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.

Introduction
   Water potential (Ψ_w, 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 (Ψ_s ), is always negative in sign. In other words, solutes decrease the water potential. The contribution of pressure (Ψ_p) 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 (Ψ_m), 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:

Ψ_w = Ψ_s + Ψ_p + Ψ_m

In this lab we will use the Chardakov and Gravimetric techniques to determine the water potential (Ψ_w) of a potato tuber cells.  We will determine the solute potential (Ψ_s ) by the Freezing Point Depression Method.  Pressure in the cells can be arithmetically calculated once Ψ_s and Ψ_w 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.

Pre-Lab:  

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:

• Chardakov Technique for Measuring Water Potential
• Gravimetric Technique for Measuring Water Potential
• Freezing Point Depression Technique to Measuring Solute Potential

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):

• Chardakov Tables 1 & 2.  Calculate the water potential of the potato based on these data.
• Gravimetric Table 2.  Plot % change in weight vs. [sucrose]; include best fit line with regression analysis data.  Calculate the water potential of the potato based on these data.  In your abstract discuss how the water potential values obtained from the Chardakov and Gravimetric techniques compare and which is likely most accurate.
• Freezing point Tables 1 & 2.  Plot temperature vs. time for both water and sap on the same graph.

3. In your abstract be sure to include:  the pressure (Ψ_p) in the potato cells and discussion whether the data you collected supported your hypotheses.

References:

• Barcelo, AR, AA Calderon and R Munoz (1994) Measuring water conductivity coefficients in plant tissues. Journal of Biological Education 28: 83 – 85.
• Bland, W. and C. B. Tanner (1985) Measurement of the water potential of stored potato tubers. Plant Physiology 79: 891-895.
• Boyer, JS (1969) Measurement of the water status of plants. ARPP 20:351-364.
• Koning, R (1999)  Web Site: Osmosis Lab.
• Kramer, P (1983) Water Relations of Plants. Academic Press, NY.
• Meidner, H (1984) Class Experiments in Plant Physiology. George Albert Unwin, Boston.
• Reiss, Carol (1994) Experiments in Plant Physiology. Prentice-Hall, Englewood Cliffs, NJ.
• Ross, C (1974) Plant Physiology Lab Manual. Wadsworth.

Lab Documents: