Seed germination

• What happens when you add gibberellins to short plants?

  • They grow
  • Promote germination and have an effect on internode length
  • Greater amount of GB- longer internodes
  • Smaller amount of GB- shorter internodes
• What evidence is there to prove GB effects?

  • Comparison of GB concs in dwarf and tall pea plants
  • Tall pea plants had higher conc of GB
  • Scientists found short plants were homozygous for recessive allele (le) and big plants were homozygous for a dominant allele (Le) (responsible for high GB concs)
• What does the Le gene do?

  • Codes for enzyme that synthesises gibberellins
  • If you graft a plant with a dominant Le gene into homozygous le plant, the plant grows tall
• Describe how GB is responsible for seed germination

  switch on genes for production of amylases and proteases. Evidence that abscisic acid (ABA) is antagonist to gibberellins and relative amounts of these two hormones affect seed germination

  • When seed absorbs water, embryo is activated and produces gibberellins
  • Gibberelins stimulate production of enzymes that break down food stores in seed
  • Seedling grows and breaks through seed coat
• Explain experimental evidence for seed germination (2)

  • Mutant varieties without the gene for GBs don’t germinate . External application of GBs causes seed to germinate
  • When biosynthesis or protein synthesis inhibitors are added, seed is unable to germinate as they cannot make gibberellins needed for them to break dormancy when inhibitors removed, or when more GBs are added seeds will germinate
• What determines whether or not a seed will germinate?

  Relative levels of ABA and gibberellin