Re: Plant Developement vs. Spectrum

Light is very important to plants since that is the source of their energy. Therefore, they must maximize their use of this valuable resource. The chlorophylls are the energy trapping molecules and use light in the blue and red regions of the spectrum (the green is reflected which is why plants usually appear green).

How does a plant get the most out of the red and blue light that is available? Plants have two other pigment systems, one (called "phytochrome")which detects the amount of red light and one (called "cryptochrome")which detects the amount of blue light.

Phytochrome means "plant pigment" and was discovered based on its effects on plant development due to red light. A plant grown in the dark will be tall and spindly, cotyledons not expanded much, and no green color. In the light, it will be shorter, with bigger, green leaves. Red light controls these and other major developmental changes in plants. Some other changes under phytochrome/red light control are germination of seeds and flowering. When enough phytochrome has been activated, telling the plant the light is sufficient for its needs (enough light to grow or even what time of year it is), the developmental changes occur. That's how plants know how to flower at the same time every year!

Whereas phytochrome helps the plant make long-term developmental decisions (Flower now? Germinate now? Start making chloroplasts?), cryptochrome ("hidden pigment") responds to more immediate environmental cues, such as which way the light is coming from (phototropism) to make the best use of the developmental changes directed by phytochrome. The blue-light work is fairly recent, much of it using a tiny relative of turnips called "Arabidopsis". Look up "Arabidopsis" and "blue light" on the internet to get some of the latest info. If you have access to a library with the journal Science, look up the article in volume 278, pages 2120-2123 for a possible identification of cryptochrome and its relation to molecules found in other organisms, even animals!

If you try this experiment (and you should!) there are a couple of other things you should know. Just to confuse things a little, phytochrome also responds somewhat to blue light! (Scientists sometime use mutants with one pigment missing to see exactly how the other behaves by itself.) And try to make sure each plant gets the same amount of light. Good luck and let me know how it turns out.

One more reference:
"From seed germination to flowering, light controls plant development via the pigment phytochrome" by J. Chory, et al. Proceedings of the National Academy of Science Vol. 93, pp 12066-12071 (1996).

You can also look up phytochrome in a college level Botany textbook.