Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 210

psychrometers are often only used in a laboratory setting.
The psychrometer method has been used extensively by
Boyer and Knipling (1965).
Another method to measure water potential involves
placing a piece of plant material into a chamber, called a
pressure bomb. Because the Cohesion-Tension theory is
based on having negative water potentials up to

potential data collected from the field or under laboratory
conditions. Because different instruments often lead to the
use of different units, confusion can ensue, which inhibits
the comparison of data collected using different approaches
from different sites. Table 9.1 shows the range of water
potentials in drying sediment and the units of measure
most commonly used.
Models to simulate the uptake of water in the root zone
and based on water potential were first applied in the 1960s
(Gardner 1960; Green et al. 2006). Gardner's approach used
an analytical solution, whereas other more recent approaches
are based on numerically solving the Darcy-Richards
equation.

1 MPa in
the xylem, the water in plants will be under tension. This
tissue is pressurized in the bomb to restore the distribution of
water potential between living and nonliving xylem cells in
the plant material. A typical instrument that can make these
measurements is the Scholander Pressure Bomb (PMS
Instruments, Corvallis, OR), named after the research of
Scholander et al. (1965). Because the act of taking a biopsy
of plant material will release any tension present when the
water column in the xylem is broken, water initially flows
into the living cells by osmosis or capillary action. Pressuri-
zation of the chamber, however, will reverse this flow of
water back to the xylem. The advantage of this instrument
over the psychrometer is that it can be used in the field.
There are those who suggest, however, that the bomb tech-
nique produces inaccurate and more negative water
potentials than expected (Zimmermann et al. 2004).
The psychrometer and pressure bomb instruments can be
used to assess the water pressure of water present in plant
tissues removed as a whole component of many different
cells. The turgor pressure of individual cells, however, also
can be directly assessed using an instrument called a pres-
sure probe (Zimmermann 1989). 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 using the Ideal Gas Law the pressure can be calculated.
The hydrostatic pressure of individual cells also can be
measured with a similar approach that uses a glass microtube
but filled with an incompressible oil rather than air. This
oil can be readily distinguished from the sap that flows
into the tube, and this sap flow can be offset by depressing
a plunger, which can indicate the hydrostatic pressure of
water in the cell.
Given the different instruments that can be used to mea-
sure water potentials, there are various ways to present water

9.1.2 Root Hydraulic Conductance

As described in Chap. 3, the term hydraulic conductivity is
used both in plant physiology and hydrogeology to describe
the movement of fluids though various media. With respect
to plants, hydraulic conductivity describes the diffusive
movement of water through the symplast from one cell
to another cell by way of the cell membranes and
plasmadesmata. Because cells are separated from each
other by a semipermeable membrane, this acts to deter the
simple flow of water. For example, if a cell at a certain water
potential is placed in contact with water at a higher potential,
water will move into the cell because it has lower water
potential. The central questions here are, what is the initial
rate of the water movement, and what controls this rate?
The resistance to water movement caused by the cell
membrane, a force that can be quantified, is referred to as
root hydraulic conductivity, L p . Root hydraulic conductivity
provides a way to assign a value to the degree of resistance to
the diffusional flow of water between cells, and has units of
volume of water per unit area of membrane per unit time per
unit driving force (m 3 /m 2 /s/mPa). The velocity of water
transport, W v , from one cell to another can be described as

W v ¼ L p DðÞ (9.1)

Table 9.1 Range of units typically used to report measurements of water potential.
Status

Osmolality a

Megapascals (MPa) Kilopascals (kPa) Millipascal (mPa) Bars Pounds per

square inch (Psi)

10 6

Wet 0.001 1 1.0 0.01 0.145 0.0004

10 7 Field capacity 0.033 33.0 3.3 0.33 4.786 0.0135

10 9 Plant available water, lower

limit (wilting point)

1.5 1,500 1.5 15 217.5 0.6157

1.0 10 11 Air dry 100 100,000 1,000 14,503 41.0494

1.0 10 12 Oven dry 1,000 1,000,000 10,000 145,037

410.4939

a Osmolality, in milliosmoles/kg