Title:  The Rate of Photosynthesis and the Incident Angle of the Sun

Anastasia Trus, Stephanie Reeves

We were interested in how the rate of photosynthesis in coniferous trees is affected by the incident angles of the Sun. We were aware that deciduous trees lose leaves when the angle of the Sun is lower in the winter and fall because they cannot absorb the light needed to drive photosynthesis. For investigation, we tested the effects of incident light angles on the photosynthesis of needles from a coniferous tree. This experiment was originally designed to compare tree leaves of different shapes, like the needles we collected from a conifer compared to a broad leaf shape collected by another group. The incident angle of the sun shouldnÕt affect the rate of photosynthesis in needles to the extent that a broad leaf would be affected. Since direct sunlight provides more intense light energy than indirect sunlight, we hypothesized that the rate of photosynthesis would decrease as the incident angle of light decreases.

To test our hypothesis, we used lamplight to model Sunlight and exposed needles to 90 and 45-degree light.  These angles represent the angles of light at Noon during winter and summer.  We attached a CO2 gas probe to a gas chamber and measured the rates of CO2 uptake for 10 min and the rate of CO2 production for another 10 minutes in the dark and calculated the rate of photosynthesis (rate of CO2 uptake in the light– rate of CO2 production in the dark).  We replicated this experiment twice.  After we performed all four trials, we calculated the mean rate of photosynthesis for each angle. We predicted the average rate of photosynthesis for the needles exposed to 45-degree light would be lower than the average rate of photosynthesis under 90-degree light.

In our results we found that the average rate of photosynthesis under 45-degree light was lower.  The mean rate of photosynthesis under 45-degree light was -2.405 while the mean rate of photosynthesis under 90-degree light was -4.215.  A greater negative slope indicates that CO2 is consumed at an increasing rate, indicating a higher rate of photosynthesis.

From our results, it appears that angles of light do affect photosynthesis, although more replicates allowing statistical analysis is necessary for more confidence with this data set.  We needed one more replicate for each angle, although the time allowed for the experiment was insufficient to collect additional replicates.  If we had three replicates for each angle we wouldÕve run a student t-test to determine whether the angle of the sun significantly affects the rate of photosynthesis in coniferous needles.  Our results demonstrate that the rate of photosynthesis is indeed, affected by the incident light angle in needle shaped leaves.  The experiment performed by another group showed a more pronounced effect when measuring the rate of photosynthesis using broad shaped leaves under the same two angles of light.   To investigate this in more detail, additional replicates, and a variation in types of leaves and angles of light used would be necessary to further our understanding of the sunÕs effect on photosynthesis.