Editing Plant physiology (section)

===Photomorphogenesis===
{{main|Photomorphogenesis}}

While most people know that [[light]] is important for photosynthesis in plants, few realize that plant sensitivity to light plays a role in the control of plant structural development ([[morphogenesis]]).  The use of light to control structural development is called [[photomorphogenesis]], and is dependent upon the presence of specialized [[Photopigment|photoreceptors]], which are chemical [[pigment]]s capable of absorbing specific [[wavelength]]s of light.

Plants use four kinds of photoreceptors:<ref name="Salisbury & Ross 1992" /> [[phytochrome]], [[cryptochrome]], a [[UV-B]] photoreceptor, and [[protochlorophyllide|protochlorophyllide ''a'']].  The first two of these, phytochrome and cryptochrome, are [[photoreceptor protein]]s, complex molecular structures formed by joining a [[protein]] with a light-sensitive pigment.  Cryptochrome is also known as the UV-A photoreceptor, because it absorbs [[ultraviolet]] light in the long wave "A" region.  The UV-B receptor is one or more compounds not yet identified with certainty, though some evidence suggests [[carotene]] or [[riboflavin]] as candidates.<ref name="Fosket 1994">{{cite book| last = Fosket| first = Donald E.| title = Plant Growth and Development: A Molecular Approach| year = 1994| publisher = Academic Press| location = San Diego| isbn = 0-12-262430-0| pages = 498–509 }}</ref>  Protochlorophyllide ''a'', as its name suggests, is a chemical precursor of [[chlorophyll]].

The most studied of the photoreceptors in plants is [[phytochrome]].  It is sensitive to light in the [[red]] and [[far-red]] region of the [[visible spectrum]]. Many flowering plants use it to regulate the time of [[flower]]ing based on the length of day and night ([[photoperiodism]]) and to set circadian rhythms. It also regulates other responses including the germination of seeds, elongation of seedlings, the size, shape and number of leaves, the synthesis of chlorophyll, and the straightening of the [[epicotyl]] or [[hypocotyl]] hook of [[dicot]] seedlings.