C3 Plant And Its Rate Of Photosynthesis

Ciara Richmond, Mike Nicastlle

CU Boulder Fall 2007


By testing the rate of photosynthesis of C3 plants under low amounts of CO2 and comparing this to the rate of photosynthesis under higher amounts of CO2, we can determine if C3 plants photosynthetic activity will vary with different concentrations of CO2. Because low levels of CO2 for C3 plants hinder photosynthesis, our hypothesis and prediction was that if C3 plantsŐ rate of photosynthesis were dependent on a certain threshold of CO2, then inadequate CO2 concentration would result in inefficient photosynthesis.

To test this hypothesis we utilized crickets along with the C3 plant. Crickets in low temperatures release more CO2, so by placing a C3 plant (blue spruce) with the crickets we could test the rate of photosynthesis under adequate amounts of CO2 and by isolating a C3 plant we could determine the rate of photosynthesis under lower amounts of CO2. Through a probe measuring CO2 concentration, we could determine the rate of photosynthesis (a decline in CO2 would indicate the action of photosynthesis).

Placing four crickets in a jar, we cooled the temperature to 15ˇC. After waiting five minutes for the CO2 concentration to accumulate, we then placed the C3 plant into the jar and under a lamp for ten minutes. Next, to avoid error, we cooled another jar to 15ˇC without crickets. Placing the C3 plant into the jar and under a lamp for ten minutes, we determined the rate of photosynthesis using the CO2 concentration probe. We repeated this procedure twice (a total of 6 trials) using different needles from the blue spruce. We predicted that the rate of photosynthesis would be greater for the blue spruce with the crickets (more CO2) than for the blue spruce without the crickets (less CO2).

Our results indicated that the rate of photosynthesis was greater without the crickets than with the crickets. The mean change in CO2 for the jar of blue spruce and crickets was 716 ppm as compared to without the crickets, which was 114 ppm. No statistics could be applied.

The results are inconsistent with our predictions. One problem that could have resulted in this inconsistency was that the rate of CO2 expulsion from the crickets was greater than the rate of photosynthesis for the blue spruce. If the crickets expelled CO2 more rapidly than the blue spruce could utilize for photosynthesis, than our data would be inaccurate and show that C3 plantsŐ rate of photosynthesis is more efficient under less CO2 concentration. We needed to increase the CO2 concentration by using the crickets, but we should have removed the crickets after reaching a certain concentration. This would allow the blue spruce to utilize a set amount of CO2 rather than the CO2 accumulating and not being used by the plant in photosynthesis.Our experiment results have allowed us to examine the possibility of a new hypothesis. Because CO2 concentration did increase with the crickets, it is possible that photosynthesis is actually inhibited by high amounts of CO2.