Plant Hormones (OCR A Level Biology): Revision Note

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

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