Photosynthesis Rates of C3 vs. C4 Plants


Jacqueline Snapp

CU Boulder, Fall 2007


Photosynthesis removes CO2 from the atmosphere.  From previous lab experiments, we observed that the amount of CO2 in a closed chamber will decrease when photosynthesis is occurring and the faster the level of CO2 decreased, the faster the rate of photosynthesis.  C3 plants obtain CO2 from the atmosphere and it directly enters the Calvin Cycle, while in C4 plants the CO2 is obtained from the atmosphere and fixed in the mesophyll cells prior to entering the Calvin Cycle.  Since C4 plants must fix the CO2 prior to entering the Calvin Cycle, we hypothesized that C3 plants obtain CO2 more efficiently than C4 plants and therefore C3 plants will have a higher rate of photosynthesis, as measured by the rate of change of CO2 over time.


In our lab investigation, we tested and compared the rates of photosynthesis of the C3 plant, Elymus glaucus, and the C4 plant, Setaria palmifolia.  To test our hypothesis, we performed three trial runs using different samples of the same species of C3 (E. glaucus) and C4 (S. palmifolia).  Each run was tested ten minutes, 5 minutes in light and 5 minutes in dark, with the various samples in a chamber attached to a CO2 gas sensor.  Each trial was normalized for mass, had a water-filled chamber as a heat sink, and identical sources of light.   


The decrease in the average rate of CO2 ppm/min/g for C3 (E. glaucus) was greater than for C4 (S. palmifolia), therefore it appeared to be consistent with our hypothesis.  Upon performing a t-test, our results indicated that the rate of photosynthesis was not significantly different (P = 0.18, P>0.05) in C3 plants (mean = -19.90 ppm CO2/min/g) vs. C4 plants (mean = -12.09 ppm CO2/min/g). 


Our results were inconsistent with predictions based on our hypothesis.  One potential problem with our experimental design, we only ran 3 trials with each species of C3 and C4 plants.  Additional trials and using more than one species of C3 and C4 plants may have resulted in more reliable data.  Results of Naegeli et al. (2004) on the EBIO 1230 website also demonstrated that C3 plants photosynthesized at a faster rate than C4 plants, but they only ran two trials.  In order to reach a more reliable conclusion we propose further testing of our hypothesis by running the experiment again, but with longer timed trials (10 minutes light, 10 minutes dark) and more than one of each species of C3 and C4 plants.