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• • The life cycle of a flowering plant involves
  • The vegetative phase
  • The part of the life cycle during which plant growth takes places
  • A plant in the vegetative phase consists of roots, stems and leaves
  • A plant in this phase can often reproduce asexually
  • The reproductive phase 
  • The part of the life cycle when flowers are produced by the shoot apical meristems 
  • A plant in the reproductive phase can reproduce sexually
• • Sexual reproduction involves two parents and the fusion of two gametes
  • In order for a plant to move from the vegetative phase to the reproductive stage, meristems in the shoot needs to be stimulated to produce a flower rather than a leaf
  • This stimulation can be brought about by changes in certain abiotic factors such as temperature or night length which alter gene expression in the shoot apical meristem
• The controlling factor in flowering is the length of the night
• It was previously thought that day length was the controlling factor
• Flowering control


  1. When the night reaches a certain critical length, genes that control flowering in the plant may be switched on or off

  2. Genes that are switched on are expressed, leading to production of the polypeptides for which they code

  3. Genes that are switched off are not expressed, so the polypeptides for which they code are not produced
• Critical length


  Determines the quantities of different forms of a pigment called phytochrome in the leaf
• Phytochrome


  A pigment that exists in two forms: PR (inactive, absorbs red light) and PFR (active, absorbs far red light)
• Phytochrome


  Reversible conversion between PR and PFR forms
• Absorption of different wavelengths of light

  Causes conversion between PR and PFR forms
• PR absorbs red light (660 nm)


  Converted into PFR
• PFR absorbs far red light (730 nm)


  Converted back into PR
• In the absence of red light

  Unstable PFR gradually converts back into PR
• During the day


  Levels of PFR rise
• Sunlight contains more wavelengths at 660 nm than 730


  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

  PFR converts slowly back to PR
• Long day plants


  Plants that flower when the nights are shorter than the critical length, e.g. in summer
• Flowering in long day plants

  1. Days are long so PR is converted to PFR at a greater rate than PFR is converted to PR

  2. The active form of phytochrome, PFR, is present at high levels

  3. High levels of PFR activate flowering

  4. PFR binds to receptors in the cytoplasm of the leaf cells, activating expression of the flowering time (FT) gene

  5. The active gene is transcribed and translated in the shoot apex

  6. The resulting protein binds to a transcription factor which leads to the expression of multiple genes associated with flowering and converts the meristem from vegetative to reproductive
• Short day plants


  Plants that flower when the nights are longer than the critical length, e.g. autumn
• Flowering in short day plants

  1. PFR inhibits flowering in short day plants when it is present at high enough levels

  2. PFR binds to receptors in the cytoplasm of the leaf cells and inhibits transcription and translation of the flowering time (FT) gene

  3. Long nights lead to low levels of PFR due to the slow conversion of PR back into PFR in the absence of red light

  4. When PFR levels are low, there is no inhibition of flowering as the FT gene can be transcribed and translated

  5. The resulting protein binds to a transcription factor which leads to the expression of multiple genes associated with flowering and converts the meristem from vegetative to reproductive
• Normal flowering times in plants

  Require the critical dark period to be uninterrupted
• Sometimes it might be necessary to induce flowering at times when plants would normally be in their vegetative state
• Growers of flowers which are sold as cut flowers


  • Can provide flowers out of season through manipulation of night length
• Flowering in short day plants

  Can be induced during the longer day season, such as during the summer, by providing a longer dark period e.g. by covering plants up
• Flowering in long day plants

  Can be induced during the winter months when days are short, by providing periods of light during the night time