Rates of Photosynthesis in C3 and C4 Plants


Alex Murray, Kristin Martell


CU-Boulder, Fall 2007


         Our experiment tested the difference in the photosynthesis rates of C3 and C4 plants in low carbon dioxide levels.  From previous experiments, it is known that C4 plants fix carbon dioxide before it enters the Calvin cycle.  The plant can therefore control both the gases and the amount of those gases that are taken by the rubisco, the primary enzyme that fixes carbon dioxide in the Calvin cycle.  Our hypothesis stated that the C4 plants will fix the carbon dioxide and limit the gases to the rubisco limiting the amount of oxygen available for photorespiration.

         Our experiment consisted of placing a sample of either a C3 or C4 plant in a gas chamber attached to a carbon dioxide sensitive probe and letting it photosynthesize under three lamps and a water bottle for ten minutes. We then measured the respiration rates of our two plants in darkness for ten minutes.  We ran two trials of each type of plant for a total of four runs.  We predicted that the C4 plant’s average slope, which represents the rate of photosynthesis, would be greater than that of the C3 plant.

         We found that the mean rate of photosynthesis, normalized for mass of the sample, for the C3 plant samples was -27.034 ppm/min/g.  The rate of photosynthesis of the C4 plant samples was 88.0075 ppm/min/g.  Our p-value comparing these two means was 0.16 and therefore not significant.  This indicated that our means were not dissimilar enough to validate our hypothesis.

         Our results did not support our hypothesis and we were forced to reject it.  However, even though our P-value was not significant, our experiment had significant problems.  On our second test of the C3 plant, the starting carbon dioxide level was about 1000 ppm higher that all the other tests.  Because of this, the carbon dioxide in the second test did not reach a “low” level as quickly as the first test.  This skewed the measure of the slope and therefore skewed our results.

         We could solve this problem by measuring our results in a different way.  In an experiment performed by O’Connell et-al. the lowest level of carbon dioxide reached was measured instead of the rate of photosynthesis.  This experiment found that the level of carbon dioxide leveled off in the gas chambers of the C3 plant samples and steadily decreased until there was almost no carbon dioxide left in the gas chambers of the C4 plant samples.  Their conclusion supports our hypothesis but not our data.  These differences may allude to our dissimilar methodologies.  Also, our means would have been more accurate and comprehensive had we run more trials.

         If we were to take this type of experiment further, we could test more samples of each type of plant and find a way to keep the carbon dioxide levels at a constant starting number.  We could also test the effect that fixing carbon dioxide has on other aspects of plant life such as moisture retention or fortitude in temperature extremes.


O'connell et al. 2005. The effects of photorespiration of C3 and C4 plants. <http://www.colorado.edu/ee