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
