Adam Redder, Adam Seligman, Robert Means Smith, Nate Reich

CU Boulder, Fall 2004

In our student project we (Adam, Adam, Robert and Nate) decided to test under which lights do plants undergo photosynthesis the best. We were shown from a previous class experiment that at different wavelengths of light plants should photosynthesize at different levels, therefore producing different rates of CO2. We were shown from this experiment that plants absorb the most light in blue wavelengths, medium amounts of light at red wavelengths and small amounts at green wavelengths.

From this we generated the hypothesis….If the results of the absorption spectrum of the 4 photosynthetic pigments are indicative to a universal absorption trend, at wavelengths 480 (blue) and 680 (red), then we can deduce that at these wavelengths the rate of CO2 consumption will be higher than at other wavelengths as a result of photosynthesis.

To test this hypothesis we collected freshly cut leaves form outside the lab building. Each weighed roughly .27 grams. We selected 3 colored filters, red, blue and green, as trial colors. We also ran a white light trial as well as a dark reaction trial.

The amounts of CO2 were read by a CO2 meter that was secured tightly to a clear jar containing each leaf sample. Each jar was held into place by a water bottle, then covered by a colored filter, and then exposed to light.

Each leaf for each trial was allowed to equilibrate underneath the light for three minutes and then the trial was run for ten minutes. As stated earlier in the hypothesis we expected to see the highest rates of Co2 consumption under red and blue light.

Our results indicated that the rate of photosynthesis was highest under red light

( -111.816 ppm/m/g) then white light (-88.979 ppm/m/g) then blue (-26.383 ppm/m/g), green (-25.141 ppm/m/g) and then the dark reaction (0 ppm/m/g) The rate of respiration was factored out of each of these in calculating the results.

Our hypothesis was inconsistent with our results. This came as a surprise as we ran a trial under blue light twice to make sure we didn’t do something wrong. I believe we took precautions to rule out as many sources of error as possible, mainly by using similarly weighted leaf samples and using a fresh one each time. It was discussed that autumn might not be the optimal time in which plants photosynthesize; this could be a possible source of error as well.

In comparing our results to other student projects we see very similar results, particularly with study 12.1 (fall 2003) on the University of Colorado Student abstract page, they ran almost the same experiment (except no dark or white light reactions) with the same results as ours.

From our results it is safe to say a new hypothesis should be generated. We have been thinking over and over that some how accessory pigments play a part in dissipating the amount of absorbed energy taken in by blue light. Blue light is very high energy, almost to the point where we believe it could damage the plant. Possibly the accessory pigments act as a "sunscreen" protecting the plant from harmful rays. This could explain the decrease in photosynthesis in blue wavelengths. All in all a worth while experiment.