The Relation of Photosynthetic Rates to Leaf Location

By Adrianna Pulver, Nakoia Paewai-Bloch, and Alec Nelson


            We tested the photosynthetic rate of leaves receiving direct sunlight (top of the plant) and leaves receiving indirect sunlight (bottom of the plant) from a plant.  Plants that grow in shady areas tend to have higher photosynthetic rates then plant in full sunlight because they have to utilize all the light they absorb.  However, when you are looking at the same plant we questioned whether this would still in fact be true.  Leaves at the bottom of the plant still carry out photosynthesis, but not necessarily at the same rate as the leaves at the top of the plant, therefore, the location of the leaf on an individual plant determines the leaf's photosynthetic rate.

            To test this hypothesis we began by collecting samples of leaves in direct and indirect sunlight.  Next we weighed the samples and placed them in gas chambers with a CO2 gas probe to measure the CO2 concentrations in logerpro.  In each trail, we measured light and dark conditions of each sample in order to subtract out respiration (Light-Dark) to get the photosynthetic rate of the leaves.  These trials ran for approximately ten minutes each for each sample of leaves  We predicted that leaves in full sunlight have a higher photosynthetic rate the leaves in partial sunlight.

            Our results displayed no significant difference in the photosynthetic rates of leaves in direct sunlight and leaves in indirect sunlight.  The mean value for the photosynthetic rate in full sunlight was -19.5625, while the mean value for the photosynthetic rate in partial sunlight was -15.4475.  This failed to indicate any substantial difference in leaf location.  Our t-test barely showed significant data.  Our observations show a slight difference with a P-Value of .044 Factors that could have contributed to this could range from control of temperature of our plant samples, running more trials, adding comparisons with samples from other plants and finally, technology problems we had with the loger pro.

            Kettle (2008) also focused on aquatic plant life and tested the reactions to different spectral resolutions and light absorption, indicating a sensitivity and higher photosynthetic rate then previously thought.  In another study, Vassiliev (2008) investigated the light harvesting in photosystem II,  focusing more closely on the molecular reactions and conversion in photosystem II; and  indicating significant variations among the photosystems I and II involving photochemicals and conversion rates.  Considering these studies and our results we propose a revised hypothesis.  Instead of the photosynthetic rate being determined by leaf location, the photochemicals and reactions to various light wavelengths is dependent on leaf location and that determines the photosynthetic rate.  Studies indicate that there would a significant comparison between light wavelength reaction and photochemicals in leaves in direct and indirect sunlight that would determine the photosynthetic rate.


Kettle, H. 2008. Modeling ocean primary production: Sensitivity to spectral resolution of    attenuation and light absorption. Progress in Oceanography, 78:135-146.


Vassiliev, S. 2008. Toward understanding molecular mechanism of light harvesting and charge separation in photosystem II. Photosynthesis Research, 97:75-89.