Biology Unit 5.4.1- Plant Responses

Why plants respond to the environment
1. Plants respond to stimuli
  1. responding to external stimuli is how plants avoid being stressed or being eaten and to help them survive long enough to reproduce
  2. Tropism- directional growth response (hormones) in which direction of the response is determined by the direction of the external stimuli
    1. Phototropism- shoot grows towards light (positive phototrophic). enables them to photosynthesise. (more shoots, less roots)
    2. Geotropism- roots grow towards pull of gravity- anchors them in soil and helps them take up water. needed for support (Turgid), photosynthesise and cool plant, water has minerals (nitrates) needed for amino synthesis
    3. chemotropism- on flower, pollen tubes grow down the style, attracted by chemicals towards ovary (fertilisation occurs)
    4. Thigmotropism- shoots of climbing plant (ivy) wind around other plants to gain support
    5. Hydrotropism
  3. Animals respond to biotic (living) and abiotic (non livivng) components of environment.
2. What controls plant hormones
  1. Hormones co ordinate plant response
  2. plant hormones reffered to as plant growth regulators because unlike animals, hormones not produced in endocrine gland but by cells in various tissues in plant
  3. Hormones move around by
    1. Active transport
    2. diffusion
    3. Mass flow- in phloem sap/ xylem vessels
3. Plant hormones and their effects
  1. Auxins (IAA- indole-3-acetic acid)
    1. promotes cell elongation, inhibits growth of side shoots, inhibints leaf abscission (falling off)
  2. Cytokinin
    1. promotes cell division
  3. Gibberellins
    1. promotes seed germination and stem growth
  4. Abscisic acid
    1. inhibits seed germination and growth, causes stomatal closures when plant is stressed
  5. ethene
    1. protoes fruit ripening
How plants respond to the environment
1. Plant Growth
  1. Cell wall of plant limit ability to expand and divide. therefore, growth occurs in particular places of plants where there are immature cells that are capable of diving. these are MERISTEMS
  2. Meristems...
    1. Apical
      1. located at tip/apices of roots, shoots. responsible for roots and shoots getting longer
    2. Lateral bud
      1. Found in bids, give rise to side shoots
    3. Lateral
      1. cylinder near outside of roots and shoots. responsible for roots and shoots widening
    4. Intercalary
      1. between nodes where leaves and buds branch of stem. growth between nodes responsible for shoot getting longer
  3. cell division occurs close to apex--> cell elongation behind apex --> auxins produced in apex--> auxins travel (diffusion, AT) --> cells in zone of elongation --> elongate/shoots grow
  4. extent to which cell elongates is proportional to conc of auxlins.. more Auxins = increase in wall stretchiness by promoting AT of H+ by ATPase on plasma membrane--> cell wall--> lowers pH therefore, optimum condition for wall loosening enzymes to work Expanisins- break cellulose bonds, increase H+ conc, disrupt hydrogen bonds with wall therefore, less rigid and expands to take in water.
2. What causes phototropism
  1. 1. shoots bend towards light- phototropism--> shaded side elongates faster than illuminated--> pushes shoot end towards light.
  2. 2. Auxins transported to shaded side, increases rate of elongation.
  3. 3. enzymes identified- Phototropin 1, phototropin 2 (promoted by blue light). there is alot of activity of P1 on light side, thought that change in gradient causes redistribution of auxins
3. Shedding leaves
  1. Cytokinins stop leaves of deciduous trees senescing (ageing). Leaf acts as sink for phloem transport, therefore, leaf garunteed good supply of nutrients.
  2. cytokinins conc drops, supply of nutrients drop, senescing begins, leaves shed (abscission)
  3. Auxins inhibit Abscission by acting on cells in abscission zone BUT..
    1. leaf sensecing causes auxin production to drop at tip
    2. makes cells more sensative in zone to other growing substances (ethene)
    3. low auxin conc, high ethene conc
    4. increase in cellulase digest cell walls in abcission zone, separating pepitole from stem
Controlling plant growth
1. Apical dominane
  1. growing apical bud at tip inhibits growth of lateral buds futher down shoot. When tip removed, auxin conc in shoot low and bud grows.
  2. Hypothesis test: applied auxin containing paste to cut end of shoot, lateral end didnt grow
  3. other factors may cause lateral bud to not grow- oxygen exposure to cut end could produce hormone promoting lateral bud growth
  4. Thimann and Skoog applied ring of auxin transport inhibitors below shoot apex. lateral bud grew. thereofre, norm auxin conc in lateral inhibits growth and decreased conc promotes growth. BUT both variables may be affected by a third.
  5. Gocal- auxin conc in lateral of kidney bean plant was high when shoot cut. 2 hormones invovled:
    1. Absicicis acid- inhibit bud growth, increase auxin conc, keep acid levels high, when tip removed, hormone decrease, bud grows
    2. Cytokinins- bud growth. direct application = override apical dominane effect. High auxin conc = shoot apex a sink for cytokinins in root. Meant that most cytokinin goes to shoot apex. When removed, cytokinin spread evenly around plant promoting growth
2. Gibberellins and stem elongation
  1. giberellic acid responsible for plant stem growth
Commercial use of plant hormones
1. Auxins
  1. 1. Taking cuttings- dip cut end into rooting powder before planting. Encourages root growth as it contains auxins, fungicides and talcum powder
  2. 2. Seedless fruits- Treating unpollinated with auxins poromotes seedless fruits, promotes ovule growth, triggers automatic auxin production
  3. 3. Herbicides- Transported in phloem to all plants for a longer effect as there are no close fit enzymes
2. Gibberellins
  1. 1. Fruit production- delays senescene in citrus, extending time for fruit picking. improves apple shape, elongates grape stalks
  2. 2. Brewing- Speed up Brewing process of alcohol
  3. 3. sugar production- spraying sugar cane stimulates growth between nodes and males stems longer thereofore more sugar yield (4.5 tonnes by hectar)
  4. 4. Plant Breeding- speed up growth processes by inducing seed fermentation in young trees
3. Cytokinins- prevents yellowing in pickled lettuce leaves, promotes bud and shoot growth. short shoots with many branches produced
4. Ethene- speeds up fruit ripening (apple, tomatoes, citrus), promotes fruit drop (cotton, cherry, walnuts), promotes female sex expression (cucumbers), decrease chances of self pollination and increase yield. lateral growth

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Biology Unit 5.4.1- Plant Responses

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	Created by Sarah Pirbhai over 11 years ago