Re: How does the absorption spectrum affect the rate of photosynthesis?

What you are looking for is called the "action spectrum", which wavelengths of light are effective in photosynthesis. For most plants where this experiment has been done, blue light is slightly more effective than red light for producing measurable photosynthetic results. The classic early experiment separated light with a prism onto a strand of spirogyra, then looked for the congregation of aerobic bacteria. They were found at the blue and red regions.

Why is the blue light more efficient? Probably it is due to a slightly higher absorption in the blue region by chlorophyll and the accessory pigments. It could also be due to the fact that blue photons are higher energy than red photons. Some of the red photons will not have enough energy to induce the reaction center P680 chlorophyll into ejecting an electron.

When you do this experiment, there are a couple of things to keep in mind.

1. Make sure each plant gets the same amount of light. Different filters absorb different amounts of light so measure the light actually reaching the plant.
2. If the experiment lasts more than a few hours, be aware there are other pigments in the plant which also use these wavelengths to control plant processes. Phytochrome, a red light receptor, controls many plant developmental processes, such as chlorophyll formation. Cryptochrome, a blue light receptor, controls plant environmental responses, such as phototropism. These could have subtle but significant effects on your results.

To find out everything you always wanted to know about photosynthesis visit the ASU Photosynthesis Center.

If you have access to a research library, look up Annual Review of Plany Physiology vol. 38:11-45 for a review of photosynthetic reaction centers.

Also a search of the internet for "photosynthesis wavelength" will give quite a few hits.