• Flowering in plants is controlled by the?stimulus?of night length
  • Nights are shorter during the spring and summer and longer in the autumn and winter
  • Some plants flower when nights are short and some flower when nights are long
• When the nights reach a certain length, genes that control flowering may be?switched on?or?off,?leading to the?activation or inhibition of flowering
  • Genes that are switched on?are?expressed, leading to?production of the polypeptides?for which they code, while genes that are switched off?are not expressed, so the polypeptides for which they code?are not produced
• The length of night can be detected by a plant because it determines the quantities of different forms of a pigment called?phytochrome?in the leaf

Phytochrome

• The phytochrome pigment exists in two forms
  • PR?is the?inactive form?of phytochrome, it absorbs light from the?red?part of the spectrum (wavelength 660 nm)
  • PFR?is the?active form?of phytochrome, it absorbs light from the?far red?part of the spectrum (wavelength 730 nm)
• Absorption of different wavelengths of light causes a?reversible?conversion between the PR?and PFR?forms of phytochrome
• • When?PR?absorbs red light?(660 nm)?it is converted into PFR
  • When?PFR?absorbs far red light?(730 nm)?it is converted back into PR
  • In the?absence of red light,?the unstable?PFR?gradually converts back into PR
• During the day?levels of?PFR?rise
  • Sunlight contains more wavelengths at 660 nm than 730 so the conversion from PR?to PFR?occurs more rapidly in the daytime than the conversion from PFR?to PR
• During the night?levels of?PR?rise
  • Red light wavelengths are not available in the darkness and PFR?converts slowly back to PR

PR?is converted to PFR?in a reversible reaction which controls flowering

E.g. long day plants

• Long day plants flower when the nights are?short?e.g. in summer
  • When nights are short, the?day length is longer, hence the term 'long day plants'
• In?long day plants?high levels of the?active form of phytochrome activate flowering
• Flowering occurs due to the following process
  • Days are long so?PR?is converted to PFR?at a greater rate?than PFR?is converted to PR
  • The?active form of phytochrome, PFR, is present at?high levels
  • High levels of PFR?activate flowering
    • PFR?activates expression of genes that stimulate flowering
    • The active gene is transcribed and translated
    • The resulting protein causes flowers to be produced rather than stems and leaves