Effect of Different Colored Lights on Photosynthesis 

Anastasia Rodionova, Cassidy Davis, Sara Cucciniello

CU Boulder, Fall 2003

Our experiment tested which color (red, blue, green) would influence the plant to produce the most amount of photosynthesis. There are four main photosynthetic pigments found in the chloroplast of the plant called chlorophyll a, chlorophyll b, xanthophylls, and carotenes. All these pigments absorb light and possibly utilize the light energy in photosynthesis. Light energy is essential for photosynthesis. An initial experiment showed that all the pigments at peak absorbance showed violet/blue light at the highest level, orange/red light as the second highest, and yellow/green having the lowest level of absorption. We hypothesized that photosynthesis was affected by the light absorption rate.

To test this we used about 5 grams of leaves for each trial, and placed them in a gas chamber. On two sides of the gas chamber we placed two clear containers filled with water to serve as the temperature regulators. Behind the water containers we placed lights directed at the plant. We ran three trails for each different leaf we used. Each trail consisted of measuring the amount of CO2, with a CO2 gas sensor under blue light, red light, and green light. We made sure to switch the order of colors in each trail as an experimental control, to minimize error. Since we know that photosynthesis requires CO2, and we know that blue light pigments absorb the most light energy, we predicted that under blue light the most CO2 would be used.

Our results showed the least amount of CO2 under blue light (mean: -8.1 ppm/min/g), medium amount in the red light (mean: -1.04 ppm/min/g), and the most amount in the green light (mean: 4.7 ppm/min/g). But our t-tests proved our results insignificant (p>0.05).

Our results are contradictory with our hypothesis, based on our statistical results. There were several problems with our experiment that could have been taken into consideration. First, when taking respiration rates the foil wasn’t covering the chamber all the way letting some external light in. Second, the colored cellophane only allows approximately 70% of light through; this might have prevented the plant from absorbing the amount of light energy needed to have a significant photosynthetic rate. Third, the fast paced moving between trials lost time and efficiency. By having short trials (2.5 min.) we might not have allowed the plant enough time to adjust its photosynthetic rate to the different wavelengths of light energy. Plus by moving the plant, and switching from cellophane to foil (or vise versa), might have screwed up the photosynthetic cycle by exposing it to white light.

 

 

Other students who did the same experiment had results that also supported that blue light was not responsible for the rate of photosynthesis. Experiments Warsh et al. 2001, Bahramzadeh et al. 2001, and Whorley and Weaver. 2002 all showed that red had much higher rates of photosynthesis than blue, but these experiments had fewer trials or shorter trials. Lots of other students also used white light as a controlled variable; this would have been a good thing to put into our experiment to give it some comparison.

Considering that not all light energy is used for photosynthesis we propose an alternative hypothesis. In a previous experiment the pigment xanthophylls absorbed significant amounts of blue light. In new research it is found that this pigment could be an important component in a process called energy dissipation rather than photosynthesis. In order to not overwhelm the plant with photosynthesis and respiration, this photon energy goes to other functions or formations of the plant. Further research on the function of xanthophylls will need to be conducted in order to understand the processes of plant function.