Plant Responses

Growth and apical dominance, seed germination, abiotic stress, Herbivory, tropisms, commercial use of hormones

  • Created by: jesscamp
  • Created on: 09-03-19 15:38

Seed Germination

  • Seed absorbs water, which stimulates gibberellin production. Enzymes breakdown food stores - ATP production.
  • Gibberellins switch on genes for amylase and protease.
  • ABA interferes with gibberellin, determining germination.
  • Experimental evidence: mutant seeds with no gibberellin don't germinate, but with external application it occurs; application of gibberellin biosynthesis inhibitors means seeds stay dormant.
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Growth and apical dominance


  • Stimulate growth of main apical shoot: auxins synthesised in meristem cells; auxins bind to receptor sites, forming vacuoles and lowering pH to increase flexibility; large central vacuoles created due to increased absorption of water; enzymes destroy auxin so no more elongation.
  • Suppresses lateral shoot growth (apical dominance): auxin inhibits lower shoot growth; apical shoot removed means lateral shoot growth due to absence.
  • Auxin in root tips stimulates growth: apical shoot removed reduces root growth, but too much inhibits.


  • Affect internode length (regions between leaves)
  • Without them, stems are shorter so there is less waste and they are less vulnerable to weather and harvesting.

Synergism - complementary hormones work together to give greater response.

Antagonism - opposite effects balance to determine response.

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Responses to abiotic stress

Leaf loss and abscission

  • Breaks dormancy of buds, timing of flowering and tubers formed for winter.
  • Phytochrome (Pr and Pfr) - different types of light and ratio depends on levels
  • Lack of light lowers auxin levels, producing ethene. Abscission zones are ethene-sensitive.
  • Enzymes digest outer layer of the zone and vascular bundles sealed off. Fatty material on stem side forms a scar to protect from pathogens.
  • Cells retain water, putting stress on outer layer, causes leaf to break off.

Preventing freezing

  • Cytoplasm and sap have low freezing points. Act as antifreeze.
  • Gene suppression/activiation on fall in temperature or day length.

Stomatal control

  • Opens for cooling and closes for water conservation. 
  • ABA stomatal closure e.g. low soil water levels - lowers potential of guard cells so turgor.
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Responses to herbivory

Physical - thorns, barbs, spikes, stings. Fibrous/inedible tissue, spiny or hairy leaves.


  • Tannins - bitter taste and toxic to insects because inactivate digestive enzymes.
  • Alkaloids - nitrogenous compounds that affect metabolism, sometimes poisons. Caffeine is toxic to fungi and insects and prevents germination of seeds. Nicotine is stored in vacuoles.
  • Terpenoids - form essential oils and is a toxin.

Pheromones - chemical which affects others of the same species. E.g. insects attack maple trees so other leaves produce callose; cabbages produce chemical to attract parasitic wasp which kills and deters cabbage white butterflies.

Folding - frightens larger herbivores, dislodges. Recovery by potassium movement into cells causing water to move in. E.g. mimosa pudica potassium into flexor cells and out of extensors.

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Phototropism practicals:

  • Unilateral light with plant shoot - bends towards the light because auxin moves laterally across the shoot (positive phototropism).
  • Light and tip removed - no response because detect stimulus or produce messenger.
  • Light and lightproof cover - no response because light stimulus needs detection.
  • Light and mica barrier - movement down shaded side causes bending towards light.
  • Light and mica on shaded side - no response because no movement.
  • Light and gelatin block between tip and shoot - movement of chemical bends towards light as gelatin allows passing of chemicals.
  • In the dark - grows directly upwards to photosynthesise due to gibberellins.

Geotropism practicals:

  • Seeds in petri dish stuck to the wall - 90 degree rotation, geotropism within two hrs.
  • Clinostat (rotating drum) rotating slowly and in the dark - stimulus applied, root straight.
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Commercial use of hormones


  • Climacteric fruit, e.g. bananas, avacadoes- ethene continues ripening after harvest. One ripe can spread effect.
  • Non-climacteric fruit, e.g. oranges, strawberries - low ethene. Ethene gas reduces transportation damage.

Hormone rooting powder and micropropogation:

  • Auxin on cut shoots stimulates root production + rooting powers increases root formation (cuttings).
  • Micropropogation produces 1000s from few cells - differentiation controlled.

Weedkillers - synthetic auxins increases growth. Simple, cheap, low toxicity, selective.

Other uses: gibberellins prevent aging and ripening; auxins for seedless fruit; ethene promotes fruit dropping e.g. cotton and walnuts; cytokinins prevent aging and micropropogation controls tissue development.

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