phytochromes

• • flowering in plants is controlled by stimulus of night length
  • some plants flower when nights are short(long day plants)and others when nights are long
• phytochromes


  Pr=inactive form of phytochrome, absorbs red light(shorter wave length)

  Pfr=active form of phytochrome, absorbs far red light

  active form stimulates flowering in the plant, inactive form does not
• when Pr absorbs RED LIGHT=Pfr

  when Pfr absorbs FAR RED LIGHT=Pr

  • sunlight contains more red light, so more conversions from Pr to Pfr are occuring and Pfr levels are higher during the day
  • during the night Pr levels rise as red light wavelengths are not availabe in darness and Pfr slowly converts back to Pr
• EXAMPLE-long day plants

  -long day plants flower when the night is short

  • high levels of Pfr(active phytochrome)activates flowering
  • days are long, sunlight hours are long so availability of RED light is high=Pr to Pfr conversions occur at greater rate than Pfr to Pr
  • Pfr activates expression of genes that stimulate flowering-active gene is transcribed and translated-resulting in a protein which causes flowers to be produced instead of stems/leaves
• describe the role of the photosynthetic pigment phytochrome in the flowering of long-day plants(2)

  • in the day, Pr is converted into Pfr by the red light
  • Pfr is the active phytochrome allowing flowering to occur, so more daylight leads to more Pfr so more flowering in these long day plants