to be known
as the Cohesion-Tension Theory (Dixon and Joly 1895).
Indication that this theory is not fully accepted is given by
textbooks in the early twentieth century. For example, Gager
(1934) describes the movement of water upward through a
plant as “not perfectly understood, but the pulling force
resulting from transpiration is, perhaps, the main factor.”
Because much of the soil moisture present in the subsur-
face above the water table is under tension, including water
in the capillary fringe, the cohesion of individual water
molecules to each other has to be stronger than the tension
holding water molecules to soil particles. Energy has to be
input, or spent, for water to move against these forces. The
force that initiates this process is the evaporation of water
and lowered water potential and the energy is from the sun.
Water movement is upward in the direction of decreasing
water potentials, or increasing negative values of water
potential,
1.9 MPa (Koch et al.
2004). Some hybrid poplars have been shown to have cavi-
tation in the xylem during prolonged droughts, assuming that
no water is removed from the water table, and that this cut-
off of water supply increases the hydraulic resistance in the
stems resulting in premature shoot death (Tyree et al. 1994).
Conversely, the water in the transpiration stream can freeze
and break. Both processes lead to gas-filled spaces, or
embolisms, in the xylem that transports the water. These
embolisms have been shown, however, to be repairable,
with water entering the gas-filled cells and continuing to
flow (Canny 1997). These results place into question the
need to explain sap flow solely in terms of measurements
of cohesion and tension, as will be discussed below. More-
over, some plant physiologists claim that water will break
when tension reaches
0.6 MPa, much lower than that
required by the Cohesion-Tension Theory to lift water in
plants more than 300 ft (91 m) tall.
We have seen that the movement of water in the xylem
follows a decrease in water potential and generally is upward
against gravity. Some water exits through the bark, however,
and this loss is discussed later in this chapter. But what path
does an individual molecule of water follow from the root
hair to the stomata? Is the path straight up the side of the tree,
or does it follow a different flow path? Some indication of
the path is suggested by the arrangement of leaves along
stems or the pattern of fissure appearance and lenticels on the
outside of some trees. The arrangement of leaves along a
common axis, in this case, a stem, is called phyllotaxis, and
the relation to a spiral pattern was introduced in the mid-
1700s by C. Bonnet in which leaves were related to a helix
winding around a cylinder, which was challenged later by a
logarithmic pattern suggested by A.H. Church (Kramer and
Boyer 1995). The spirals can be either left- or right-handed.
Much less obvious is the sometimes spiral construction of
xylem beneath the bark. Waisel et al. (1972) reported a study
of the path that water that had been stained with dye takes in
. If no evaporation occurs, the water column
does not move. Unlike the plumbing system of most homes
in which water is supplied through the pipes under pressure
and flow to faucets is created when the pressure is relieved to
a lower level, movement of water in plants from roots to
leaves in the direction of lower tensions, and flow through
the plant is initiated by decreasing the leaf water potential.
Experimental evidence of the high negative tensions nec-
essary to support transpiration streams 10s of ft (1s of m)
above ground was provided by P.F. Scholander, using the
pressure bomb that he devised to measure water potentials
c
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