Phytochrome system
The biologically-inactive form of phytochrome (Pr) is converted to the biologically-active form Pfr under illumination with red light. Far-red light and darkness convert the molecule back to the inactive form.
Source
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"OpenStax College, Plant Sensory Systems and Responses. October 17, 2013."
http://cnx.org/content/m44711/latest/Figure_30_06_01.jpg
OpenStax CNX
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