5.5.3 Plant Hormones

Plant Hormones

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

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)

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 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