Plant Responses Flashcards
What are the main reasons plants respond to their environment
Avoid abiotic stress
Maximise photosynthesis
Obtain more light, water, minerals
Avoid herbivory/grazing/disease
Ensure germination in suitable conditions, or pollination, or seed dispersal
What are the three chemical plant defences against herbivory
- Tannins - toxic to microbes and taste bitter to animals
- Alkaloids - taste bitter and are sometimes poisonous e.g. nicotine
- Pheromones - some plants produce these signalling molecules in response to grazing to communicate with other plants
Define tropism
A directional growth plant response
Determined by the direction of external stimuli
What are the types of tropism
Phototropism - light
Geotropism - pull of gravity
Chemotropism - attracted by chemicals
Thigmotropism - shoots of climbing plants wind around solid structures
Define nastic responses (nasties)
Non-directional response to external stimuli
Define photoperiodism
Responses to day length or photoperiod
What controls plant responses
Hormones / plant growth regulators
- chemical signals
- target cells/ tissues
- receptors
NOT produced in endocrine glands
Produced in a variety of tissues
Very small quantities
What are the main plant hormones/ growth regulators (5) and what do they do
- auxins - inhibit leaf abscission, cell elongation and inhibit lateral growth
- gibberellins - seed germination, stem growth
- abscisic acid - stomatal closure, inhibits seed germination and growth
- ethene - promote fruit ripening
- cytokinins - promote cell division
Where are auxins produced then transported to
At the apices (apex)
Transported by diffusion or active transport
To the Zone of Elongation
Describe phototropism (positive)
Illumination causes auxins to move down the shoot tip and causes elongation of cells
If the illumination is only on one side, auxins will move to the shaded side causing elongation and the shoot bends towards the light
What occurs in the zone of elongation
Build up of hydrogen ions accumulates in cell wall (due to hydrogen pump)
Causing a low pH which is optimum for expansins (wall loosening enzymes) - breaking bonds within cellulose
And increased H+ conc disrupts hydrogen bonds
Cell walls become less rigid and cell expands as it takes on water
What are the differences between negative and positive phototropism
Negative - in the roots, auxins inhibit elongation on the shaded side root bends away from the light
Positive - in the apex of shoots, auxins promote elongation on the shaded side, shoot bends towards light
Define abscission
A leaf or fruit fall from a Plant
What is the cause of abscission
Auxin production decreases,
ETHENE production increases
Causes production of CELLULASE - digests cell walls in the abscission zone
Petiole separates from stem
Define apical dominance
Auxins from the apex (apical bud) inhibit lateral growth
Describe how gibberellins promote seed germination
- seed absorbs water and embryo releases gibberellin
- gibberellin causes amylase to be produced
- amylase breaks down starch into glucose
- glucose used for respiration so embryo can grow
How does abscisic acid promote stomatal closure
ABA triggers stomatal closure when there is a lack of water
ABA binds to receptors on the guard cell membranes
Causes K+ ions to leave the cell
Raises water potential of cells
Water leaves cell via osmosis and stomata closes
How is leaf loss (abscission) in deciduous plants controlled?
- As leaf ages, cytokinin and auxin levels lower, ethene levels increase
2.triggers production of cellulase enzymes which weaken leaves by breaking down cell walls in abscission layer - Leaves break from the branch. Below abscission layer, Suberin layer forms to prevent entry of pathogens
List the functions of gibberellins
Stimulate:
Germination
Elongation at cell internodes
Fruit growth
Rapid growth/ flowering
List the functions of auxins
Involved in trophies responses e.g. IAA
Control cell elongation
Suppress lateral buds to maintain apical dominance
Promote root growth e.g. in rooting powders
How are auxins and cytokinins used commercially?
Auxins: rooting powder, growing seedless fruit, herbicides, low concentrations prevent leaf and fruit growth, high concentrations prevent leaf and fruit growth, high concentrations promote fruit drop
Cytokinins: prevent yellowing of lettuce leaves, promotes shoot growth
How are gibberellins and ethene used commercially?
Gibberellins: delay senescence in citrus, elongation of apples and grape stalks, brewing beer for malt production, increase sugar cane yield, speed up formation in conifers, prevent lodging
Ethene: speeds up ripening, promotes lateral growth, promotes fruit drop Cytokinins
What statistical test is most appropriate for testing whether there was a significant different between the stem lengths of the seedlings in group a and group b
And why
(Unpaired) T-test
Idea of comparing two means
