16 Plant responses Flashcards
What does auxin do?
- control cell elongation
- prevent leaf fall (abscission)
- involved in tropisms
- maintain apical dominance
- prevent lateral bud growth
What does gibberellin (growth hormones) do?
- Gibberellins stimulate stem elongation and seed germination
- Taller plants have more Le gene which codes for enzymes that forms gibberellins
- stimulate pollen tube growth in fertilisation
- Seed absorbs water, water reaches endodermis layer and stimulates release of gibberellins. Gibberellins stimulate seed germination by triggering the breakdown of starch into glucose. Glucose used as substrate for respiration by the plant embryo and can release energy needed for growth.
- They also stimulate side shoot formation
- gibberellins switch on genes which code for amylases and proteases- the digestive enzymes required for germination.
What does ethene do?
Causes fruit ripening, promotes abscission
What does ABA (Abscisic acid) do?
Maintains dormancy of seeds and buds, stimulates stomatal closing
What are examples of auxins?
Indoleacetic acid (IAA)
What are tannins?
phenols, toxic to insects: they bind to the digestive enzymes produced in saliva and inactivate them.
What are alkaloids?
- Alkaloids are a large group of very bitter tasting, nitrogenous compounds found in many plants.
What hormones play a role in seed germination?
Gibberellins and ABA
How does seed germination occur?
1) When seeds absorb water, the embryo is activated and begins to produce gibberellins.
2) They in turn stimulate the production of enzymes that break down the food stores found in the seed.
3) The embryo plant uses these food stores to produce ATP for building materials so it can grow and break through the seed coat.
4) Evidence suggests that gibberellins switch on genes which code for amylases and proteases- the digestive enzymes required for germination.
Where is indoleacetic acid produced?
Indoleacetic acid is produced in the shoots and roots in flowering plants.
What is the role of auxins such as indoleacetic acid?
- They stimulate the growth of the main, apical shoot.
- Evidence suggests that auxins affect the plasticity of the cell wall; the presence of auxins means the cell wall stretches more easily.
What is apical dominance?
- When high concentrations of auxins suppress the growth of lateral shoots.
- The apical bud (the shoot at the top) is dominant over the lateral buds.
- Prevents side shoots from growing
What does synergism mean?
When different hormones work together and complement each other, giving a greater response than on their own.
Give an example of when hormones are synergistic?
Auxins and gibberellins work together to help plants grow very tall.
What does antagonism mean?
When hormones have opposite effects
Give an example of when hormones are antagonistic.
Gibberellins stimulate growth of side shoots but auxins inhibit the growth of side shoots.
What do low concentrations of auxins promote?
Root growth
What is abiotic stress? Give examples.
Abiotic stress is anything that’s potentially harmful to a plant e.g changes in day length, cold and heat, lack of water, excess water, high winds
What is photoperiodism?
When plants are sensitive to a lack of light in their environment
How do leaves respond to the falling auxin concentration?
They produce the gaseous plant hormone ethene
What is the abscission zone?
- base of the leaf stalk is a region called the abscission zone
- made up of two layers of cells sensitive to ethene.
What does ethene do?
Ethene seems to initiate gene switching in cells resulting in the production of new enzymes.
- These digest and weaken the cell walls in the outer layer of the abscission zone (separation zone)
How do leaves fall off?
- phytochromes detect low light levels, higher levels of ethene and low levels of auxins, ethene triggers production of digestive enzymes like cellulase in the abscission zone. Cellulase digests the cell wall in the separation zone
- The vascular bundles which carry materials into and out of the leaf are sealed off. At the same time fatty material is deposited in the cells on the stem side to form a protective scar when the leaf falls, preventing entry of pathogens.
- Further abiotic factors such as low temperatures, strong winds puts further strain and leaf separates from the plant.
- A neat waterproof scar is left behind
How do leaves prevent freezing?
- The cytoplasm of the plant cells and the sap in the vacuoles contain solutes which lower the freezing point.
- Some plants produce sugars, amino acids and even proteins which act as antifreeze to prevent cytoplasm from freezing
- Most species produce chemicals which make them hardy and frost resistant during winter months
