Environmental Engineering Reference
In-Depth Information
3.6.2 Measurement of Water Potential
(pre-dawn) when the plant roots and soil moisture are in
equilibrium and a daily high at noon. A plant sample, such
as a leaf, is taken by cleanly cutting it with a sharp knife.
Because the water in the xylem is always under tension, or
negative water potential, no water will flow from the cut;
however, sap can flow because it is under positive pressure
in the phloem. To exude the water and to record the pressure,
the sample is placed in the chamber but with the cut end
outside at ambient pressure. When the water is exuded under
pressure, it equates to the plant's original tension. This can
be done in the field with portable instruments.
The pre-dawn water potential of plants is made under
conditions of closed stomata and, hence, the plant water
potential will be equal to the soil water potential. In the
afternoon, however, stomata are open and water flows from
the soil to the atmosphere through the plant, and measure-
ment of the water potential of the plant is an indicator of
water demand in the air. Phreatophytes tend to have a more
constant supply of water and, therefore, have a higher and
more consistent pre-dawn water-potential value over time
(Snyder and Williams 2000).
The turgor pressure of individual cells can be directly
assessed by using a pressure probe. An air-filled glass tube
sealed at only one end can be inserted into a cell. The
pressure in the cell compresses the gas in the glass tube,
and the pressure calculated using the ideal gas law. The
hydrostatic pressure of individual cells also can be measured
by using a similar approach in which a glass microtube is
filled with incompressible oil. This oil can be readily distin-
guished from the sap that flows into the tube, and the sap
flow can be offset by depressing a plunger, which can indi-
cate the hydrostatic pressure in the cell.
Other field instruments include tensiometers that can be
installed directly in the field to measure the water potential
of the water in the soil near plant roots. A tensiometer
consists of a tube with a porous ceramic cup attached to
one end. It is filled with water and capped with rubber septa
prior to installation in the field. If the soil is drier than the
water-filled porous cup, water will flow out of the tensiome-
ter, and the change in pressure in the headspace above the
water level in the tube can be measured with a transducer.
Why go to all this trouble to define water potential? Why
doesn't a simple measurement of just the water content of
soil, or even the percentage of soil moisture, suffice? This is
because soil water content and moisture percentage can indi-
cate the relative difference in water amounts between two
soil samples, but neither measurement can indicate in which
direction water will flow. For example, the water content of
soil can be the same as the water content of roots, but no
indication of flow direction is suggested. Moreover, some
water is tightly bound to soil particles so a high soil moisture
content does not necessarily indicate that the water is bio-
available. The percentage of soil moisture at
As important as water potential is, it would be meaningless
as an indicator of plant water status if it could not be
measured readily. Fortunately, water potentials can be
measured using instruments that measure pressures or
tensions, such as psychrometers, hygrometers, and tensio-
meters. In most cases, a device that measures pressure used
to stop the movement of water, the hydrostatic pressure, is
used to measure water potentials.
With the psychrometer method, a piece of plant material
of unknown water potential is placed in a sealed chamber
that also contains a droplet of a solution of known water
potential. As the name of the method implies, if the plant
material has a lower water potential than the droplet of
solution and, hence, a lower vapor pressure by way of a
higher solute concentration, preferential evaporation from
the droplet cools its surface. Conversely, if the droplet of
solution has a lower water potential than a less-concentrated
plant-material sample, the sample will evaporate and warm
the droplet. Hence, if the water potential of a particular
solution is known and it results in no net movement of
water to cool or warm the droplet, the sample of plant
material must have the same water potential. Because a
change in temperature can cause a change in water potential
(for example a change of 0.01
C
¼
0.1 MPa, 0.1 MPa
¼
1 bar, 1 bar
14.5 psi), the chamber must be kept at con-
stant temperature and, therefore, is primarily a laboratory
method. This method has been used extensively by Boyer
and Knipling (1965).
Another method involves placing a piece of plant mate-
rial in a chamber that can be pressurized in order to restore
the distribution of water potential between living and non-
living xylem cells in the plant material. Because the act of
taking a biopsy of plant material releases tension when the
water column in the xylem is broken, water initially flows
into the living cells by osmosis. Pressurization of the cham-
ber, however, can reverse this flow of water back to the
xylem. This method has been used since the mid-1960s,
after widespread use by P.F. Scholander and others (1965),
and can be used in the field. The main advantage to water
potential measurements with the pressure chamber approach
is that it incorporates the linkage of the atmospheric demand
for water, the soil's or sediment's supply of water, and the
plant reaction to both. Another method applies pressure to
the whole plant rather than to tissue samples, for example:
the branch or trunk is cut, sealed off, and pressure is applied.
The amount of pressure applied is related to the water-flow
characteristics of the plant.
Because the components that compose water potential
can vary, the total plant water potential is not a constant.
Reference times for collecting water potential, however,
have been established and include a daily low at dawn
¼
least can
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