Physical Characteristics of Plants and their Effects on Photosynthesis

Nicole Captanian and Richard Cornelius

CU Boulder, Fall 2006

Why do some plants have needles, some have leaves, and some have a structure in between? Why do some have leaves densely packed together, while others have leaves spread far apart? Also, why do some plants have all leaves facing one direction, while others have them sorted at random? These questions were the inspiration for our experiment and guided us to the conclusion that these characteristics must be attributed to a biological function. Because photosynthesis is arguably the most essential process in plants, we figured that the plantsŐ structure could be for photosynthetic purposes. This led to our hypothesis and prediction—if some plants have a biological advantage over others in their particular locale because of their leavesŐ shape, size, and orientation, then these traits would affect the plantsŐ habits in photosynthesis.

We chose three plant samples with different physical traits to test: Juniperus, Berberis (Barberry), and Padus virginiana (Choke Cherry). We tested the rate of photosynthesis for each using a gas chamber containing the plant and CO2 sensor attached to the Lab Pro Interface. We placed two high intensity lights over the clear gas chamber and a transparent container of water between the lights and chamber to regulate temperature. The computer tracked the amount of CO2 for fifteen minutes. We then removed the lights and covered the gas chamber with tinfoil. The computer then tracked the amount of CO2 in the dark for ten minutes. We obtained the rate of photosynthesis by subtracting the slope of the line divided by the mass of the plant in darkness from the slope of the plant divided by its mass in light to account for respiration. We obtained three rates of photosynthesis for each plant sample.

Our results are statistically significant (P = 0.000247). Therefore, when we compare our values for each plant sample, we can conclude that the rates of photosynthesis are different. The compared averages (in CO2/ppm/g) were: -54.79 for Juniperus, -72.75 for Berberis, and -48.89 for Padus virginiana.

There are no experiments on the web page that match ours exactly; however there is one that compares. In 2000, four CU students found that the photosynthetic rates of spinach leaves and jade leaves were different. These plants have different structures to their leaves and therefore can support our results.

Our results support our hypothesis. However, there are several limitations in our experiment. First, we were only able to test three plants. Second, we couldnŐt obtain leaves from significantly different environments. Third, we could not reproduce the plantsŐ natural surroundings in the lab. Lastly, we couldnŐt find whether the plants were all C3, though we assumed they were. In retrospect, there were too many complications to make our experiment completely accurate. These problems donŐt necessarily invalidate our data, but they do note areas in which the experiment could have been designed more carefully.

Our hypothesis remains the same, though if we were to perform the experiment again we would change several aspects of it to deviate some of the potential problems.