Summary

John Boyer

Definitions

Pressure chambers measure the water potential and its components by using an external pressure to determine the tension on water in the xylem of an excised leaf. When the external pressure is raised to balance the tension, the xylem solution appears at the cut surface and remains steady without flow in or out of the leaf. Because the tension is the main component determining the water potential of the leaf, the method often is used as a simple, rapid estimate of the leaf water potential (Boyer, 1967b).

Terminology and equations

Water potential is the sum of the forces acting on water in the liquid. The water potential is

where is the chemical potential (energy/mol), V is the partial molal volume of water (m3/mol), and the subscripts w and o are water in the system and in the reference, respectively. V is considered constant over most of the biological range of conditions. ℼw is thus proportional to the difference in chemical potentials. ℼw has units of energy/m3, i.e., force/m2 = pressure (Kramer and Boyer, 1995).

The energy of liquid water is affected by several components, specifically solute, pressure, matrix (solid) and gravity, and their algebraic sum determines the water potential

where subscripts s, p, m, and g indicate solute, pressure, matrix (solid) and gravity effects on the energy of the water. The reference is defined to be pure water (no solute), at atmospheric pressure, free (no solids), a defined gravitational position, and the same temperature as the sample. The water potential of the reference is zero and by comparison, solute and solid components always decrease the energy (always less than the energy in the reference, i.e., negative) while pressure and gravitational components can be positive or negative. Units of the water potential and its components are usually megapascals (MPa) or kilopascals (KPa) where 1 MPa = 1000 KPa = 10 bars = 9.87 atmospheres.

In plant tissues, there are two compartments that must be considered for these components. The liquid in the cell walls (apoplast) is in equilibrium with that in the cytoplasm (symplast), but the components of the water potential can be different, e.g., low solute concentrations in the apoplast and large concentrations in the symplast, or negative pressures (tensions) in the apoplast but positive ones (turgor pressures) in the symplast. This compartmentation needs to be considered when measuring the component potentials.

Pressure chambers measure only one component of the water potential – the tension on water in the xylem (Scholander et al., 1964; 1965). The xylem is in liquid continuity with the cell walls and thus the apoplast. Porous solids such as walls contain surfaces that attract water (Boyer, 1967a) and hold it in the interstices of the wall matrix, resisting removal of the water by the tension in the xylem. This attraction is the matric component of the water in the apoplast. Normally, the apoplast is exposed only to atmospheric pressure and has an external pressure of zero but the wall solution may contain solute in addition to the tensions caused by the wall matrix. When gravitational effects can be ignored (see below), the apoplast water potential is

where the subscript a denotes the apoplast compartment (Boyer, 1967b). If the wall solution is dilute, the solute component may be ignored and

i.e., the apoplast water potential is dominated by the matric tension measured by the pressure chamber. Although the apoplast components differ from those in the symplast, the water potentials are in equilibrium with each other. Therefore, in many leaves, the pressure chamber becomes a convenient measure of the water potential of the whole leaf (Boyer, 1967b).

Measurement approaches

Instruments

Pressure chambers consist of a chamber holding an excised leaf which extends through a seal in the chamber top so the cut surface can be observed outside while the blade is exposed to pressure inside. Compressed air enters the chamber through a tube. A gauge measures the pressure inside relative to the atmospheric pressure outside (gauge pressure). As the pressure builds inside the chamber, water is forced out of the cells into the apoplast until the xylem solution appears at the cut surface of the xylem. With small adjustments in the pressure, the balancing pressure can be found at which the xylem solution neither accumulates nor retreats from the cut surface. The solution is then at balance and at the same position as before the leaf was detached from the plant. The balancing pressure is m(a) controlled by the tension on the solution in the xylem.

With a microliter osmometer such as that provided by an isopiestic thermocouple psychrometer (Boyer, 1967b), a small sample of the xylem exudate can be collected and s(a) measured. The water potential of the leaf is then

. In many situations, s(a) is small enough to ignore and m(a) is a good approximation of the leaf water potential.

Pressure chambers involve high pressure and are heavily made. They differ primarily in the nature of the seal. In one design, the seal is on the outside of the chamber top and contains rubber in a well topped by a plate that can be screwed more tightly onto the rubber, squeezing it more tightly onto tissue extending through the seal. During a measurement, the operator tightens the seal just enough to prevent gas from escaping from the chamber around the plant tissue. Another design has a pressure-actuated seal consisting of a rubber stopper in a cone on the underside of the chamber top. As pressure builds in the chamber, the stopper is pushed farther into the cone, tightening the seal. Of the two, the outside seal is preferred because the tightness is controlled by the operator and excessive tightening is avoided. Seal tightness is important because it can collapse the xylem or tear delicate leaf tissue, preventing accurate measurements.

Most pressure chambers are designed to allow the pressure to be adjusted after the first appearance of the xylem solution. This is preferred because pressure can build ahead of the appearance of the solution, giving a false reading unless time is taken to adjust the pressure until a true balance occurs. Other chambers are designed to stop gas entry at the first appearance of xylem solution. This method is faster than the balancing method, but is less desirable because the first appearance may not reflect the equilibrium condition between apoplast and symplast.

Seals can vary in diameter or shape to accommodate different tissue dimensions passing through the seal. Petioles and stems usually pass through a round opening in the seal while grass blades are held in a slit in the seal.

Components of water potential

Solute – although the pressure chamber measures conditions in the apoplast, the equilibrium between the apoplast and symplast compartments allows the osmotic potential of the symplast to be obtained. Advantage is taken of the behaviour of solutions when water is removed from them. Assuming the solute content is unaltered by the removal, the osmotic potential follows the relation

where V is the volume of water in the symplast and k is a constant (Scholander et al., 1964; 1965; Tyree and Hammel, 1972). This relation is the equation of a straight line, and plant tissue from which water is removed will follow this relation. Assuming gravitational effects can be ignored, the osmotic potential and turgor pressure are the only two components normally affecting the energy of water in the symplast because the hydration is too high to form a significant matric potential. Accordingly, the balancing pressure in the pressure chamber indicates

where the subscript (s) indicates the symplast compartment in equilibrium with the apoplast compartment. By over-pressuring the tissue in a pressure chamber, water can be removed until the turgor pressure is zero, and

. A plot of the reciprocal of the pressure chamber readings (1/ s) versus the volume removed gives a straight line that can be extrapolated to any V, thus giving the symplast osmotic potential for any hydration of the tissue.

Pressure – if the plot of the osmotic potential described above includes the region where turgor pressure exists, the turgor pressure can be obtained from

. Pressures measured this way appear quite accurate (Boyer and Potter, 1973).

Matrix – the pressure chamber directly measures the matric force in the apoplast, described above.

Gravity – the mass of water becomes important in columns such as the vascular system of trees where water at the bottom is under more pressure than at the top. For example, a tree 10 meters high will have a gravitational potential 0.1 MPa higher at the bottom than at the top in the xylem solution. To measure the gravitational potential, samples of leaves are collected from various heights at the same time when the tree is not transpiring, and the gravitational potential is assessed from

because the gravitational differences is the only component altering the water potential in the apoplast (in hydraulic contact with the xylem solution whose tension is measured with the pressure chamber). The non-transpiring condition is needed to minimize water movement through frictional resistances in the tree, thus allowing the weight of the water to be the only remaining force affecting the water potential of the xylem solution (as in Koch et al., 2004).

Ranges of values

The pressure chamber is robust and capable of rapid measurements, making it ideal for use in the field. The best approach is to place the chamber next to the plant to be measured in order to minimize the time between excising the leaf and sealing the chamber. In general, the chamber should be loaded within 10 s after excision. If more time is required, a glove box should be constructed and the tissue loaded into the chamber top in a saturated atmosphere inside the glove box, then rapidly placed in the pressure chamber.

The pressure chamber returns the xylem solution to its position in the intact plant using gas applied to the leaf exterior. The cut surface is thus a reference position and should not be modified or re-cut.

The gas entering the chamber passes into the leaf and presses on the external surface of the cells, causing water to exit into the xylem. No dew or external water drops should be on the leaf surface because they can be pressed into the tissue by the gas.

Compressed air should be used as the pressurizing gas in pressure chambers because it supplies oxygen to the tissue inside. This is particularly important for long exposures during measurements of the osmotic potential (see above).

Gas entering the pressure chamber is dry and will dehydrate the tissue, causing tensions to be larger than in the intact plant (Puritch and Turner, 1973). To avoid this problem, the walls of the chamber should be covered with wet blotting paper and the incoming gas should be bubbled through water on the bottom of the chamber. Alternately, the leaf blade can be enclosed in a plastic bag, excised, and loaded in the chamber with the bag enclosing the blade during the measurement (Turner and Long, 1980).

Gas entering the chamber becomes warm as it re-compresses, which heats the tissue. On a warm day in the field, the temperatures can be high enough to disrupt cell membranes. In order to avoid this problem, wrap the chamber in a wet paper towel and let the water evaporate to the atmosphere, cooling the chamber exterior.

Gas entering the chamber dissolves in the water in the tissue and is released in the xylem, which is at atmospheric pressure. The release can be observed as small bubbles at the cut surface or occasionally as foam if the xylem solution contains surfactants. If foam occurs, touch to break it up. If the foam persists, the measurement may need to be abandoned.

Health, safety and hazardous waste disposal considerations

Always pre-test a pressure chamber by completely filling it with water and over-pressurizing. If it withstands the pressure, it should be safe to use with gas.

Always observe the cut surface from the side, never from overhead. Wear eye protection when observing the cut surface. Occasionally, plant tissue comes loose without warning and can be dangerous.

Boyer JS. 1995. Measuring the Water Status of Plants and Soils. Academic Press: San Diego. 178 pp. https://udspace.udel.edu/handle/19716/2828

Kramer PJ, Boyer JS. 1995. Water Relations of Plants and Soils. Academic Press: San Diego. 495 pp. https://udspace.udel.edu/handle/19716/2830

Boyer JS. 1969. Measurement of the water status of plants. Annual Review of Plant Physiology 20, 351-364.

Literature references

Boyer JS. 1967a. Matric potentials of leaves. Plant Physiology 42, 213-217.

Boyer JS. 1967b. Leaf water potentials measured with a pressure chamber. Plant Physiology 42, 133-137.

Boyer JS. 1995. Measuring the Water Status of Plants and Soils. Academic Press: San Diego. 178 pp. https://udspace.udel.edu/handle/19716/2828

Boyer JS, Potter JR. 1973. Chloroplast response to low leaf water potentials. 1. Role of turgor. Plant Physiology 51, 989-992.

Kramer PJ, Boyer JS. 1995. Water Relations of Plants and Soils. Academic Press: San Diego. 495 pp. https://udspace.udel.edu/handle/19716/2830

Puritch GS, Turner JA. 1973. Effects of pressure increase and release on temperature within a pressure chamber used to estimate plant water potential. Journal of Experimental Botany 24, 342-348.

Scholander PF, Hammel HT, Bradstreet ED, Hemmingsen EA. 1965. Sap pressure in vascular plants. Science 148, 330-346.

Scholander PF, Hammel HT, Hemmingsen EA, Bradstreet ED. 1964. Hydrostatic pressure and osmotic potential in leaves of mangroves and some other plants. Proceedings of the National Academy of Sciences USA 52, 119-125.

Turner NC, Long MJ. 1980. Errors arising from rapid water loss in the measurement of leaf water potential by the pressure chamber technique. Australian Journal of Plant Physiology 7, 527-537.

Tyree MT, Hammel HT. 1972. The measurement of the turgor pressure and the water relations of plants by the pressure-bomb technique. Journal of Experimental Botany 23, 267-282.