The Physiology of Flowering Plants - KHAM PHA MOI

WATER POTENTIALS OF PLANT CELLS AND TISSUES 65Fig: 3:2 Generalized quantitativerelationship between overall cell C,C p , C p and cell volume. Note thatthe x-axis scale runs from amaximum value for the volume onthe left, at full turgor, to a minimumvalue at the point of limitingplasmolysis/ wilting. The extent towhich the cell volume variesbetween these points dependsgreatly on the extensibility of thecell wall; hence no numerical valuesare given, but if the volume at fullturgidity is taken as 100%, theminimum values quoted fordifferent cells vary from about 95%to 70%. The dashed lines indicatewhat happens if the cell wall caves inunder tension after limiting wilting:the volume decreases still further,C p becomes negative (top sectionof graph), and the cell C falls belowC p (bottom section of graph). If thewall does not cave in, but the cellwrinkles, C continues to equal C pwith further water loss.the incubation period; the tissue C equals that of the environment inwhich it neither gains nor loses water.Another method for measuring tissue water potential is thethermocouple psychrometer. The tissue is allowed to equilibratewith the atmosphere in a small chamber which houses a thermocouplejunction and which is incubated at constant temperature. TheC of the chamber atmosphere will become equal to that of the tissue.