Effects of Blue and Red Light on the Rate of Photosynthesis

Braddock, B., S. Mercer, C. Rachelson, and S. Sapp.

CU Boulder, Fall 2001.


We tested the effects of blue and red light on the rate of plant photosynthesis. We hypothesized that light absorption by the plant and the energy level of different wavelengths of light are positively correlated to the rate of photosynthesis. Thus, because blue light has a higher absorbance by plant photosynthetic pigments and has a higher energy wavelength than red light, we predicted that juniper needles placed in blue light would photosynthesize faster than juniper needles placed in red light. We measured the rate of change in CO2 concentration due to juniper needles. For each sample, we placed the needles into a chamber connected to the CO2 monitor and measured the rate of change of CO2 concentration for 10 minutes under red light and then 10 minutes under blue light. We ran three independent trials and alternated which color of light to which the leaves were first exposed. We weighed the juniper needles in each sample so that we could control for differences in mass; the rates of change of CO2 concentration were calculated per gram of juniper needles. We did not test the rate of respiration of the juniper needles in the absence of light because we assumed that the rate of respiration was constant for each sample of juniper needles. We monitored the rate of change in CO2 concentration of an empty chamber as a control to demonstrate that any change in CO2 concentration was a result of the juniper leaves and not the chamber itself changing the concentration of CO2. The rate of change of CO2 concentration in the empty chamber was nearly 0, so we did not have to correct/adjust any values during the experiment due to this control. Plants in red light produced less CO2 over time (photosynthesized faster) than the plants in the blue light for each of our three trials. Two of the three trials in the red light were negative values, reflecting a decrease in the concentration of CO2. These values of the photosynthesis (plus respiration) rates in red light were 0.443, -0.141, and -1.1 ppm/g/min with a mean value of -0.27 ppm/g/min. The values of photosynthesis (plus respiration) rates in blue light were 2.449, 1.667, and 2.997 ppm/g/min with a mean value of 2.36 ppm/g/min. A t-test comparing the mean photosynthetic rates under red and blue light indicated no significant difference (p=0.068). However, this value is close to being significant, so with additional trials of our experiment it is possible that we would come up with a significantly faster rate of photosynthesis under red light compared to blue light. Based upon our results, we rejected our hypothesis. Blue light does not make plant needles photosynthesize faster than red light, and we see a trend towards faster rates of photosynthesis under the red light. Other student projects done in previous years produced similar results. One study found a decreasing rate of photosynthesis in blue light (Mae et. al. 2000). Another study found that the rate of photosynthesis occurred fastest in red light and that the reason for this was because xanthophylls were dissipating the excess energy associated with blue light (Brins et. al. 2000). One possible explanation for our results is that due to the high-energy nature of blue light, some of the blue light shining onto the juniper needles is absorbed by plant pigments other than the chlorophylls and is not transferred to the photosynthetic reactions. Xanthophylls and carotenes are possibly dissipating the high-energy blue light because xanthophylls and carotenes absorb only in the blue spectrum. These energy dissipation mechanisms operate in the blue spectrum because high energy blue light may be damaging to the plant. Further experimentation should be performed to verify our results and to test new hypotheses. In the future, more trials of our experiment should be run to test whether red light is photosynthesizing significantly faster than blue light. New experiments examining how and where blue light is absorbed by juniper needles are needed in order to better understand the effects of blue light on the plant.