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F215 Plant Responses OCR A2 Biology
Terms in this set (27)
A directional growth response in which the direction of the response is determined by the direction of the external stimulus.
Shoots grow towards light (they are positively phototropic), which enables them to photosynthesise.
Roots grow towards the pull of gravity. This anchors them in the soil and helps them to take up water, which is needed for support (to keep cells turgid), as a raw material for photosynthesis and to help cool the plant. There will also be minerals, such as nitrate ions, in the water, needed for the synthesis of amino acids.
On a flower, pollen tubes grow down the style, attracted by chemicals, towards the ovary where fertilisation can take place.
Shoots of climbing plants, such as ivy, wind around other plants or solid structures and gain support.
Why are plant hormones called plant growth regulators?
Because unlike animal hormones, they are not produced in endocrine glands, but by cells in a variety of different tissues in the plant.
Promote cell elongation; inhibit growth of side-shoots; inhibit leaf abscission (leaf fall).
Promote cell division.
Promote seed germination and growth of stems.
Inhibits seed germination and growth; causes stomatal closure when the plant is stressed by low water availability.
Promotes fruit ripening.
Located at the tips or apices (singular: apex) of roots amd shoots, and are responsible for the roots and shoots getting longer.
Lateral Bud Meristems
Found in the buds. These could give rise to side shoots.
Found in a cylinder near the outside of roots and shoots and are responsible for the roots and shoots getting wider.
Intercalary Meristems (some plants)
Located between the nodes (where the leaves and buds branch off the stem). Growth between the nodes is responsible for the shoot getting longer.
Groups of immature cells that are still capable of dividing.
The role of auxins in apical dominance
- High concentrations of auxins inhibits lateral bud growth and low levels of auxins encourages lateral bud growth
- Auxins are produced by the apical bud and move down the stem.
- At the top of the stem the auxin concentration is high and constantly produced The auxins inhibit the growth of lateral buds which remain dormant.
- Further down the stem when the concentration of auxins is lower the lateral buds are stimulated to grow; this means plants grow into a cone shape so higher leaves don't shade lower ones.
- If the apical bud is removed, the dormant lateral buds grow due to the lack of auxin.
- If the apical bud is removed and auxins are placed on the cut tip the lateral buds will remain dormant.
The role of gibberellin in stem elongation
Gibberellin (a protein hormone) brings about stem elongation in most plants by loosening cell walls and therefore causing cell elongation and also by stimulating cell division by stimulating the production of a protein that controls the cell cycle.
Gibberellin works by affecting gene expression. Gibberellin moves through the plasma membrane and into the cells nucleus where it binds to a series of receptor proteins eventually resulting in the breakdown of DELLA protein allowing transcription of the gene. DELLA proteins bind to transcription factors inhibit cell division and expansion.
Experimental evidence comes from investigation using a species of pea plant which has two forms: dwarf and tall.
When genetically dwarf plants are treated with gibberellic acid they elongate considerably and resemble tall plants.
The Le allele codes for an enzyme used to synthesise gibberellic acid.
The le allele does not code for the enzyme and therefore dwarf plants (homozygous recessive) do not produce gibberellins.
Fruit ripening (Use of Ethene)
Fruit is picked and shipped before it is ripe, then it is hung in rooms with ethene gas in the atmosphere to promote ripening.
Encourage fruit drop (Use of Ethene)
Ethene is released in orchards to make all fruits ready for picking at the same time.
Keeping lettuce green (Use of Cytokinins)
Cytokinins delay leaf senescence so leafy vegetables and cut flowers stay green for longer.
Micropropagation (Use of Cytokinins)
Cytokinins promote bud and shoot growth in small tissue samples taken from the parent plant.
Sugar production (Use of Gibberellins)
Gibberellins elongate the stems of sugar canes meaning more sugar.
Extending fruit shelf life (Use of Gibberellins)
Gibberellins delay the ripening of citrus fruit.
Stimulate seed germination (barley malting)
Taking cuttings (Use of Auxins)
Dipping the end of the cutting in auxins promotes the growth of roots.
Producing seedless fruit (Use of Auxins)
Treating unpollinated flowers with auxins stimulates fruit growth without fertilisation; consequently the fruit has no seeds.
Herbicides (Use of Auxins)
Auxins promote shoot growth to such an extent that the stem falls over and dies.
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