Light is vital for photosynthesis. It is necessary to direct plant growth and development. Light acts as a signal to initiate and regulate photoperiodism and photomorphogenesis.
There are two light-sensing systems involved in these responses: a) the red light sensitive system (Phytochrome system) * Phytochrome is a plant photoreceptor. It is found in the leaves of most plants (including all higher plants) and some bacteria. It is a pigment that plants use to detect light. Phytochrome is sensitive to light in the red and far-red region of the visible spectrum. It is a special pigment with two forms, Pr and Pfr, which absorb red light and far red light respectively, giving off a green to blue hue. Phytochrome can be converted from one form to another by different types of light.
Most of the flowering plants use phytochrome to regulate the time of flowering based on the length of day and night (photoperiodism) and to set circadian rhythms. Besides, phytochrome also regulates other responses including elongation of seedlings, controlling what shape and size leaves form, how many leaves form, the germination of seeds (photoblasty), the synthesis of chlorophyll, and the straightening of the epicotyl or hypocotyl hook of dicot seedlings.
b) the blue light sensitive system
Cryptochromes are blue, ultraviolet-A photoreceptors that mediate various light-induced responses in plants and animals. But, they are more important in fungi and primitive plants. Cryptochromes are flavoproteins similar in sequence to photolyases, which catalyze the repair of UV light-damaged DNA, but do not have photolyase activity. Cryptochromes are involved in the circadian rhythms of plants and animals, and in the sensing of magnetic fields in a number of species. Besides, they effect phototropisms (directional growth), stem length, and sleep movements.
The two genes Cry1 and
References: Carol L Thompson and Aziz Sancar (2002, December 16). Photolyase/cryptochrome blue-light photoreceptors use photon energy to repair DNA and reset the circadian clock. Retrieved November 16, 2012 from http://www.nature.com/onc/journal/v21/n58/full/1205958a.html Chentao Lin (2000, August). Plant blue-light receptors. Retrieved November 17, 2012 from http://elmu.umm.ac.id/file.php/1/jurnal/P/PlantScience/Plant%20Science_BioMedNet/001-020/015.pdf Chentao Lin (2012, November 18). Blue Light Receptors And Signal Transduction. Retrieved November 19, 2012 from http://www.plantcell.org/content/14/suppl_1/S207.full.pdf+html Click4Biology. Click4Biology: Topic 9 Plant Science. Retrieved November 17, 2012 from http://click4biology.info/c4b/9/plant9.3.htm#6 Missouri Botanical Garden (2006, April 16). Phytocrome. Retrieved November 18, 2012 from http://www.mobot.org/jwcross/duckweed/phytochrome.htm Wikipedia, The Free Encyclopedia (2012, November 2). Cryptochrome. Retrieved November 16, 2012 from http://en.wikipedia.org/wiki/Cryptochrome Wikipedia, The Free Encyclopedia (2012, October 9). Phytochrome. Retrieved November 16, 2012 from http://en.wikipedia.org/wiki/Phytochrome