Module 5: Plant responses Flashcards
(15 cards)
How do plants coordinate growth and responses?
Plant hormones (like auxins, gibberellins, cytokinins) regulate growth, development, and responses to environmental stimuli. Auxins promote cell elongation and can direct growth toward light.
Environmental Cues: Plants respond to light (phototropism), gravity (gravitropism), and water availability (hydrotropism), adjusting their growth patterns accordingly.
Also, plant cells communicate through plasmodesmata, allowing the exchange of signalling molecules and nutrients.
Examples of abiotic stress for plants?
Freezing, drought.
Example of biotic stress for plants?
Being eaten, so plants produce chemicals. E.g. alkaloids (caffeine, nicotine) are bitter tasting, either deterring or killing herbivores.
Pheromones are signals to nearby plants that its species is under attack, triggering other defences.
Define a tropism.
Directional growth response due to stimulus from environment or something?
Positive tropism - towards a stimulus.
Negative tropism - away from a stimulus.
3 examples of tropisms?
Phototropism
Geotropism/gravitropism
Hydrotropism
Experimental evidence on plant hormones.
- Prepare 3 shoots: one with foil cap, one without, one with the base wrapped in foil.
- Set up shoots in front of light source, each at same distance from light source.
Use of ethene.
Ripens climatic fruits like banana, mango. The fruit is picked and transported unripe as it’s less likely to be damaged in cool conditions. When fruits are on sale, they’re exposed to ethane gas.
Ethene also stimulates the breakdown of cell walls in the abscission layer at the base of the leaf stalk (made up of parenchyma cells), causing leaf to drop off. Auxins inhibit leaf loss and are produced in young leaves, making the leaf stalks insensitive to ethene. But 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.
=> Hormone abscisic acid (ABA) is produced by plants to stimulate stomatal closure. This can happen in high temps or low water supplies.
=> Guard cells have ABA receptors on their cell surface membranes. ABA binds with these, inhibiting the proton pumps and therefore stopping the active transport of hydrogen (H+) ions out of the guard cells.
=> ABA also causes Ca2+ ions to move into cytoplasm of guard cells through cell surface membrane.
Ca2+ ions act as secondary 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 water potential of guard cells. Water leaves guard cells by osmosis.
=> Guard cells become flaccid, so stomata close.
The role of gibberellins in seed germination and stem elongation.
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.
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
Experimental evidence for the role of gibberellin in stem elongation.
=> Gibberellins are hormones that stimulate cell division and elongation in the stem.
=> Dwarf plants have low levels of gibberellins due to a mutation in a gene involved in the synthesis of gibberellins.
Experimental evidence for the role of gibberellin in seed germination.
Seeds of mutant varieties of the Arabidopsis plant that do not produce gibberellins can be induced to germinate if gibberellins are applied.
Seeds of certain lettuce varieties that require light in order to germinate can be made to germinate in the dark if gibberellins are applied.
Auxins DIFFUSE to different sides of the shoot/root.
=> Fewer side chains as auxins inhibit side chain growth. If tip is removed, more cell division can take place so side shoots grow.
=> Auxins stimulate apical shoot growth.
=> High concentration of auxins in roots suppress cell elongation, resulting in shorter fruits.
=> Gibberellins stimulate cell elongation in plant stems.
=> Auxins prevent leaf loss as they’re antagonistic to absiscic acid which causes leaf loss.
=> Cytokinins prevent food aging.
=> Gibbs cause fruit to increase in size by causing cells to divide more. GIbbs delay ripening, imporving size and shape. Not sure.
What is apical dominance?
Caused by auxins.
Gibberellins => discovered by Japanese scientist after investigating Bakanae disease in rice.
Found in stem tips. Stimulates cell division, germination, elongation.
Commercial uses of plant hormones.
=> Selective weed killers: HIGH concentrations of auxins can cause such rapid growth that plant tissues become damaged, allowing pathogens to enter.
=> Rooting powders: at low doses, auxins can be used to stimulate cuttings to grow new roots. The lower end of the cutting is dipped in the powder before being planted in compost and with the correct environmental conditions, roots begin to grow shortly afterwards.
=> Control ripening: ethene used to stimulate fruit to ripen. So bananas can be harvested when unripe (so harder), transported and then ripened artificially using ethene during transport or when they have arrived at their destination.
