Plant Hormones (OCR A Level Biology)

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Plant Hormones

The role of hormones in leaf loss

  • Deciduous plants lose their leaves in very hot and dry environmental conditions, in order to reduce water loss
  • Deciduous plants in temperate climates also do this during winter when absorption of water is difficult due to frozen soils
    • The shedding of leaves at this time is also due to photosynthesis being limited by low temperatures and reduced light

  • Hormones are responsible for this leaf loss
    • In temperate climates, these hormones are produced in response to shortening day length in the autumn

  • A layer of cells known as the abscission layer develops at the base of the leaf stalk
    • This is a layer of parenchyma cells with thin walls, making them weak and easy to break

  • The plant hormone ethene stimulates the breakdown of cell walls in this abscission layer, causing the leaf to drop off
  • Auxins also play a role in leaf loss
    • Usually, auxins inhibit leaf loss and are produced in young leaves, making the leaf stalks insensitive to ethene
    • The concentration of auxins in leaves decreases as they age until leaf loss can once again occur in response to ethene

The role of hormones in stomatal closure

  • During times of water stress, the hormone abscisic acid (ABA) is produced by plants to stimulate the closing of their stomata
    • Certain environmental conditions can cause water stress, such as very high temperatures or reduced water supplies

  • Guard cells have ABA receptors on their cell surface membranes
  • ABA binds with these receptors, inhibiting the proton pumps and therefore stopping the active transport of hydrogen (H+) ions out of the guard cells
  • ABA also causes calcium (Ca2+) ions to move into the cytoplasm of the guard cells through the cell surface membranes
  • The calcium ions act as second messengers:
    • They cause channel proteins to open that allow negatively charged ions to leave the guard cells
    • This stimulates the opening of further channel proteins that allow potassium (K+) ions to leave the guard cells
    • The calcium ions also stimulate the closing of channel proteins that allow potassium (K+) ions to enter the guard cells

  • This loss of ions increases the water potential of the guard cells
  • Water leaves the guard cells by osmosis
  • The guard cells become flaccid, causing the stomata to close

Closure of a stoma in response to abscisic acid (ABA)

The role of hormones in seed germination

  • Gibberellins are a type of plant hormone involved in controlling seed germination and stem elongation
  • When a seed is shed from the parent plant, it is in a state of dormancy (contains very little water and is metabolically inactive)
  • This allows the seed to survive harsh conditions until the conditions are right for successful germination (eg. the seed can survive a cold winter until temperatures rise again in spring)
  • The seed contains:
    • An embryo – will grow into the new plant when the seed germinates
    • An endosperm – a starch-containing energy store surrounding the embryo
    • An aleurone layer – a protein-rich layer on the outer edge of the endosperm

  • When the conditions are right, the barley seed starts to absorb water to begin the process of germination
  • This stimulates the embryo to produce gibberellins
  • Gibberellin molecules diffuse into the aleurone layer and stimulate the cells there to synthesise amylase
    • In barley seeds, it has been shown that gibberellin does this by regulating genes involved in the synthesis of amylase, causing an increase in the transcription of mRNA coding for amylase

  • The amylase hydrolyses starch molecules in the endosperm, producing soluble maltose molecules
  • The maltose is converted to glucose and transported to the embryo
  • This glucose can be respired by the embryo, breaking dormancy and providing the embryo with the energy needed for it to grow

Gibberellin in barley germination (1), downloadable AS & A Level Biology revision notesGibberellin in barley germination (2), downloadable AS & A Level Biology revision notes

The role of gibberellin in the germination of a barley seed

  • Abscisic acid has the opposite effect to gibberellins, maintaining dormancy by inhibiting amylase production
  • The start of germination is therefore determined by the balance of abscisic acid and gibberellins present in the seed

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Alistair

Author: Alistair

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.