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

Gravitropism in Dandelion Scapes

Introduction: 
     In this lab we will study the gravitropic response of the hollow flowering stalk (scape) of dandelion (Taraxacum officinale).   The dandelion provides an excellent model system for studying gravitropism (Clifford and Oxlade, 1991). 

Plant Material:
    Cut the dandelion stalks off at the base with a knife or razor blade.  The scapes should be maintained in an upright position in water until used.  Attempt to collect scapes while the flower is in bud because the gravitropic sensitivity declines once the inflorescence opens.  Upon fruiting - formation of the head or "clock" - the scapes become strongly gravitropic.  Decapitate the inflorescence.

Study Apparatus:
    Insert a few centimeters of the scape into a tube filled with water.  Mount the tube on a horizontal surface with tape or place vertically in a test tube rack.  Protractors will be available to measure the angles of curvature.  Microscopes and slides will also be provided.

Procedure:
    Working with a partner, design an experiment to answer a question about gravitropism using the dandelion scapes. Some possible questions are provided below.  When designing your experiment, consider:  what is your hypothesis? what is your biological rationale for this hypothesis?  what data do you need to collect to test this hypothesis?  How many samples will you need?  what type of tables and/or graphs will you use to summarize your data?  How will you analyze your data?  Will your data be appropriate to support or refute your hypothesis?

Questions:  Consider the following...

1. How rapidly does the scape bend?  What is the time course of bending (plot angle of curvature vs. time)?  Is there a lag period in the bending response?  If so, how long?
2. What is the effect of temperature (or light) on the bending response?
3. What is the presentation for the response (i.e., how long do the scapes need to be gravistimulated to initiate the bending response (hint:  try 1 - 20 minutes).
4. Are statocytes visible in the scape?
5. What is the position of the amyloplasts (=statoliths) in a vertically-oriented scape?  How does this change if the scape is placed on its side?
6. How quickly do the amyloplasts sediment?  How can you measure the rate of sedimentation?
7. What is the effect of applying auxin paste to the scape?  Or calcium? or EDTA?
8. Where does the bending responses take place?  Near the inflorescence? base?
9. Why does the gravitropic sensitivity change over time?

References:

• Barclay, GF & PE Clifford (1991) Using light microscopy to study geotropism.  American Biology Teacher 53: 285 - 293.
• Clifford PE & EL Oxlade (1991) Using dandelion flower stalks for gravitropic studies. American Biology Teacher 53: 290 - 293.
• EL Oxlade & PE Clifford (1981) Experiments in geotropism.  Journal of Biological Education 15:  137 - 142.

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