Lauren Roth, Matt Rose
CU Boulder, Fall 2008
                We tested photosynthesis efficiency of C3 plants and C4 plants in low CO2 concentrations. We knew C4Õs have one CO2 fixation before entering the Calvin Cycle, making them more efficient in low CO2 concentrations. We also knew C3Õs use rubisco, an enzyme that works best in a higher CO2 concentration, to fix CO2. Due to the CO2 fixation undergone before entering the Calvin cycle, we hypothesized that C4Õs will photosynthesize more efficiently at lower CO2 concentrations than C3Õs.
               To test our hypothesis we placed a C3 in an empty bottle surrounded with aluminum foil with two lights shining overhead to maximize the amount of light received by the plant. We then used a CO2 gas probe to measure CO2 levels inside the bottle. We watched CO2 levels inside the bottle decline until leveling out. Time wasnÕt a factor in our experiment since we werenÕt seeing how long it took for the plants to use up CO2; we simply had to wait until the CO2 rates leveled out. This allowed us to know when photosynthesis stopped and cellular respiration began. The lowest reading of CO2 showed the lowest CO2 level the plant could be at and still continue to undergo photosynthesis. We repeated the same test, 3 tests for both C3Õs and C4Õs.
                Our results showed C4Õs can photosynthesize significantly better in lower concentrations of CO2 (mean=23.67ppm/CO2) whereas C3Õs need higher concentrations (mean=39ppm/CO2; t=0.034, p<0.05).
               Our results are consistent with predictions from our hypothesis. Despite being consistent, out results are not reliable. This could be due to our inconsistent use of materials. Due to time constraints, we had had to test over two days. During these two days used different plants, different monitors, different probes, and a different lab.  We believe our biggest problem was using different monitors. Each monitor measures differently, so our results could have been skewed by using two, increasing our percent error. The rooms could have affected our experiment because they had different amounts of light coming in and different temperatures, and both can affect photosynthesis. 
               Experimental results of Poorter et al. 2003 showed that when C3Õs and C4Õs are placed in high CO2 concentrations, C3Õs flourish while C4Õs hold constant. These werenÕt surprising results, based on our knowledge of rubisco. Curtis et al. 1999 attempted to mimic the effects of rising global temperatures on C4Õs by placing them in high CO2 concentrations too. The conclusive results of this experiment werenÕt consistent with their predicted results of C4Õs performing poorly (there was no real fluctuation in photosynthetic rate). 
               It makes sense based on our results that C4Õs are perfect for mass production(like corn) since they do well in low CO2 environments(such as a corn field). And now that we know C3Õs thrive in high CO2 concentrations and C4Õs in low, we would like to discover the ideal CO2 environment for both to maximize their rates of photosynthesis by placing both in controlled CO2 environments at varying intervals.