Photosynthesis Rates of CAM vs. C4 plants



Pamela Williams

Kelly Salance

Naomi Zelin


CU Boulder, Fall 2007



The question that our group wished to test was “Do C4 plants photosynthesize more efficiently than CAM plants?”  We then developed a broad hypothesis and prediction of: “If C4 plants have a high rate of photosynthesis, than there will be less CO2 in the chamber.”  From this more general statement, we developed our final hypothesis and predictions for the results: If C4 plants have a higher rate of photosynthesis compared to that of CAM plants, than there will be less CO2 left in the chamber of the C4 plant samples because of the presence of PEP Carboxylase, which enables CO2 to be taken into the plant very quickly .

In order to test this hypothesis, we took three different samples of each type of plant (heveria xpatriata for CAM, and setoria palmifola, common name palm grass, for C4) and placed them into essentially airtight chambers with a CO2 monitor inside.  We than ran each sample for seven minutes under two lights (with a clear container of water to act as a heat absorber so the samples wouldn’t heat up), and than for seven minutes wrapped in foil (dark).  For each type of plant, we kept the same CO2 monitor and chamber in order to minimize variability in the results.  We also used the same scale to weigh all samples as well as performed the entire experiment in the same room; in order to keep the same ratio of oxygen/ CO2 in the container analogous.

Our calculated photosynthesis values for the CAM samples are 12.527, 9.440 and 2.017 with a mean of 7.995.  Our calculated photosynthesis rates for the C4 samples are   -8.85, -12.46, and -35.24 with a mean of -18.85.  We calculated the rate of photosynthesis by taking the slope of line at seven minutes (sample exposed to light) and subtracting the slope of the line at fourteen minutes (sample exposed to dark), then dividing by the mass of the sample.  Our results indicated that the rate of photosynthesis was significantly greater for CAM samples (mean = 7.995 ppm CO2/min/g) compared to C4 samples (-18.85 ppm CO2/min/g; t = .0178, p < .05).

Our results are consistent with the predictions based on our hypothesis.  Some problems that arose with our experiment are the first CO2 probe and the Logger Pro interface that we used operated inaccurately, so we had to start over.  We also didn’t use different species of CAM and C4 plants, just different samples for each trial.  Also, because of time constraints, we ran the CAM samples with one set of equipment and the C4 samples with another.  In order to reduce variability in the equipment, with the proper amount of time we would have used all the same equipment to test all the samples.  In order to correct this problem we should have used different species and run three trials for each.  No other students have compared photosynthesis rates of CAM and C4 plants and we therefore cannot compare our results with others.  But, based upon articles found in the school library system, it states that neither C4 or CAM plants photosynthesize more efficiently ( estid  =156341).

A more specific hypothesis relevant to this experiment would address exactly how much more efficiently C4 plants photosynthesize compared to CAM plants (2X as much? 3X as much?)  Although there aren’t as many plants in dry climates, more CO2 absorption occurs because of their efficiency in producing ATP.   We would also want to test the samples at different temperatures because both C4 and CAM plants are adapted to dry conditions and it is therefore harder to compare their rates of photosynthesis at one temperature.  This data would result in a more comprehensive study of their photosynthesis rates.