Brian Ahlstrand, James Allen, and Kristina Fournier

CU Boulder, Fall 2004

Our group explored how different wavelengths of light affect photosynthesis. In a previous lab, we discovered that chlorophyll a was the main photosynthetic pigment in plant cells. Chlorophyll a was shown to absorb most wavelengths of light, except green, and was found to absorb blue wavelengths the most. These findings bring us to our hypothesis that juniper sprigs should use the most amounts of CO₂, thus producing the highest rate of photosynthesis, when exposed to blue light, the second most when exposed to red light, and lastly a limited amount when exposed to green light.

To test our hypothesis we started by assembling an airtight container with a carbon dioxide gas sensor attached. We then ran a control with no sprigs in the container to make sure the gas senor was running correctly. Next, we placed weighed amounts of freshly picked Juniper sprigs in the container. We exposed the Juniper sprigs to white, red, blue, green light, and no light each for four minutes and then calculated the CO₂ used in parts per million per minute per gram (ppm/min/g). We repeated the entire process one additional time with a different amount of Juniper sprigs. Because blue light yields the highest amount of energy, and is most readily absorbed by chlorophyll a, we expected to have it use the highest amount of carbon dioxide.

Our results showed that the highest mean rate of CO₂ consumption occurred when the sprigs were exposed to red light (-9.19 ppm/min/g). In a close second, blue light used —7.19 ppm/min/g and lastly when exposed to green light —2.63 ppm/min/g of CO₂ was used. We then ran three t-tests to find if there was any significance between the mean differences of white light verses the three colors. White and red had a p value of .26 thus showing no significant difference, but both white to blue (p = .045) and white to green (p= .011) showed a significant difference between their means. It should also be noted that when red and blue were tested a p value of .66 was calculated showing no significant difference.

Our results were inconsistent with our hypothesis. The main problem that we had in the experiment was a lack of time, and therefore a limited amount of trials. In addition, it would have been wise to run the trials for longer periods of time to obtain more accurate slopes of carbon dioxide consumption. However, other results on the CABLE website (Warsh et al. 2001, Cucciniello et al. 2003, and Braddock et al. 2001) showed similar results with red wavelengths producing the highest rates of photosynthesis.

We have rejected our hypothesis and now believe that both red and blue play almost an equal role in the rate of photosynthesis. Chlorophyll a absorbs large amounts of red and blue light and uses the energy from each to carry out the light dependent reactions in photosynthesis. It would be nice to repeat this experiment and run more trials for longer periods of time to test this new hypothesis.