Incipient plasmolysis

Figure 2-12. Responses of an initially turgid cell (internal hydrostatic pressure P1 > 0) placed in pure water (n° = 0) to changes in the external osmotic pressure (IT0). As IT0 increases, P decreases. At the point of incipient plasmolysis (plasma membrane just beginning to pull away from the cell wall), P is reduced to zero, which is hence also called the turgor loss point. Constancy of cell volume and water equilibrium are assumed at each step as 11° is raised (actually, IT1 increases a few percent as IT° is raised from 0 to n°JasmoJysjs because the cell shrinks slightly accompanying the decrease of P to zero).

Equation 2.20 suggests that a relatively simple measurement (n°lasmolysis) is sufficient to estimate the osmotic pressure (lTp,asmo,ysis) occurring inside an individual plant cell at incipient plasmolysis. Under natural conditions, desiccation of plants can lead to cellular plasmolysis in leaves, roots, and stems,... 

Figure 2-13. Relative position of the plasma membrane for a plant cell undergoing plasmolysis (a) turgid cell with the plasma membrane pushing against the cell wall (b) cell just undergoing plasmolysis, i.e., at incipient plasmolysis and (c) cell with extensive plasmolysis, as the plasma membrane has pulled away from the cell wall over a large region.

Figure 2-14. A Hbfler diagram showing the relationship between the water potential (T1), the hydrostatic pressure (P ), and the osmotic pressure (IT) in a plant cell for various protoplast volumes. Assuming that solutes do not enter or leave the cell, the internal osmotic pressure increases as the protoplast volume decreases. For a group of cells, relative water content is often used in such diagrams instead of relative protoplast volume. The nearly 10% decrease in volume from full turgor to incipient plasmolysis (at T1 = -1.0 MPa) is characteristic of many plant cells. Note that T), and 4 are defined by Equation 2.13b.

Figure 2-15. Relation between the reciprocal of leaf water potential determined with a pressure chamber (Fig. 2-10) and the volume of xylem sap extruded as the air pressure in the chamber is progressively increased. The solid line indicates a typical range for data points for material initially at full turgor (TJ, = 0). The reciprocal of the internal osmotic pressure (1/n1) including the value at full turgor (1 /nj,), the internal hydrostatic pressure (P1), the point of incipient plasmolysis and turgor loss, and the volume of symplastic water (VSympiasm) can all be determined from such a P-V curve.

C. What is the water flux density at the point of incipient plasmolysis ... 

In Chapter 2 we used classical thermodynamics to derive the condition for incipient plasmolysis (n°lasmolysis = rTplasmolysis, Eq. 2.20 also see Fig. 2-13), which occurs when the hydrostatic pressure inside a plant cell P1 just becomes zero. The derivation assumed equilibrium of water (i.e., equal water potentials) across a membrane impermeable to solutes. However, the assumptions of water equilibrium and solute impermeability are often not valid. We can remedy this situation using an approach based on irreversible thermodynamics. 

Measurements of incipient plasmolysis can be made for zero volume flux density (Jv = 0) and for a simple external solution (x° = 0) at atmospheric pressure (P° = 0). In this case, Equation 3.41 is the appropriate expression from irreversible thermodynamics, instead of the less realistic condition of water equilibrium that we used previously. For this stationary state condition, the following expression describes incipient plasmolysis (P1 = 0) when the solutes can cross the cell membrane ... 

Because a° depends on the external solutes present, Equation 3.42 (a corrected version of Eq. 2.20) indicates that the external osmotic pressure n° at incipient plasmolysis can vary with the particular solute in the solution surrounding the plant cells. Suppose that solute i cannot penetrate the membrane, so o/ equals 1, a situation often true for sucrose. Suppose that... 

Figure 3-22. Diagrams of sections through a cell showing a cell wall (shaded region) and a plasma membrane (line) for various external osmotic pressures (a) point of incipient plasmolysis in the presence of a nonpenetrating solute (for clarity of showing the location of the plasma membrane, a slight amount of plasmolysis is indicated), (b) point of incipient plasmolysis with a penetrating solute, (c) extensive plasmolysis, and (d) cell under turgor. Consider Equation 3.41 with x1 equal to 0.

On the other hand, if n° were 2A for a nonpenetrating external solute, extensive plasmolysis of the cell would occur, as is illustrated in Figure 3-22c. Because o-°n° is 2A in this case, Equation 3.41 (o-°n° = a-1 1 — P when x1 = 0) indicates that t/n1 must also be 2A, so essentially half of the internal water has left the cell. Finally, if the reflection coefficient were 0.5 and the external osmotic pressure were A, o-°n° would be 1 A, and we would not be at the point of incipient plasmolysis. In fact, the cell would be under turgor with an internal hydrostatic pressure equal to lA (Fig. 3-22d), at least until the concentration of the penetrating solute begins to build up inside. We must take into account the reflection coefficients of both external and internal solutes to describe conditions at the point of incipient plasmolysis and, by extension, to predict the direction and the magnitude of volume fluxes across membranes. 

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