CASE STUDY 15.2 Pressure–volume curves

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Byron Lamont

Xylem elements supplying water to a transpiring leaf are normally under tension. When such a leaf is excised, xylem sap retreats towards the transpiring leaf blade to an extent that varies according to that tension. If the excised leaf is now sealed into a pressure chamber with the cut petiole protruding (Figure 1a) and pressure applied (Figure 1b), xylem sap can be forced back to the cut surface (Figure 1c). That point, or balancing pressure (BP), is taken as numerically equivalent (although opposite in sign) to the original xylem tension.

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Figure 1 Operation of a pressure camber for measurement of bulk leaf xylem water potential. (a) Insertion of a detached leaf into a chamber with cut petiole protruding through a sealed aperture in the lid. In practice, this leaf would be held within a plastic bag prior to excision and during subsequent measurement to avoid moisture loss. Gas pressire is applied (b) until the cut surface of the protruding petiole moistens (c) (normally viewed with a binocular microscope for safety and as an aid to precision. Excess pressure has been applied here to emphasise sap extrusion. (Photograph courtesy E. A. Lawton) 

At equilibrium, water potential of the xylem sap (Ψxylem) should equal that of the surrounding leaf cells (Ψ) so that:

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Recall Section 4.3 for a walled cell showing:

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where P = turgor pressure and Π = osmotic pressure. Now consider a drying leaf that reaches the turgor loss point where turgor pressure (P) becomes zero (denoted P0). At that point,

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Some physiologists believe that water content as a percentage of what could be held at saturation (relative water content, RWC) is a better index of water status than Ψ. Since the vacuole (internal sap) occupies at least 80% of the volume of the mature cell, the water content of the vacuole largely controls the volume of the protoplast. Hence RWC is an index of the relative volume of the symplasm. The relationship between BP and RWC is therefore known as a pressure–volume curve. It is produced by starting with a near-saturated leaf or shoot (BP → 0, RWC → 100) and allowing it to dry out on the laboratory bench between readings of BP in the pressure chamber. The material is weighed between readings. BP increases rapidly at first but slows down as the wilting point is approached. Eventually, the turgor-loss point (0) is reached whose P0 = 0 and –BP = Ψ0 = Π0. Further reduction in Ψ is much slower and is linear in the absence of the turgor component. After several points have been obtained for the linear section, the material is oven dried and RWC calculated for each reading:

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Figure 2 Pressure-volume curves for shoots from seedlings of Hakea polyanthama that were establishing after a fire on sandplains north of Perth. The winter curve (solid symbols) shows a higher relative water content at any applied balancing pressure than occurs in summer shoots (open symbols) (Original data courtesy B.B. Lamont)

Instead of plotting RWC against BP, it is usually plotted against the inverse, 1/BP, with reduction in water content proceeding from left to right (Figure 2). A straight line is fitted to the lowest (below turgor) part of the curve and extrapolated to the y-axis to produce the inverse of the osmotic potential at full turgor (Π100) (Figure 3a) where the rounded part of the curve that departs from the straight line locates the positions of 1/Π0 and RWC0. The real reason for inverting BP is that the line can also be extrapolated on the right to the x-axis — this is the RWC beyond which no amount of tension will remove the water adhering to the cell wall polysaccharides, and is called the apoplasmic or bound (B) water. Its calculation is tentative at best (because the slope is so shallow small errors could have a large effect on the value obtained).

The final parameter calculated is the bulk elastic modulus (e). This is how much strain (reduction in relative cell volume) occurs in response to a certain stress (in this case a lowering of the water potential). It is actually an inverse measure of how elastic the bulk of the tissues are. In its simplest form, it is the slope between the two endpoints: saturation (RWC100, Ψ100) and turgor-loss point (RWC0, Ψ0) (Figure 3a). As it is the protoplasts that actually change volume, the formula is usually corrected for apoplasmic water (B), that is, water outside the protoplast:

 

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Table 1

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Figure 3 Key parameters of tissue water relations can be derived from pressure-volume curves. By plotting the inverse of balancing pressure (as shown in (a)) the curve can be extrapolated to the x-axis to obtain a value for water bound to cell walls (B) as well as a value for the inverse of osmotic pressure at saturation (P100), and at turgor-loss point (P0) as well as RWC0 (identified where the curve starts to become linear). In (b), the level of water stress imposed as water potential drops from saturation, Ψ100, to turgor-loss point, Ψ0, creates a strain in the tissues which is registered as a reduction in cell volume from saturation (Ψ100) to RWC0. The slope of this relationship represents the bulk elastic modulus (ε) (Original drawings courtesy B.B. Lamont)

Preparation of pressure–volume curves enables identification of a number of key water relations properties of plants. They show how different species and ecotypes respond to the same growing conditions, and how the same species changes its water relations properties as water availability changes (for example, see Table 1). These values are derived from pressure–volume curves as shown in Figure 2. The osmotic potential of the vacuolar sap is more negative under the drier summer conditions than in winter. This means more solutes have been secreted into the vacuole, a response called osmotic adjust-ment (∆P). In this case, ∆P is –1.75 – (–2.57) = 0.82 MPa. The greater the plant’s ability to osmotically adjust, the greater its drought tolerance, for this increases its ability to maintain turgor and prevent desiccation. Drought-tolerant plants usually have a lower P100, P0 and RWC0 than drought-sensitive or drought-avoiding plants. However, just because a plant wilts (i.e. it drops below RWC0) it won’t necessarily die, for this may depend on the ability of the protoplast to withstand various levels of desiccation (see Feature essay 15.1). Against intuition, drought-tolerant plants tend to have higher B and e. This is because their cells are smaller and have thicker, denser walls and smaller vacuoles, so that more water is apoplasmic and the cells are less elastic. Perhaps this works by preventing tissue collapse once RWC0 is reached.

Further reading

Radford, S.P. and Lamont, B.B. (1992). An Instruction Manual for ‘TEMPLATE’: A Rapid, Accurate Program for Calculating and Plotting Water Relations Data Obtained from Pressure–volume Studies, School of Environmental Biology, Curtin University: Perth.

Turner, N.C. (1981). ‘Techniques and experimental approaches for the measurement of plant water status’, Plant and Soil, 58, 339–366.

Witkowski, E., Lamont, B., Walton, C. and Radford, S. (1992). ‘Leaf demography, sclerophylly and ecophysiology of two banksias with contrasting leaf life spans’, Australian Journal of Botany, 40, 849–862.

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