Introduction
A convenient way to investigate the effect of light intensity on the rate of photosynthesis is to compare the rate of oxygen production by a plant at different levels of illumination. We will use leaves from several different species of plant for this investigation. One of the simplest ways of determining the rate of photosynthesis is with the floating disk assay. In this investigation, we will fill the air spaces in leaves with a solution of Sodium bicarbonate, a convenient source carbon for photosynthesis. Leaf disks, which normally float on water will sink when filled with a solution of
Sodium bicarbonate. As the plant tissue photosynthesizes, and oxygen is released into the interior of the leaf, the leaf will become more buoyant and will cause the disk to rise to the surface of the water. By timing how quickly the disks rise to the surface, we will be effectively measuring the rate of photosynthesis.
Question
Will light intensity affect the rate of photosynthesis of leaf tissue as measured by the leaf disk assay method?
Materials
Cup/beaker
Liquid soap
Hole punch
Sodium bicarbonate (NaHCO
3
)
1 mL pipette
Syringe
Forceps
Light source
Leaves (various species)
Microtiter plate
Timer
Methods
Set up
1.
Prepare a 100 mL of a 0.2% solution of bicarbonate solution [Remember: (grams of solute/volume of solution) X 100 = % solution].
2.
Add 1 drop of liquid soap to this solution.
3.
Fill 10 wells in a microtiter plate with the bicarbonate solution and set aside.
4.
Choose one type of leaf. Using the hole punch, make 10 uniform leaf disks.
Fill the air space in the leaf with sodium bicarbonate solution
5.
Remove the plunger on the syringe and place the leaf disks in the syringe barrel. Carefully replace the plunger making sure not to crush the leaf disks. Push on the plunger until there is room for only a small amount of air and the disks in the syringe barrel.
6.
Fill the syringe about half way with the bicarbonate solution. Tap the syringe to make sure the disks are suspended in the solution.
7.
Hold your thumb over the syringe opening and pull back the plunger creating a vacuum.
Hold this vacuum for about 20 seconds.
8.
If you have filled the air spaces in the leaves with bicarbonate solution, the leaves should sink. If they do not sink repeat step 6 until the leaves sink in the solution.
Performing the leaf disk assay
9.
Empty the contents of the syringe back into the cup with the bicarbonate solution.
10.
Using forceps, place the ten leaf disks into the ten wells on the microtiter plate with the bicarbonate solution.
11.
Record the length of time necessary for the disks to float to the surface of the bicarbonate

solution.
12.
Repeat this process using a new set of leaf disks and the light provided. You will need to suspend the light from one of the support rods at your work station.
Results (complete in your lab notebook)
1.
Record your results in an appropriate table in your lab notebook.
2.
Graph your results in your lab notebook. You should create a scatter graph for each of the two sets of data (with and without light).
3.
Add a trendline for each set of data.
4.
Make sure you have included a complete table and figure title and labeled all appropriate table columns and/or graph axis.
Conclusion (complete in your lab notebook)

Provide a valid conclusion, with justification, based on a correct interpretation of the results.
Relate back to the purpose of your investigation and answer the question.

What other variables, besides light intensity, could you test using this assay?