Chapter 16 Plant Hormones Flashcards
What is a tropism?
Directional growth in response to environmental cues such as gravity and light
What are the general roles of auxins in plants?
(IAA is an example of an Auxin)
Cell elongation- ph, plasticity
Apical Dominance
Tropisms
Inhibit abscission by inhibiting ethene production increasing
Fruit ripening
What are the general roles of gibberellins in plants?
Stem elongation
Mobilisation of food stores in seeds at germination
Pollen tube growth
What are the general roles of ethene in plants?
Fruit ripening
Abscission ( leaf loss)
What are the general roles of ABA in plants?
Maintain dormancy
Cold responses
Stomatal closing
Why is it difficult to understand the role of hormones in plants?
Some effects are hormonal and environment- hard to isolate what is causing the change
Work at very low concentration
How do hormones affects seed germination?
When a seed absorbs water, it becomes activated
Gibberellins switch on genes for amylases and proteases, needed for germination, digest food stores
ABA antagonist to Gibberellins
What is the experimental evidence for the role of gibberellins within germination?
Mutants without the gene for gibberellins won’t germinate unless external gibberellin is added
If gibberellin synthesis inhibitors are added, germination cannot continue
Chemically, how do auxins stimulate the main growth of the apical shoot?
Auxin binds to cell surface receptors, causing the pH to fall to 5
Optimum pH for enzymes which keep the cell wall flexible so expand
As cells mature, auxin is destroyed, pH rises, enzymes less efficient less stretchy so cannot grow
So only near top can grow as there is where the auxin is made and diffuses down from
Physically, how do auxins enable apical dominance, with respect to other shoots? and what evidence is there?
Inhibit growth of lateral shoots, move down the stem
Near the bottom, lower conc of auxin so lateral can grow
Evidence: if apical is removed, lateral shoots can grow. If auxin applied, apical
How does auxin affect roots? How does this compare to lateral buds and the apical shoot
Low concentration promote growth
There comes a point when if the concentration is too high, the auxins will inhibit growth rather than promote it
The same thing will occur to lateral buds and the apical shoot, however, the concentration of auxins must be far higher to inhibit growth, more for apical shoot
How do gibberellins affect growth of the plant?
Increases the length of the region which is effectively the stems
Without gibberellins, plants are much shorte
What is synergism and antagonism?
Synergism- hormones compliment each other, giving a greater response
Antagonism- hormones have opposite effects, with the balance determining the response
Why do trees lose their leaves?
In winter there are short days and long nights. And low temperatures
There comes a point where the amount of glucose respires to maintain leaves and anti freeze is greater than the amount of glucose produced by photosynthesis
And trees with leaves are more likely to be damaged by winter gales
So trees lose their leaves
What is photoperiodism and how does it work within plants?
A sensitivity to a lack of light due a pigment of phytochrome
Phytochrome r and Phytochrome Fr
Ratio of the two changes depending on the level of light
What is the mechanism of abscission?
Lower light levels means a lower concentration of auxin
In response, cells produce and secrete ethene
Ethene has target cells in the abscission zone, two layers of cells between the base of the stalk and the leaf
Ethene turns on the genes to produce digestive enzymes to digest the cells walls of the outer layer of the abscission layer, called the separation layer
The vascular bundles are sealed and fatty material deposited on the stem side, when leaves fall, a protective scar will form
Cells within the separation layer retain more water due to hormonal cues
With wind, the strain is too much and the leaves fall
How does cold protection within the leaves work?
The cytoplasm and vacuole contain solutes which lower the freezing/melting point, as impurities effectively
Sugars, polysaccharides and amino acids act as antifreeze
Sustained cold weather activates genes to produce chemicals which make the plant hardy and tough
What is the mechanism for stomatal closure?
In response to abiotic stress, the leaves and roots can release ABA
Bind to receptors in the cytoplasm of guard cells, inhibiting proton pumps, preventing H+ leaving the guard cells
Also allows Ca2+ to enter the cell, acting as secondary messengers
-Open channels to allow negative ions to leave, and others to allow K+ to leave and shut those that allow K+ in
Lower solute concentration, water leaves via osmosis, flaccid
What is herbivory? What are some physical defences to herbivory?
The process by which plants are eaten by animals
Thorns, Barbs, Spikes, Inedible tissue, hairy leaves
What are some examples of chemical plant defences?
Tannins- bitter, toxic to insects as they bind to digestive enzymes and deactivate them
Alkaloids- bitter, nitrogenous compounds like caffeine and morphine, can affect metabolism
Terpenoids- essential oils but toxins to insects and fungi
What are pheromones? What do plants use instead?
Chemicals which affect the social behaviour of members of the same species
Plants use volatile organic compounds to signal to other species
What is an example of how a plant uses VOCs?
Cabbages attacked by caterpillar
Plants secrete a compound which attracts parasitic wasps which lay eggs in the caterpillars, killing them
How does folding in response to touch help plants?
Dislodges insects, frightens herbivores
Potassium ion movement into cells means water osmoses in, returns to original placement
What is a phototropism? What is a geotropism? What is a chemotropism? Why do tropisms occur?
Response in growth to light, gravity, chemicals
Enables maximum use of environmental conditions e.g keep growing within the light, roots anchored in
