Spring.wmf (18300 bytes) Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Measuring the Water Status of Potato Tubers

Objectives Upon completion of this laboratory you should be able to:

  1. determine the water potential of a plant tissue by the Chardokov and gravimetric techniques and understand the underlying theory.
  2. determine the osmotic potential of a plant extract by the freezing point depression method and understand the underlying theory.
  3. describe techniques for measuring the pressure potential of a plant tissue.
  4. describe techniques for measuring plant water status such as the pressure bomb, vapor pressure osmometer, and psychrometer.

   Water potential (Ψw, psi), which is a measure of the energy state of water is affected by dissolved solutes, pressure and matrix particles. The contribution to water potential by dissolved solutes, termed osmotic potential (Ψs ), is always negative in sign. In other words, solutes decrease the water potential. The contribution of pressure (Ψp) may be positive, negative or zero, but is generally positive since most plant cells are turgid (turgor pressure). The contribution due to the binding of water to colloidal particles (matric) and surfaces, termed matric potential (Ψm), also lowers the water potential. Although it is often small enough to be ignored, matrix potential is important when considering soil water relations. Thus, the water potential of a plant system can be arithmetically represented by the equation:

Ψw = Ψs + Ψp + Ψm

In this lab we will use the Chardakov and Gravimetric techniques to determine the water potential (Ψw) of a potato tuber cells.  We will determine the solute potential (Ψs ) by the Freezing Point Depression Method.  Pressure in the cells can be arithmetically calculated once Ψs and Ψw are known.  If time permits, we will also measure the water conductivity of potato tubers, determine the Q10 for water transport into potatoes and prepare a Hofler diagram.


  1. Email to me before lab begins the answers to questions 1-6 in the Pre-Lab Assignment
  2. Print copies of this exercise and the following handouts and bring them all to lab:

Post-Lab Assignment

  1. Write an abstract of this experiment
  2. Append to your abstract the following (be sure to include appropriate captions for your tables and graphs):
  1. In your abstract be sure to include:  the pressure (Ψp) in the potato cells and discussion whether the data you collected supported your hypotheses.


Lab Documents:

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Last updated:  01/07/2009     � Copyright  by SG Saupe