The Hofler diagram and the pressurevolume curve

Water relations parameters of plant tissues can be presented by the Hofler diagram and the pressure-volume curve (PV curve). The Hofler diagram shows the relationship between water potential and relative water content (Fig. 2.2a). The PV curve is a plot between the reciprocal of water potential (-1/^) and RWC or water loss (1 - RWC) during desiccation (Fig. 2.2b). Both the Hofler diagram and the PV curve are widely used to characterize water relations of plant tissues. To construct a Hofler diagram or a PV curve, the changes in water potential and RWC are monitored as the tissue is dehydrating. Several important parameters can be obtained by analysing the components of cell water potential, including the osmotic potentials at full turgor and at the partially dehydrated state, the apoplastic and symplastic water volume in tissues, a plot of turgor pressure (i.e. hydrostatic water potential in Equation (6)) as a function of RWC, and the tissue bulk modulus of elasticity. Without knowing these biophysical metrics, it would be impossible to identify different kinds of cellular stresses associated with the loss of water in the tissue and to examine the significance of an array of biochemical and physiological responses during desiccation. Moreover, valid comparisons of the response of cell function to water stress among different organisms cannot be made without such knowledge.

2.4.1.1. Change of cell turgor pressure during desiccation

In fully turgid cells, turgor pressure is equal to the osmotic potential (with opposite

Hofler Diagram

Fig. 2.2. Cellular water relations. (a) Hofler diagram showing the components of cell water potential. Intercellular or external water (RWC > 1.0) in many plant tissues is held at near-zero water potential and, during the initial dehydration, cell water potential and turgor pressure (^p) do not change significantly (the horizontal dashed line). Maximum osmotic potential is found at the point of full turgor (RWC = 1.0), where ^p = As the plant tissue loses water, turgor pressure decreases, and at the turgor-loss point (RWC = ~0.8), ^ = (the vertical dashed line). At RWC < ~0.8, the relationship between RWC and follows a rectangular hyperbola (RWC = a + b/^n). Osmotic potential at RWC = ~0.8-1.0 is extrapolated from the rectangular hyperbola relationship. Turgor pressure is the difference between the measured water potential and the extrapolated osmotic potential. (b) The pressure-volume curve showing the relationships between and during dehydration. The reciprocal of water potential is plotted against (1 — RWC).

Beyond the turgor-loss point (incipient plasmolysis), the relationship between (1 — RWC) and 1/^ (or 1/^n) is linear. The extrapolation of this linear relationship toward the y-axis intercept gives osmotic potential (the dashed line) of the tissue when the tissue is still turgid. The difference between the measured water potential and the extrapolated osmotic potential is turgor pressure (inset).

Fig. 2.2. Cellular water relations. (a) Hofler diagram showing the components of cell water potential. Intercellular or external water (RWC > 1.0) in many plant tissues is held at near-zero water potential and, during the initial dehydration, cell water potential and turgor pressure (^p) do not change significantly (the horizontal dashed line). Maximum osmotic potential is found at the point of full turgor (RWC = 1.0), where ^p = As the plant tissue loses water, turgor pressure decreases, and at the turgor-loss point (RWC = ~0.8), ^ = (the vertical dashed line). At RWC < ~0.8, the relationship between RWC and follows a rectangular hyperbola (RWC = a + b/^n). Osmotic potential at RWC = ~0.8-1.0 is extrapolated from the rectangular hyperbola relationship. Turgor pressure is the difference between the measured water potential and the extrapolated osmotic potential. (b) The pressure-volume curve showing the relationships between and during dehydration. The reciprocal of water potential is plotted against (1 — RWC).

Beyond the turgor-loss point (incipient plasmolysis), the relationship between (1 — RWC) and 1/^ (or 1/^n) is linear. The extrapolation of this linear relationship toward the y-axis intercept gives osmotic potential (the dashed line) of the tissue when the tissue is still turgid. The difference between the measured water potential and the extrapolated osmotic potential is turgor pressure (inset).

signs). During dehydration, the PV curve of a plant tissue initially displays a concave region, beyond which the curve is linear (Fig. 2.2b). The loss of turgor is marked by the point at which the relation of to (1 - RWC) deviates away from linearity. Turgor pressure is calculated as the difference between the extrapolated linear portion of the PV curve and the water potential actually measured, and is often plotted as a function of RWC. The relationship between turgor pressure and RWC can be described sufficiently by a quadratic or cubic function.

Certain plant tissues might develop negative turgor pressure before the cells collapse and can become zero under severe water stress. When negative develops, the PV curve would fall below the extrapolated linear portion of the graph (Fig. 2.3a). If the cells are sufficiently strong, do not collapse and the plasma membrane remains firmly attached to the cell wall, the formation of an intracellular gas bubble will increasingly become possible (cavitation). The development of negative turgor pressure and intra-cellular cavitation appear to play some roles in desiccation tolerance of certain cells. A good example of a cell surviving large negative turgor pressure and cavitation is the ascospore of Sordaria (Milburn, 1970). The volume of Sordaria ascospores changes very little, and the protoplast remains in contact with the spore wall at all times. Under water stress (by air-drying or in osmotic solution), these cells might generate negative as much as -4 MPa. Beyond this negative turgor pressure, a small bubble appears inside the protoplasm suddenly, which increases slowly in size and approaches the walls quite closely without losing its spherical appearance. Honegger (1995) and Scheidegger et al. (1995) also showed that ascomycetous lichen myco-bionts form large intracellular gas bubbles when desiccated. More recently, the PV analysis by Beckett (1997) suggested the existence of negative turgor pressure in vegetative cells of several desiccation-tolerant (poikilohydric) plants (e.g. Dumortiera hirsuta and Myrothamnus flabellifolia). PV curves of most plants do not show any indi cation of negative turgor pressure. It is conceivable that the development of negative turgor pressure may reduce mechanical damage on cellular structures by preventing collapse of the cells.

2.4.1.2. Change of osmotic potential during desiccation

When cell turgor pressure falls to zero during desiccation, the water potential of the cell is equal to the osmotic potential (see Equation (6)). As desiccation continues, osmotic potential and cell water potential are equal and inversely proportional to the volume of osmotically active water. The relationship between RWC and the reciprocal of osmotic potential is a straight line. The osmotic potential at full turgor is calculated from the extrapolation of the linear portion of the PV curve to the RWC at full turgor (i.e. RWC = 1.0 in Fig. 2.2b). In the Hofler diagram, the relationship between osmotic potential and RWC is represented by a rectangular hyperbolic function to the data points corresponding to the linear part of the PV curve (dashed part of the in Fig. 2.2a).

2.4.1.3. The volume of water in symplast, apoplast and intercellular spaces

In hydrated plant tissues, water may exist in the symplast, in the apoplast (i.e. the porous spaces in the cell wall) and in the intercellular spaces (large voids) as discussed before. Intercellular water, also called 'external' cell water by some workers, may account for up to 35% of total water in certain plant tissues, such as lichens, liverworts, mosses and fern fronds (Beckett, 1997; Proctor, 1999) and developing embryos of higher plants (W.Q. Sun, unpublished data). During desiccation, water potential and turgor potential do not fall with initial water loss at RWC >1.0 (Fig. 2.2a and b inset). The volume of water that is lost before turgor pressure starts to fall is assumed to be intercellular water. The volume of symplastic water represents the amount of osmotically active water in the tissue, and is obtained by subtracting the apoplastic water volume from the water

Hofler Diagram

Fig. 2.3. (a) The pressure-volume curves of plant tissues that develop negative turgor pressure (curve 1) and intracellular cavitation (curve 2) during desiccation. The inset in (a) shows the change of cell turgor pressure (^p) during the early stage of drying. When intracellular cavitation occurs, the suddenly changes to zero (curve 2, inset), and ^ is equal to (curve 2). If intracellular cavitation does not occur, the cell wall will collapse or deform when the develops beyond the threshold to which the cell wall can resist (i.e. (1 — RWC) > 0.15). The collapse or deformation of the cell wall will lead to a gradual increase in ^ (curve 1) and (curve 1, inset). (b) The semi-logarithmic plot between RWC and tissue water potential. The high RWC break point corresponds to the turgor-loss point, whereas the low RWC break point corresponds to the volume of apoplastic water. Drawn with data from Quercus rubra seeds (Sun, 1999).

Fig. 2.3. (a) The pressure-volume curves of plant tissues that develop negative turgor pressure (curve 1) and intracellular cavitation (curve 2) during desiccation. The inset in (a) shows the change of cell turgor pressure (^p) during the early stage of drying. When intracellular cavitation occurs, the suddenly changes to zero (curve 2, inset), and ^ is equal to (curve 2). If intracellular cavitation does not occur, the cell wall will collapse or deform when the develops beyond the threshold to which the cell wall can resist (i.e. (1 — RWC) > 0.15). The collapse or deformation of the cell wall will lead to a gradual increase in ^ (curve 1) and (curve 1, inset). (b) The semi-logarithmic plot between RWC and tissue water potential. The high RWC break point corresponds to the turgor-loss point, whereas the low RWC break point corresponds to the volume of apoplastic water. Drawn with data from Quercus rubra seeds (Sun, 1999).

content at full turgor. Symplastic water generally declines over a range of water potential from about —0.5 to —10 MPa, in line with that of osmotic potential.

Apoplastic or osmotically inactive water is present in very small pores and strong water-binding sites of biological surfaces in plant tissues. This fraction of water is held by matric and molecular forces, and lost only when plant tissues are desiccated to a water potential less than -15 MPa (Meidner and Sheriff, 1976). The loss of apoplastic water in some species extends to approximately -800 MPa. The amount of such matrix-bound water in plant tissues can be as high as 0.1-0.2 RWC or up to 0.25-0.35 g g-1 dw. This fraction of water does not generally act as a solvent in cells, and is not readily freezable. From the Hofler diagram, the apoplastic volume is estimated from the fitted hyperbolic function. From the PV curve, the volume of apoplastic water is commonly estimated by extrapolation of the linear relationship between RWC and the reciprocal of osmotic potential to the (1 - RWC) axis after the loss of turgor pressure. However, the simple extrapolation from the PV curve is not a reliable method of estimating the apoplastic volume, and in some cases gives negative values (Proctor et al., 1998).

The apoplastic volume of water should be derived with data from the isothermal sorption study at low water potentials (water activity), rather than the extrapolation method, because the linear relationship between RWC and the reciprocal of osmotic potential does not hold for the apoplastic volume of water (which is osmotically inactive). Compared to the removal of osmotically active (symplastic) water, the measured osmotic potential (including the term of matric potential) declines much more rapidly when the apoplastic water is removed. Therefore, the volume of apoplastic water is marked by the point at low water content at which the relationship of 1/" to (1 - RWC) again deviates away from linearity (Fig. 2.3b). The volume of apoplastic water roughly corresponds to the primary hydration in tissues (including both strong and weak water-binding sites).

One can expect that plant tissues would respond differently to the loss of external, symplastic and apoplastic water. The loss of symplastic water can cause osmotic perturbation of physiological and biochemical processes, whereas the loss of apoplastic water may disrupt the structure and func tion of cellular membrane and molecular assemblies. So far, workers have paid little attention to the location of water in plant tissues. The difference in the relative volume of external, symplastic, and apoplastic water should be taken into account in the comparative studies on mechanisms of desiccation tolerance among cells, tissues or plants. A similar analysis of water relations was found to be very useful in developing a mechanistic understanding of the role of dehydration in freezing tolerance in earthworms (Holmstrup and Zachariassen, 1996).

2.4.1.4. Volumetric elasticity of the cell wall

The cell wall may undergo elastic expansion or contraction. Elastic (mechanical) properties of cell walls play an important role in cell water relations. For example, the negative turgor pressure that can develop in a cell largely depends on the mechanical properties of the cell wall. The elasticity of the cell wall is represented by the volumetric elasticity module e, where € depends on both " (turgor pressure) and V (cell volume) and is defined as:

where AV is volume change caused by a given pressure change A"p. Equation (11) indicates that a high value of e implies a rigid cell wall, whereas a low value implies a more elastic cell wall. The e value can be calculated from the relationship between "p and RWC (Steudle et al., 1977; Stadelmann, 1984). The change of e as a function of RWC is given by the first derivative of the quadratic or cubic function of turgor pressure on RWC. The value of the "p/RWC derivative curve at RWC = 1.0 is usually taken as the bulk modulus of elasticity and used for purposes of comparison.

A pressure probe technique can be used directly to determine the turgor pressure and the e for individual plant cells. This technique is useful for continuous measurement of cell turgor pressure, cell wall elasticity and hydraulic conductivity of the cell membrane in single cells (Husken et al., 1978). The intracellular hydrostatic pressure is transmitted to the pressure transducer via an oil-filled microcapillary introduced into the cell, which transforms into a proportional voltage. This technique permits volume changes and turgor pressure changes to be determined with an accuracy of 10"5-10-6 pl and 3-5 X 10"3 MPa, respectively.

At present, very little information is available on cell wall properties of desiccation-tolerant plant tissues. Proctor (1999) found that two highly desiccation-tolerant liverworts have low values of bulk elastic modulus. He thought that extensible cell walls might be a part of structural adaptation to rapid changes of cell volume in their intermittently desiccated habitats. Ultrastructural studies on dry mesophyll cells of desiccation-tolerant Selaginella lepidophylla by Thomson and Platt (1997) showed highly folded cell walls and continuous apposition of plasmalemma to the walls. Vicre et al. (1999) studied the cell wall architecture of leaf tissues of Craterostigma wilmsii (a resurrection plant), and also observed extensive folding of the cell wall during desiccation. The folding of the cell wall allows the plasma membrane to remain firmly attached to the wall as the cell loses water. Biochemical modifications of the cell wall were observed during desiccation and rehydra-tion, leading to the change in its tensile strength that may prevent the total collapse of the walls in the dry tissue and avoid rapid expansion upon rehydration. The change in cell wall elasticity during desiccation can be determined easily by taking the first derivative of the function of turgor pressure on RWC.

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  • judy wakefield
    How to interpret a hofler diagram?
    9 years ago
  • silke
    What is pressurevolume curves in plants?
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  • Tom
    How to plot hofler diagram?
    9 years ago
  • elizabeth
    How to construct a hofler diagram?
    8 years ago
  • danyl
    What does hofler diagram represent?
    4 years ago
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    What are the main point in hofler diagrame?
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