Playing around with the Floating Disk Assay—Light Response Curves

Over the years I’ve made the claim that the floating leaf disk assay is quite possibly the best way for students to explore how the process of photosynthesis. The method is inexpensive, accurate, reliably replicable and most importantly accessible for all levels of students from 5th grade to university. However, I’ve got to say that even I was surprised at some data I collected, yesterday. Recently, while working on new AP Biology Labs, I revisited the original (and still the best) paper that first discussed this technique. (or at least the earliest I can find.)

Wickliff, J. L., and R. M. Chasson. 1964. Measurement of photosynthesis in plant tissues using bicarbonate solutions. BioScience 14, no. 3: 32–33.

In this article I saw this graph of a photosynthesis light response curve that got me to thinking:

Last year, the UKanTeach program where I teach acquired a couple of PAR (photosynthetically active radiation) meters to measure photon flux. PAR meters are typically on the expensive side but this model from Apogee runs about $300. I hadn’t taken time to try them out and decided that now was the time.

Yesterday, I went out the north side of Haworth Hall and picked an ivy (Hedera helix) leaf that was growing in deep shade under a shrub.

English Ivy leaf, shade adapted

I picked a shade adapted leaf figuring that a leaf adapted to shade would likely reach photosaturation earlier than a sun adapted leaf. I wasn’t sure whether or not my light source was bright enough to induce photosaturation.

My light source is a clamp shop light with an 8 inch reflector and an 100 watt equivalent compact fluorescent bulb. Actually I found that if I put my meter within a couple of inches of the bulb I can get a flux reading equivalent to a summer’s day. I was sure my light was bright enough for the leaf I had picked.

I modified the technique that I presented here by placing the infiltrated disks in shallow petri dishes instead of plastic cups. I also modified the data collection procedure. Instead of counting disks floating at the end of each minute, I actually attempted to time each disk–a bit of a challenge that I wasn’t quite up to the first time. I should have used a video camera or at least used a computer timer program capable of timing 10 or more “laps” or intervals.

Modified technique

It is real easy to record the first movements of the disks with this technique.

In low light conditions, I started by carefully cutting about 80 disks from one leaf. I then infiltrated ten disks at a time with a dilute bicarbonate solution with a vacuum created with a 10 ml syringe. I placed the 10 sunken disks in separate petri dishes with a total of 30 mls of bicarbonate solution. The dishes with the disks were then placed under a box lid to exclude any light. I then tested 6 of the sets of 10 disks under different light intensities. The data from the highest light intensity are not included because I neglected to use a water heat sink filter to keep the infiltration solution temperature constant. The higher temperatures on this replication affected the outcome. It was only when the light was very close to the petri dish that this was a problem but I need to account for this next time.

Here’s the results:

Note that I’ve plotted plus or minus two estimated Standard Errors for each mean. I was impressed. This is a classic response curve and the parameters of this curve are consistent with data reported in the literature for shade grown English Ivy. I’m more convinced than ever that the floating leaf disk assay is a very valuable tool for a biology teaching laboratory. With this technique students can start their exploration of photosynthesis but the same technique is powerful enough to explore more sophisticated concepts.

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