Karthik Ramachandran, Katie Olson, Stephanie Persutte

University of Colorado at Boulder, Fall 2004

The purpose of our experiment was to test the different CO₂ respiration rates in C3, C4 and CAM plants. More specifically, we wanted to know the CO₂ production or consumption rates in light verses dark conditions. In a prior lab experiment, we established that in C3 light dependent and independent reactions occur in the light, while cellular respiration predominates in the dark (although it is a continual process).

CAM plants are known for their separation of light dependent reactions which occur during the day, and light independent reactions which occur at night, because of their existence in desert climates. Reasons behind this phenomenon arise due to stomata remaining closed during the day to prevent evaporation of water and hindering CO₂ from entering, and opening during cooler temperature at night to allow gas exchange. If CAM plants perform light independent reactions exclusively in the dark, while C3 and C4 perform light independent reactions in the light, then the presence of light will have a considerable effect on the photosynthetic rates of these plants.

To test out hypothesis we placed sets of leaves into separate gas chambers and attached a CO₂ probe to measure the amount of CO₂ production or consumption using software called Logger Pro. A water tank was placed between the light source and the gas chamber to absorb any heat emitted by the light, and cause a change in temperature in the gas chamber. Using C3, C4, and CAM plant leaves, we place them into the separate gas chambers and took their weights. Three trials lasting a total of 24 minutes were done on each plant type. In each trial, the leaves were exposed to 8 minutes of light, followed by 8 minutes in the dark, and finally 8 minutes of light. To simulate dark conditions, the chamber was covered with aluminum foil.

The computer collected the data and upon completion, a slope was analyzed for the 3 different lighting segments for each plant. The slope of the line was then divided by the mass of the sample to determine the rate of change in CO₂ levels per gram of leaves. In order to isolate the photosynthetic rate from the cellular respiration rate in C3 and C4 plants, the rate in the dark was subtracted from the average rate in the light. Inversely, since we predicted that the light-independent reactions for CAM plants occur in the dark, we subtracted the average rate in the light from the rate in the dark.

Our results indicated that the rate of photosynthesis in CAM plants remained constant throughout the experiment (photosynthetic rate = 0 ppm CO₂/min/g), while an obvious change in photosynthetic rate was observed in C3 and C4 plants (photosynthetic rate in C3 = -21.7 ppm CO₂/min/g; photosynthetic rate in C4 averages -32.83 ppm CO₂/min/g).

Our experimental results are inconsistent with our hypothesis about CAM plants, but supported our predictions about C3 and C4 plants. This was consistent with other studies found on the CABLE website. Results of Faulkner and Ehlers et al. 2002, suggested the same results regarding CAM plants with no significant difference from light to dark. Upon further research, we recognized that not only does light effect a CAM plant, but the effects of temperature may play a role in photosynthesis and respiration. CAM plants are designed for environments in which extreme heat during the day and cooler temperature at night, and due to the need to conserve water in such an environment, the cells may react differently.

Upon rejecting our hypothesis, we discussed possible problems with our experiment including the temperature factor, humidity, and the inability to provide as close to a natural environment as possible. Other problems may include sample size or inadequate time amount. With all this in mind, we proposed a new hypothesis regarding whether a change in temperature will affect the photosynthetic rates in CAM plants and whether this effect is the determining difference between C3 and C4 plants, and CAM plants.