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The Floating Leaf Disk Assay for Investigating Photosynthesis

(A resource page)

Brad Williamson

Introduction:

Trying to find a good, quantitative procedure that students can use for exploring photosynthesis is a challenge. The standard procedures such as counting oxygen bubbles generated by an elodea stem tend to not be “student” proof or reliable. This is a particular problem if your laboratory instruction emphasizes student-generated questions. Over the years, I have found the floating leaf disk assay technique to be reliable and understandable to students. Once the students are familiar with the technique they can readily design experiments to answer their own questions about photosynthesis. 

The biology behind the prodedure:

Leaf disks float, normally.  When the air spaces are infiltrated with solution the overall density of the leaf disk increases and the disk sinks.  The infiltration solution includes a small amount of Sodium bicarbonate.  Bicarbonate ion serves as the carbon source for photosynthesis.  As photosynthesis proceeds oxygen is released into the interior of the leaf which changes the buoyancy--causing the disks to rise.  Since cellular respiration is taking place at the same time, consuming oxygen, the rate that the disks rise is an indirect measurement of the net rate of photosynthesis.

Basis for leaf disk

Materials:

Optional:

Procedure:

Bicarb photo
soap
Leaf Disks
Disks in syringe
Evacuating leaf disks



Sunk
Start
Rising

Data Collection and Analysis

These data are from an demonstration investigation using grape ivy leaf disks.  

Minutes Disks
1 0
2 0
3 0
4 0
5 0
6 0
7 1
8 1
9 1
10 1
11 4
12 7
13 8
14 10

The point at which 50% of the leaf disks are floating (the median) is the point of reference for this procedure.  By extrapolating from the graph, the 50% floating point is about 11.5 minutes.  Using the 50% point provides a greater degree of reliability and repeatability for this procedure.  As Steucek, et. al. (1985) described this term is referred to as the ET50.


Graph

The problem with ET50 is that it goes down as the rate of photosynthesis goes up--it is an inverse relationship and creates the following type of graph (data from Steucek, et al. 1985.):

Inverse

To correct for this representation of the data and present a graph that shows increasing rates of photosynthesis with a positive slope the
ET50 term can be modified by taking the inverse or 1/ET50.  This creates a graph like this(data from Steucek, et al. 1985.):

Direct

Extension:

In this graph, the light was turned off at 14 minutes and the cups with their floating disks (grape ivy) were placed in the dark.

Photosynthesis and Respiration


Every minute, I removed the dark cover and counted how many were still floating.  Then I stirred the disks.  Note that after a while the disks begin to sink.  Why?  Cellular respiration removes the oxygen from the cell spaces.  The rate that the disks sink is an indirect measure of the rate of cellular respiration.   Can you think of a way to how you might measure the gross rate of photosynthesis with this technique?


Print and Web Resources:


Wickliff, J.L. and Chasson, R.M. 1964. Measurement of photosynthesis in plant tissues using bicarbonate solutions. Bioscience, 14: 32-33.

Steucek, Guy L. Robert J. Hill and Class/Summer 1982. 1985. Photosynthesis I: An Assay Utilizing Leaf Disks. The American Biology Teacher, 47(2):96-99.

Tatina, Robert E. 1986. Improvements to the Steucek and Hill Assay of Photosynthesis. The American Biology Teacher, 48(6): 364-366.

Juliao, Fernando and Henry C. Butcher IV. 1989. Further Improvements to the Steucek and Hill Assay of Photosynthesis. The American Biology Teacher, 51(3): 174-176.

Armstrong, Joeseph E. 1995. Investigation of Photosynthesis using the Floating Leaf Disk Assy. http://www.bio.ilstu.edu/Armstrong/biolab/cellbio/psynex1.htm

Rukes, Kari L. and Timothy J.Mulkey. 1994. Measurement on the Effects of Light Quality and Other Factors on the Rate of Photosynthesis. Bioscene, 20(3): 7-11. http://www.acube.org/volume_20/v20-3p7-11.pdf

Greenler, John. 1990. Exploring Photosynthesis with Fast Plants. WisconsinFast Plant Notes, 4(1): 4-5. http://www.fastplants.org/pdf/activities/exploring_photosynthesis.pdf

BioPi listserv archives. http://listserv.ksu.edu/archives/biopi-l.html
Enter the "Leaf Disks" for a search to review a thread on the technique. Dan Mott attached a copy of his lab using this technique to one of his postings.

Richard, David S. Measure of Photosynthetic Rate In Spinach Leaf Disks  http://www.susqu.edu/FacStaff/r/richard/photosynthlab.html