Definition of water potential (Ψ)
The water status in plant systems is measured in terms of water potential (Ψ), which is a
measure of free energy available to do work. Water moves spontaneously only from regions of
higher water potential to regions of lower water potential. As water moves down its potential
gradient, it releases free energy so that such a flow has the potential to do work.
(This potential has the units of J m -3 (energy units), the dimensions of which are the same as
those of pressure).
The state of water in all of its sites, from the soil through the plant and into the atmosphere can
be described by the appropriate SI unit for pressure, which is the pascal (Pa), the name for N m 2. (1 Pa = 1 N m-2 = 10-5 bar). Water potentials will normally be expressed as MPa (1 MPa = 10 6
pascals = 10 bar).
Ψ of pure water is defined as having a water potential of 0.0 MPa at a standard temperature of
25°C and at atmospheric pressure.
Definition of total water potential (Ψt )
Total water potential (Ψt ) is the central parameter of plant water relations and describes the
energy state of water in the soil - plant - atmosphere continuum (SPAC).
Flow through the Soil - Plant - Atmosphere Continuum
Water movement from the soil, through the plant, to the atmosphere may be treated as a series
of interrelated processes.
The rate of water absorption from the soil is affected by both the rate of water loss from the
leaves to the atmosphere and the rate at which water can move from the soil to the root surface.
This movement of water may be described with water transport equations analogous to the
Ohm's Law in electrical engineering:
Flux = (Driving Force) / Resistance
or Water flux = ( Delta Ψ) / Resistance
where Delta Ψ is the difference in water potential and R is the resistance between the two points
where water potential is being considered.
In 1948, based on earlier work of Huber, Van den Honert proposed equations for describing
water flow from the soil to the atmosphere in a linear fashion. A major weakness in modeling flow
with such an equation is that flow is described in terms of a catena or unbranched chain,
whereas a tree is a branched system that also may store water.
Water flow is driven by the difference in water potential between the soil (Ψsoil) and the
atmosphere (Ψair) (from Ewers & Cruiziat, 1990). The potential gradient in the SPAC is the
driving force for water transport through the plant.
Current models have been expanded to incorporate the presence of stored water or
capacitance. The capacitance of any part of the system may be defined as the ratio of the
change in tissue water content to the change in water potential. In leaves and in herbaceous
stems and roots, the mechanism of water storage is elastic, that means, as tissue water potential
increases or decreases, the volume of the tissue increases or decreases respectively.The
importance of stored water has also been considered for trees. Most of the diurnal flow of water
from storage is provided by the phloem and the bark, while on a seasonal basis, the sapwood is
the most important source.
Components of water potential
Total water potential is the link between two equations describing different aspects of water
relations. They form a system of demands and responses.
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Demands come from the soil - plant - atmosphere continuum and may be divided into three
components, which have, like total water potential, a negative sign or are at most zero:
(-)Ψt = (-)ΨS + (-)ΨG + (-)ΨF
Ψ S is static substrate potential, Ψ G gravitational potential, and ΨF frictional potential.
This equation deals with factors in the soil-plant-atmosphere continuum which reduce the energy
of water below the value for distilled water at standard temperature and pressure. Water
somewhere in the pathway cannot have an energy content higher than that of the substrate, thus
static soil water potential sets the base value. Soil water potential will generally change slowly,
but during a dry period and on shallow sites may reach quite negative values.
A further reduction in total water potential in the continuum is due to gravity: holding the water
column above the absorption zone of the roots will require 0.01 MPa per metre of height; this
value may be calculated from the specific weight of water and the gravity constant. In most
cases ΨG is of minor importance and, therefore, can be ignored, but must be included when
considering movement in tall trees.
Finally, the mass flow of water will encounter frictional resistances which further reduce the
energy content of the water before it arrives at the evaporating surfaces in a leaf. Frictional
potential losses are responsible for the daycourse patterns of total water potential on a clear day
in the field. Friction can arise from water movement in the soil and in the plant body. Equation
(1a) explains this:
Point X
F = - fi * ri
Substrate
(Frictional potential is the sum of the products of fluxes and resistances along the pathway from
the soil to the measuring point).
The water pathway has to be divided into sections where both fluxes (f i) and resistances
(r i) differ. Examples for such sections in the plant body are small rootlets, with a high resistance
r i due to the small diameter and the restricted number of conducting elements, but also a small
portion of water taken up per unit time, i.e., a small flux f i. Each one of the sections in the
pathway reduces the potential of the water it transfers to the next section. The potential drop in a
given section of the pathway is thus a function of both the amount of water transported and the
efficiency of the conduit, that is, the number, structure and functional state of the xylem
elements.
The second equation describes the response mechanisms adjusting total water potential to the
value preset by potential losses in the continuum.
-t = (-)0 + (+/-)P
Ψo, osmotic potential, is always negative because osmotically active particles reduce the water
potential of solutions below that of distilled water. Ψo
solutes concentrate during cell dehydration. This is a passive increase of solute concentration
due to water loss. During the process of osmotic adjustment, which has been observed in many
species as response to water stress, the osmotic potential becomes more negative because of
active accumulation of solutes.
Ψp, pressure potential, may have a positive or a negative sign depending on the compartment
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under consideration. In a turgescent cell, pressure potential is positive representing turgor
pressure generated by diffusion of water into the protoplast of the cell, which is enclosed by a
relatively rigid wall that resists expansion.
In the water conducting elements of xylem the pressure potential is usually negative.
It depends on the specific compartment in the plant body which numerical values we will find for
the two components on the right side of equation 2. A single total water potential can be
measured, for instance, for a whole leaf but it is impossible to do the same with osmotic potential
or pressure potential. Different combinations of osmotic and pressure potential are found in
xylem conduits, cell walls and protoplast.
To my opinion, the pressure chamber is the instrument best suited for measurement of Ψ t,
especially in field studies. This method was first used in 1914 by Dixon to measure the water
status of leaves. Scholander et al. (1965) rediscovered the technique and made it quite popular.
The pressure chamber design has changed little since the improvements of Scholander and coworkers and is described in detail by Turner (1988). Therefore, I will not give a description of the
single parts of the apparatus now, but persons who are interested in this technique are invited to
discussion after the lecture.
Measurement Procedure
A leaf, part of a leaf, a twig or root is cut from the plant and immediately placed in the chamber
with the cut end protruding through a rubber gasket which seals the chamber. Then compressed
air is led slowly into the chamber thus increasing the pressure inside gradually. Pressure is
applied until water begins to return to the cut surface. This 'balance pressure' required to force
water back to the cut end is equal in magnitude but opposite in sign to the tension in the xylem
that existed in the intact plant material prior to excision. Since the osmotic potential of xylem sap
is usually less than 0.02 MPa, the negative of the balance pressure therefore equals the original
water potential of the intact organ.
Pressure-volume (PV) curves
Pressure-volume (PV) curves describe the relationship between total water potential (Ψ t) and
relative water content (R) of living organs. According to the Boyle’s and Van’t Hoff’s laws, the
equation
Ψo x V = constant
says that the product of osmotic potential and volume of solution should be a constant for any
given amount of osmotically active solutes in an ideal osmotic system. Methods for measuring
the ‘pressure’ or potential parameter and the ‘volume’ or water content term vary. Linear
relationships may be obtained by converting either potential or water content to its reciprocal. We
can plot Ψt vs. 1/R (type I transformation) or 1/Ψt vs. R (typ II transformation).
Both of these transformations have certain advantages.
A type I transformation allows analysis of the whole turgor range.
A type II transformation is generally superior for estimating osmotic potentials at full saturation
from linear regression analysis.
A PV curve can be used to estimate 3 significant water relations parameters. First, the potential
at which the straight ‘osmotic line’ intercepts the vertical at R -1 = 1, the ‘saturation line’ gives the
osmotic potential at full saturation (Ψo(sat)). Secondly, the point where the line curving down from
a water potential of 0 meets the ‘osmotic line’ is the turgor loss point (Ψ o(tlp)). To the right of Ψo(tlp),
the cell or organ is flaccid. Thirdly, the intersections of the straight ‘osmotic line’ with abscissa
and ordinate provide an estimate for the ‘ideality’ of a given set of data. The larger the distance
of these intersections from the origin, the larger is the deviation from an ideal osmotic system).
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PV curves may be established on plant material repeatedly sampled at different times of day or
during the growing season. Analysis of the curves may reveal osmotic adjustment.
An addition of solutes to a given amount of vacuolar solution should lead to lower osmotic
potentials both at full saturation and the turgor loss point. The angle between the coordinates
and the saturation line is changed.
For pressure-volume measurements, samples need not be killed. For the construction of PV
curves thermocouple hygrometers may be used, but in most cases the pressure chamber is
more convenient.
PV curves with the pressure chamber may be established by two methods:
Sap collection method:
If additional pressure is applied to a leaf or twig beyond the initial balancing pressure, xylem sap
is expressed. This sap can be collected and its volume determined. By doing this in a step-wise
manner, a pressure-volume curve can be established.
Bench drying method:
A leaf or twig is allowed to dehydrate on the lab bench between the readings. After an initial
balance pressure is obtained, the leaf is quickly weighed and then left to dry until a new balance
point is established.
Kikuta, Kyriakopoulos & Richter (1985):
After careful washing the leaves were resaturated overnight by standing them in a closed vessel
with the petioles immersed in distilled water.
The leaf was wrapped in a plastic bag to reduce transpiration into the chamber. Air pressure in
the chamber was increased at a rate not higher than 0.02 MPa s -1. After the first appearance of
water at the cut surface, the pressure was again reduced so that the water disappeared
completely. The pressure was increased and reduced again until a stable reading was obtained,
usually 0.03-0.05 MPa below the pressure at which the water had appeared first. Gas was then
released at a rate of about 0.02 MPa s -1 to avoid injury to the leaf cells which may occur at higher
rates. The actual fresh weight for each pressure chamber measurement was assumed to be the
average of weighings before and after insertion into the chamber.
Measurement of pressure potential (Ψp )
The last component we have to deal with in equation 2 is the pressure potential which may bei
either positive (in turgescent, living cells) or negative (in the conducting elements of the xylem).
The positive pressure in living cells may be indirectly estimated as the difference between total
water potential and osmotic potential calculated from equation 2.
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