Figure 26.11 Photomorphogenesis and Red Light
The effects of red and far-red light on lettuce seed germination were demonstrated by an experiment in which lettuce seeds were exposed to the different light sources. Most of the seeds that were exposed to only red light were observed to germinate, whereas just a few of the seeds exposed to a period of red light followed by a period of far-red light germinated. Seeds were then exposed to alternating periods of red light for 1 minute and far-red light for 4 minutes. The results indicated that if the final exposure was to red light, then most of the seeds germinated. By comparison, if the final exposure was to far-red light, then most of the seeds remained dormant. In each set of experiments, the final light exposure was found to determine the seed’s germination response. Importantly, it was noted that the effects of red and far-red light on germination are mutually reversible. Later studies revealed that the basis for these effects is due to the interconversion between two forms of phytochromes, Pr and Pfr. Phytochrome Pr absorbs red light and is converted to Pfr, and Pfr absorbs far-red light and is converted to Pr. Note that, while it is exposure to red light that promotes subsequent germination, the resulting biological effects are actually due to the action of the Pfr form of the phytochrome.
Original Paper
Borthwick, H. A., S. B. Hendricks, M. W. Parker, E. H. Toole, and V. K. Toole. 1952. A reversible photoreaction controlling seed germination. Proceedings of the National Academy of Sciences 38: 662–666.
http://www.pnas.org/content/38/8/662.full.pdf+html
Links
Koning, Ross E. 1994. Plant Physiology Information Website: Phytochrome.
http://plantphys.info/plant_physiology/phytochrome.shtml
Home Page for Ross Konig: Plant Physiology Information Website: The Effect of Light on Lettuce Seed Germination
http://plantphys.info/research/experimenta.html
Kimball’s Biology Pages: Photoperiodism
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Photoperiodism.html
Missouri Botanical Garden: Phytochrome
http://www.mobot.org/jwcross/duckweed/phytochrome.htm
USDA: Agricultural Research Service Timeline: Tripping the Light Switch Fantastic
http://www.ars.usda.gov/is/timeline/light.htm
Newswise: NU Chemist Finds Key to Light’s Effect on Plant Growth
http://www.newswise.com/articles/chemist-finds-key-to-lights-effect-on-plant-growth
James Shinkle: Basic Photomorphogenesis
http://www.photobiology.info/Shinkle.html
Botany online: Plant Responses to Light: Phototaxis, Photomorphogenesis, and Photoperiodism
http://www.biologie.uni-hamburg.de/b-online/e30/30.htm
Plants-In-Motion: Photomorphogenesis
http://plantsinmotion.bio.indiana.edu/plantmotion/earlygrowth/photomorph/photomorph.html