Y ye ee eoeo yje214

in which ym is now a function of e. Equation 21.4 gives us a relation between the water potential and the relative water content of the cell. ym(e) represents the relation between the water content and the matric potential. Growth can be expected to cause some departure from the expression used in deriving Equation 21.4, but to the extent that the assumptions are valid, Equation 21.4 gives a unique relation between the total water potential and the relative water content of the leaf.

In practice, it is easier to make the measurements needed to test Equation 21.4 on tissue rather than on single cells. Therefore, Gardner and Ehlig (1965) used tissue [leaves of cotton (Gossypium hirsutum L.), bell pepper (Capsicum frutescens L.), sunflower (Helianthus annuus L.), and birdsfoot trefoil (Lotus corniculatus L.)]. The plants were grown in a greenhouse. To obtain different values of water potential, they withheld water from the plants until their leaves wilted to the desired extent. Water potential and osmotic potential were determined with thermocouple psychrometers. The relative water content was determined by using the method of Barrs and Weatherley (1962).

Figure 21.3 shows the relation between the relative water content and osmotic potential for the four plant species, as determined by Gardner and Ehlig (1965). The data are plotted on a logarithmic scale and the straight line has a slope of 45 degrees, as would be predicted if the solute content were to remain constant and the amount of bound water were negligible.

If a plant is growing in a saline soil solution, then over a period of time, the solute content of the cells tends to adjust accordingly. The rate of adjustment varies from species to species. Figure 21.4 shows the relation between the total water potential and the osmotic potential for bell pepper on both saline and nonsaline substrates (Ehlig et al., 1968).

If we neglect the matric and gravitational potentials, we can use Equation 21.1 to obtain the turgor potential by subtracting the osmotic potential from the total water potential. All three potentials are plotted as a

FIG. 21.3 Leaf relative water content as a function of the average osmotic potential in the plant leaf. The straight lines represent the relation expected if the solutes behave ideally and there is no bound water. (From Gardner, W.R., and Ehlig, C.F., Physical aspects of the internal water relations of plant leaves. Plant Physiology 40; 705-710, ©1965, American Society of Plant Physiologists. Reprinted by permission of the American Society of Plant Biologists, Rockville, Maryland.)

FIG. 21.3 Leaf relative water content as a function of the average osmotic potential in the plant leaf. The straight lines represent the relation expected if the solutes behave ideally and there is no bound water. (From Gardner, W.R., and Ehlig, C.F., Physical aspects of the internal water relations of plant leaves. Plant Physiology 40; 705-710, ©1965, American Society of Plant Physiologists. Reprinted by permission of the American Society of Plant Biologists, Rockville, Maryland.)

function of relative water content for nonsaline plants (Fig. 21.5). Of particular interest is the abrupt change in slope of the pressure potential (turgor potential) at a leaf relative water content of about 0.85. In pepper, for example, this corresponds to a total water potential of about -11 bars ( -1.1 MPa) and coincides with the appearance of marked symptoms of visible wilting. The change in slope corresponds to a change in the elastic modulus of the leaf tissue and explains the wilting symptoms. This also corresponds roughly with the point at which the stomata are almost completely closed (Baver et al., 1972, p. 398).

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