Rates of Photosynthesis of Broad Leaves and Needle Leaves

 

Holly Schor, Stanley Wun

 

CU Boulder, Fall 2006

 

         This experiment compared the rates of photosynthesis between broad leaves and needle leaves. Leaves need light and CO2 to perform photosynthesis. Broad leaves have a larger surface area than needle leaves. Broad leaves should be able to absorb more light and more CO2 than needle leaves because broad leaves have a larger surface area. Therefore, it was hypothesized that the greater the surface area of the leaf, the faster the rate of photosynthesis of the leaf.

Three species of broad leaves and three species of needle leaves collected from trees on campus were used in this experiment. Each type of leaf was put into a gas chamber where a gas probe measured the amount of CO2 in the chamber. Each leaf was spread out so that it was not blocked from the light by another leaf. The rate of change of the amount of CO2 (ppm/min) was collected while the leaves were under light and in the dark. The rate of change of CO2 while the leaves were in the dark was factored out of the rate of change of CO2 while the leaves were under light by subtracting the rates since photosynthesis and respiration occurs under light and only respiration occurs in the dark. The rate of photosynthesis was then divided by the mass of the leaves so the results could be compared.

The results indicate that there is a difference between the rates of photosynthesis depending on the surface area. The mean rate of photosynthesis for broad leaves in the experiment was 35.35 ppm CO2/min/g and the mean rate of photosynthesis for needle leaves was 14.469 ppm CO2/min/g. Broad leaves, leaves with a greater surface area, were able to perform photosynthesis faster. A p-value of 0.046 from a t-test indicates that there was a significant difference between the rates of photosynthesis of broad leaves and needle leaves.

The results support the hypothesis; however there were some problems with the experiment. The needle leaves that were used in the experiment sat around longer than the broad leaves that were used. The needle leaves were not as alive as the broad leaves so the rate of photosynthesis could be less. The leaves should have been gathered off of the trees or plants right before they were to be tested. Another problem with the experiment was that the leveling off level of CO2 was not measured. The rate of photosynthesis is affected by the amount of CO2 in the gas chamber and the rate is less when the amount of CO2 is near the leveling off level. The last problem with the experiment is that there could have been other factors other than the leave’s shape that affect the rate of photosynthesis. An attempt was made to only use C3 plants in the experiment but there was no way to determine if the plant was C3, C4, or CAM. C4 and CAM plants fixate CO2 differently so the rate of photosynthesis could be different. Results of Nguyen et al. 2001 and Murray et al. 2004 on the CABLE website showed that there is no significant difference in the rates of photosynthesis of broad leaves and needle leaves but their mean rates of photosynthesis for broad leaves were greater than their mean rates of photosynthesis for needle leaves. The p-values in these experiments are greater than 0.05 probably because of low number of trials.

New ideas arise from the experiment. The rates of photosynthesis for C3, C4 and CAM plants could be tested and determine if they have a different rate of photosynthesis since they fixate CO2 differently. Another idea that arises is how the age of the leaves affect the rate of photosynthesis. Some of the leaves used in this experiment were becoming yellow and getting ready to fall off since fall leaves were used. If the experiment was done in the spring when the leaves are green, then the rate of photosynthesis may be different. So color could also be a factor to be tested. Other hypotheses could be tested for this experiment. One possible hypothesis is leaves with a high surface area can absorb more CO2 from the air; therefore, broad leaves will perform photosynthesis longer than needle leaves in a gas chamber. The same method would be used only the emphasis and focus of the experiment would change.