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5.1.5 Plant Responses (Auxins (Mechanism of auxin's effect…
5.1.5 Plant Responses
Auxins
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commercial use
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rooting powder: after taking cuttings, dip the end into rooting powder before planting to encourage root growth
seedless fruit: treat unpollinated flowers to promote growth of seedless fruit; auxin promotes ovule growth; triggers automatic production of auxin by tissues in developing fruit
herbicides: kill weeds; plants find them more difficult to break down so act in plant for longer; promote shoot growth so much that the stem cannot support itself; buckles and dies
regulate plant growth; inhibit leaf abscission (fall); inhibit growth of side-shoots; promote cell elongation
apical dominance: inhibition of lateral buds further down the shoot by chemicals produced by the apical bud at the top of the plant
if apical bud is removed, the plants grow side shoots (lateral buds), and auxin levels in shoot drops
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root and shoot cells in the elongation zone have different responses to same concentration of auxin; concentrations that stimulate shoot growth inhibit root growth
Roles of plant hormones:
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hormones bind to complementary receptors on the plasma membrane of target cells to bring about a response
move around plant via active transport, diffusion, or mass flow in phloem sap or xylem vessels
plant growth occurs:
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intercalary meristems between nodes where leaves and buds branch off the stem; growth between nodes responsible for shoot getting longer
normal plant cells have a cell wall which limits their ability to divide and expand; meristem cells don't have a cell wall so are the only cells capable of dividing in a plant
Abscisic acid
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inhibit bud growth; high auxin in shoot may keep abscisic acid levels high; when tip is removed, abscisic acid levels drop and bud starts to grow
Gibberellins
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commercial use
fruit production: delay senescence in citrus fruit so can be left on the shelf for longer; can make apples elongate to improve shape; elongate grape stalks to make grapes less compacted and get bigger grapes
brewing: barley seeds germinate and aleurone layer of seed produces amylase enzymes to break down starch into maltose; genes for amylase production are switched on my gibberellin; adding more gibberellin can speed it up; malt produced by drying and grinding up the seeds
sugar production: spray sugar cane with gibberellins to stimulate growth between the nodes; stems elongate; sugar cane store sugar in cells of internodes, so more sugar is in each plant
plant breeding: induce seed formation on young trees; spray gibberellin synthesis inhibitors on plants to keep flowers short and stocky, so internodes of crop plants stay short to prevent lodging
lodging: stems bend over because of weight of water collected on ripened seed heads; happens in wet summers; crop is difficult to harvest
causes growth in the internodes by stimulating cell elongation (by loosening cell walls) and cell division (by stimulating production of a protein that controls the cell cycle)
promote seed germination: seed absorbs water, embryo releases gibberellin which travels to aleurone layer in endosperm region of seed; gibberellin enables production of amylase so starch can be broken down into glucose; glucose is respiratory substrate for embryo so it grows; glucose also used for protein synthesis
Plant responses
abiotic stress
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e.g. if temp too high, thicker waxy cuticle to prevent transpiration
tropisms:
phototropism: response to light; shoots grow towards light (+), which allows them to photosynthesis
geotropism: response to gravity; roots grow towards pull of gravity (+); anchors them in the soil and helps take up water and minerals in the soil
chemotropism: response to chemicals; pollen tubes grow down the style in a flower, attracted by chemicals towards the ovary, for fertilisation
thigmotropism: response to touch; shoots of climbing plants wind around other plants or structures for support
a directional growth response in with the direction of the response is determined by the direction of the external stimuli
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herbivory
tannins: toxic to microorganisms and larger herbivores; found in upper epidermis in leaves, make leaf taste bad; prevent infiltration of pathogenic microorganisms in the root
alkaloids: derived from amino acids, taste bitter; feeding deterrent; located in growing tips and flowers and peripheral cell layers of stems and roots
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nastic response: non-directional response to stimuli; leaves fold up in response to touch (Mimosa plants) - thigmonasty
Cytokinins
commercial use
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tissue culture: mass produce plants; promote bud and shoot growth from small amount of tissue from parent plant; short shoot with many side branches produced and can be split into many individual plants
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promote bud growth; can override apical dominance if applied directly to buds; high levels of auxins means high levels of cytokinins go to shoot apex; when apex is removed, cytokinin spreads evenly around the plant
Ethene
commercial use
speed up fruit ripening in apples, tomatoes, citrus fruits
promote fruit drop in cotton, cherry, walnut
promote female sex expression in cucumbers; reduce chance of self pollination (making them taste bitter), and increasing yield
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