Limitations Of The Cohesion Theory

Even though most plant physiologists feel that the cohesion theory is probably the correct explanation for the rise of water in plants, the theory has

FIG. 19.2 The cohesion theory of the ascent of sap summarized. (From Salisbury, F.B., and Ross, C.W., Plant Physiology, 2nd ed., p. 58, ©1978. Wadsworth Publishing Company, Inc: Belmont, California. Reprinted with permission of Brooks/Cole, a division of Thomson Learning: www.thomsonrights.com. Fax 800 730-2215.)

FIG. 19.2 The cohesion theory of the ascent of sap summarized. (From Salisbury, F.B., and Ross, C.W., Plant Physiology, 2nd ed., p. 58, ©1978. Wadsworth Publishing Company, Inc: Belmont, California. Reprinted with permission of Brooks/Cole, a division of Thomson Learning: www.thomsonrights.com. Fax 800 730-2215.)

limitations. The main difficulty is that it postulates a system of potentially great instability and vulnerability, although it is clear that the water-conducting system in plants must be both stable and invulnerable. Objections to the theory include three major points (Kramer, 1983, p. 283; Salisbury and Ross, 1978, p. 58-60):

1. The tensile strength of water is inadequate under the great tensions necessary to pull water to the top of plants, especially tall plants.

2. There is insufficient evidence for the existence of continuous water columns (that is, water columns under tension are not stable and they cavitate, or form cavities, hollows, or bubbles).

3. It seems impossible to have tensive channels in the presence of free air bubbles, which can occur when trees in cold climates freeze and then thaw.

Let us consider each point. First, is the tensile strength of water adequate to pull water to the top of plants? Tensile strength is defined as the "resistance to lengthwise stress, measured by the greatest load in weight per unit area pulling in the direction of length that a given substance can bear without tearing apart" (Webster's New World Dictionary of the American Language, 1959).

Nobel (1970, pp. 35-36, 40; 1974, pp. 46-47, 52-53) calculates the tensile strength of water. Let us do the calculations that Nobel does. We must consider the structure of ice (Nobel, 1974, p. 46). Ice is a coordinated crystalline structure in which essentially all the water molecules are joined by hydrogen bonds (see Fig. 3.2). When heat is added so that the ice melts, some of these intermolecular hydrogen bonds are broken. The heat of fusion of ice at 0°C is 80 cal/gm or 1.44 kcal/mole. (Remember: 18 gm/mole for water; 80 cal/gm x 18 gm/mole = 1,400 cal/mole = 1.44 kcal/mole.) The total rupture of the intermolecular hydrogen bonds involving each of its hydrogens would require 9.6 kcal/mole of water. The 9.6 kcal/mole is a given value. Nobel (1974, p. 46) gives references for the value. He cites work by Eisenberg and Kauzmann (1969; e.g., see p. 145, 269) and Pauling (1964; e.g., see p. 456) for references on the hydrogen bond energy. Nobel (1974, p. 46) points out that the actual magnitude of the hydrogen bond energy assigned to ice depends somewhat on the particular operational definition used in the measurement of the various bonding energies. Therefore, the quoted values vary somewhat.

The heat of fusion thus indicates that (100) (1.44)/(9.6), or at most 15%, of the hydrogen bonds are broken when ice melts. Some energy is needed to overcome van der Waal's attractions, so that less than 15% of the hydrogen bonds are actually broken upon melting (Nobel, 1974, p. 46).

Conversely, over 85% of the hydrogen bonds remain intact for liquid water at 0°C. Because 1.00 cal is needed to heat 1 gram of water 1°C, (1.00 cal/gm°)(25°)(18 gm/mole)(0.001 kcal/cal), or 0.45 kcal/mole is required to heat water from 0 to 25°C. If all of this energy were used to break hydrogen bonds, over 80% of the bonds would still remain intact at 25°C. (Note: 0.45/9.6 = 0.047, which is less than 5%; so 85% - less than 5% = greater than 80%.) The extensive amount of intermolecular hydrogen bonds present in the liquid state contributes to the unique and biologically important properties of water, including its high tensile strength, which is of interest to us now.

If 80% of the hydrogen bonds are intact in water at 25°C (Nobel, 1974, p. 52), then the energy will be (0.80)(9.6) or 7.7 kcal/mole, which is (7.7 kcal/mole)/(18 gm/mole) or 0.43 kcal/gm of water. For a density of 1.00 gm/cm3, and replacing kcal by 4.184 x 1010 ergs, we calculate that the energy of the hydrogen bonds is 1.8 x 1010 ergs/cm3. (Remember: 1 joule = 107 ergs; 1 cal = 4.184 joule; therefore, 1 cal = 4.184 x 107 ergs; 1 kcal = 4.184 x 1010 ergs.) The tension that is applied to a water column acts against this attractive energy of the hydrogen bonds.

When the fracture is just about to occur at each hydrogen bond, the maximum possible tensile strength is developed. Thus, the maximum tensile strength would represent an input of 1.8 x 1010 ergs/cm3. Since an erg = dyne-cm and a bar = 106 dyne/cm2, the maximum tensile strength of water corresponds to 18,000 bars.

Nobel's (1974) theoretical considerations, therefore, show that the calculated value for the tensile strength of water is large (18,000 bars) and would permit rise of water in plants even under great tensions. Tensions in higher (more evolved) plants probably never exceed 100 atm. Lower plants such as fungi apparently can grow in soil with a tension (or absolute value of matric potential) of 14001 bars (Harris, 1981, p. 26). What values of the tension of water have been measured experimentally? Dixon and Joly (1895) estimated that water entrapped in glass tubes of small diameter could withstand tensions exceeding 200 atm without fracture. Ursprung (1929) calculated that tensions on the order of 300 atm were reached in annulus cells of discharging fern sporangia. Briggs (1950) employed a centrifugal method to obtain values of about 220 atm for the tensile strength of water. (See Fig. 19.3 for a method of measuring the cohesive properties of water using a bent centrifuge tube.)

In contrast to the foregoing rather large values, a number of other investigators have demonstrated that water may have a relatively low tensile strength. Loomis et al. (1960) suggested that Ursprung's values of

FIG. 19.3 Method of measuring the cohesive properties of water using a centrifuged Z-tube. Small arrows indicate direction of centrifugal force and principle of balancing due to the Z-tube. These tubes are centrifuged causing tension on the water at the center of the tube. The tension present when the water column breaks can be calculated. (From Salisbury, F.B., and Ross, C.W., Plant Physiology, 2nd ed., p. 59, ©1978. Wadsworth Publishing Company, Inc: Belmont, California. Reprinted with permission of Brooks/Cole, a division of Thomson Learning: www.thomsonrights.com. Fax 800 730-2215.)

FIG. 19.3 Method of measuring the cohesive properties of water using a centrifuged Z-tube. Small arrows indicate direction of centrifugal force and principle of balancing due to the Z-tube. These tubes are centrifuged causing tension on the water at the center of the tube. The tension present when the water column breaks can be calculated. (From Salisbury, F.B., and Ross, C.W., Plant Physiology, 2nd ed., p. 59, ©1978. Wadsworth Publishing Company, Inc: Belmont, California. Reprinted with permission of Brooks/Cole, a division of Thomson Learning: www.thomsonrights.com. Fax 800 730-2215.)

the tensile strength of water were open to question because of a confusion of adsorption forces with cohesion. Scholander et al. (1955), through centrifugation in glass tubes, observed tensive values from 10 to 20 atm without producing cavitation of water. When the experiments were repeated using plant material, they observed much lower values (1-3 atm). Also, they were unable to fit hydrostatic pressures in transpiring grape vines into a pattern that followed the cohesion theory. Measured pressure did not indicate cohesion tension at any time, and hydrostatic pressures in transpiring tall vines were higher at the top rather than lower, as they should have been if the transpiring stream were under tension. Measurements taken on Douglas fir trees, however, did follow the pattern that one would expect if water were rising in the plants according to the cohesion theory (Scholander et al., 1965) (Fig. 19.4). That is, the hydrostatic pressure at the top of the trees was more negative than at the bottom of the trees. Greenidge (1957) discusses different techniques used to measure the tensile strength of water that yield values ranging from 0.05 to 10 atm.

It appears that, experimentally, water can withstand negative pressures (tensions) only up to about 300 bars without breaking (Nobel, 1974, p. 52). The observed tensile strength depends on the wall material, the diameter of the xylem vessel, and any solutes present in the water. Local

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Responses

  • james
    What are the weakness of tension theory?
    2 years ago
  • Nibs
    What are the limitations of cohension tension theory?
    3 days ago

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