Kristin Maggard, Jennifer Martin, and Jessica Read

CU Boulder, Fall 2004

We tested the effects of light intensity on the rate of photosynthesis in apple leaves. Our observations from a previous lab indicated that there was a change that occurred when the CO2 gas chamber was completely covered in darkness compared to when it was exposed to direct light. Our interest in these results led us formulate other possible experiments we could perform with photosynthesis. We believed that if the rate of photosynthesis is affected by the amount of light available, then the rate of photosynthesis (tested indirectly by a CO2 gas sensor) would be directly proportional to the amount of light introduced to the plant.

To test our hypothesis we used apple leaves (4.78g) that we placed in a CO2 gas chamber. We then placed a CO2 gas sensor (which was attached to the computer) inside the gas chamber. The computer then ran our data in a program that would interpret the CO2 gas level, both in a table and on a graph. We used the same light source for each trial, and we used a container of water to stabilize the temperature.

To test the light intensity, four identically sized glass panes were used. The first pane was completely clear. We used that pane as our control group, because 100% of the light was introduced to the plant. The other panes of glass were covered in increasing proportional percentages by two different colored coats of paint. The 67% and 33% covered panes were painted in strips by taping off the parts of the glass that were needed for light transmittance. The 0% light pane was completely covered in paint. We ran each trial for 10 minutes, totaling 40 minutes. Since it takes light energy and CO2 to induce photosynthesis, we predicted that the more light there was, the more CO2 would be absorbed, producing a negative slope.

Our results indicated that the amount of light is, in fact, proportional to the light introduced to the plant (100% light = -50.08 ppm/min, 67% light = -28.48 ppm/min, 33% light = -17.11 ppm/min, and 0% light = 61.75 ppm/min; "r_ value" mean was .9025 or 90.25%; "sig. F" mean 4E-34 : P< 0.05)

Our results are consistent with our predictions based on our derived hypothesis. The variables are, in fact, significant and the data fit the best-fit line generated by the regression analysis. Our experiment was both valid and reliable.

The first potential problem that we came across in this analysis was that there were some small holes in the paint that let a small amount of light through the panes. A second potential problem was that the overhead light could have affected our results.

One proposed remedy would be to make an opaque box that is large enough to house the water, the chamber and the light panes. This would eliminate the problem of any outside light penetration. In addition, it would also be beneficial to go over each pane of glass with a marker to make sure that no light would infiltrate.

Unfortunately, we were unable to find any similar experiments completed on the CABLE web site. The closest experiment that was done (#12.1, Clover and Curil. 2000. Effects of low carbon dioxide on rates of photosynthesis in C3 plants. CU Boulder) showed that "the rates of photosynthesis slowed down at lower levels of CO2".

In a revised version of this experiment, we would use leaves or needles from various areas to test if there is a difference in light intensity and CO2 depending on the climate.