Photosynthesis and Respiration Under Filtered Light


Lisa Wilding


CU Boulder, Fall 2007


Does the rates of photosynthesis change when Juniper sprigs are placed under red and blue light filters?  Even though red light has a lower energy compared with blue light, blue light can be damaging to a plant and, therefore, may be filtered by the plant to protect itself.  In "light" of this, I hypothesized that photosynthesis rates will be highest under red filters, as the plant would be able to absorb all of the red wavelengths safely whereas the plant may be filtering some of the damaging blue wavelengths.


[NG1] To test this hypothesis, data needed to be collected for this photosynthesis rates under red and blue light.  hypothesis, data needed to be collected from the control variable (Juniper sprigs under normal white light).  The experiment consisted of Juniper sprigs placed into a chamber with a CO2 gas sensor attached.  The CO2 gas probe was the attached to the computer to record the rate of change of CO2 in light versus dark to estimate the rate of photosynthesis.  The chamber was wrapped in a red-filtered bag.  A water bottle was placed on top of the chamber to provide a heat sink.  High-intensity, full-spectrum lights were directed at the Juniper sprigs for 710 minutes, then the lights were turned off and the chamber was wrapped in tin foil for 710 minutes of darkness.  Another 710 minutes under the lights was sufficient to collect data for red-filtered light.  Data was collected for sprigs under a blue filter using the same procedure.  The entire test cycle was repeated three times for an averagenormal white light rate of photosynthesis..


If photosynthesis rates are higher under the red filter, then CO2 levels (indicating respiration) will be the lowest when juniper sprigs are under red light filter.


To test photosynthesis rates of red and blue light, the above methods were followed with the addition of alternating red and then blue filters placed around the chamber holding the Juniper sprigs.  Data was recorded for sprigs under the light with each filter for 7 minutes then in the dark for 7 minutes, repeating each test cycle three times for an average[NG2] .  Prediction goes hereÉ


According to the means[NG3] , photosynthesis rates were higher under red light filters (6.79 ppm/min/g)  compared to blue light filters (4.63 ppm/min/g).  This seemed to clearly support my hypothesis; however, T-tests were run on the mean rates of photosynthesis, as well as the respiration rates.  Difference in rates of photosynthesis between red and blue light filters was NOT not significant (p-value=0.1145); however, the difference in respiration rates under between red and blue light filters WAS was significantly greater[NG4]  (p-value=0.035). 


Sprigs under blue light filters were breaking down the sunlight and making sugars at such a high rate that photosynthesis rates under this filter was were masked by high respiration rates.  I have, therefore, have to rejected my hypothesis.


Experimental methods which could potentially lead to errors in data collection:  time switched from light to dark was not always exact; amount of air fanned in the chamber between testing; amount of Juniper sprigs used; possible holes in the filters or not complete darkness allowing white light to leak in; lights not set in exact same place each time; and the filters used may not have represented the exact red and blue wavelengths of natural light.


These findings are consistent with the alternative hypothesis that photosynthesis rates will be the highest under blue light filters.  This modified hypothesis is also supported by Cusack et al. 2006.

 [NG1]What does white light have to do with the comparison of blue and red light?  White light is all colors and doesnŐt really impact the red vs blue light comparisonÉ

 [NG2]Of what??

 [NG3]You need to give the means (in ppm/min/g)

 [NG4]Significant how – which as significantly greater?