Deciduous vs. Coniferous Leaves

Kelsey Henneke, Lindsay Holman, and Heidi Lord

CU-Boulder, Fall 2008

We tested the relationship of morphology to the rate of photosynthesis in leaves. Morphology determines the leaf life span. Deciduous leaves, such as those from an oak tree, have seasonal life spans whereas coniferous leaves have a much larger leaf life span. We hypothesized that leaf life span effects the rate of photosynthesis. This led to the prediction that deciduous leaves would have a higher rate of photosynthesis than coniferous leaves.

To test our hypothesis, we first placed 0.5g of coniferous needles into a gas chamber attached to a CO2 probe. The chamber was placed under three lights to simulate sunlight, and the temperature of the chamber was kept constant by filtering the light through a plastic container of water. CO2 concentration was recorded for five minutes, then the chamber was covered with tinfoil to stop the process of photosynthesis, and simulate night. We recorded CO2 concentrations for an additional five minutes to determine the rate of cellular respiration. We repeated this process a total of four times, twice with coniferous leaves and twice with deciduous leaves.

The means of our data (coniferous = -14.66 CO2ppm/min/g, deciduous = -25.15) suggested that the rate of photosynthesis was higher in deciduous leaves than coniferous matching our prediction and supporting our hypothesis. In contrast, the results from our t-test (P = 0.214) showed that the differences were not significant. It was not possible to know if our results were valid with only 4 samples.

Although our original hypothesis was somewhat supported by the trends of the results, a more complete study of leaf life span in relation to rate of photosynthesis would examine many different samples of leaves over their entire lifespan. However, additional information lead us to conclude that deciduous leaves have a higher rate of photosynthesis in direct light, because the majority of chlorophyll is located on the top surface of the leaf. Photosynthetic rates are lower in coniferous leaves in more direct light but the entire surface area of the needle contains chlorophyll to aid in capturing indirect or reflected light (i.e. snow). This led to our decision to modify our hypothesis; morphology has an effect on rate of photosynthesis due to quantity of chlorophyll on the surface.

Waring, et al. (1979) stated that coniferous trees have an advantage over deciduous trees in locations farther north because they can complete photosynthesis more efficiently during the cold months. This supported our hypothesis. Our second article also shed some light on the structure and morphology of leaves. The article, by Smith, et al. 1997, says that plants respond to the amount of light in a habitat by evolving their leaf structure. Morphological structure of leaves is generally associated with amount of sun or shade exposure or water stress. The relationship between photosynthetic leaf form and function is still being investigated and researched, and scientists began to draw correlations only as recently as the mid 1990’s. Scientists have yet to attain an actual mechanism between leaf structure and photosynthesis (Smith et al. 1997).