Plant Responses

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Why Plants Respond

Biotic - Living Factors
Abiotic - Non-living factors 

Tropism - a directional growth in response to an external stimulus
Phototropism - shoots grow towards light
Geotropsim - roots grow towards the gravitational pull
Chemotropism - chemicals

Nastic Movement - rapid response of a plant to a stimulus, usually brought about by changes in the turgitity of specialised cells

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How Plants Respond

  • Plant hormones coordinate plant responses to environmental stimuli
  • They are not made in glands but in groups of cells in certain parts of the plant
  • Hormones bind to receptor sites on the target cells membrane which results in a response
  • They can move around via active transport, diffusion or mass flow in phloem or xylem


  • Light receptors known as phototropins sit in plasma membrane of cells in plant shoots
  • When blue light hit one side of the shoot these cells become phosphorylated, which brings about a movement of auxin to the shady side of the shoot
  • The presence of auxin causes the cells on one side to enlongate - auxin binds to the receptor on the plasma membrane, causing a build up of hydrogen ions in the cell wall
  • pH lowers activating enzymes which break cross links in the cell wall, making it easier to become stretched 
  • water moves into the cell via osmosis causing it to swell and become longer
  • this causes the plants shoot to bend towards the light
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How Plants Respond - 2

Apical Dominence

  • A shoots apical meristem lies at the top of the shoot and contains cells that are contranly diviving causing the shoot to increase in length
  • Lateral buds are meristems on the side of the plant but arnt usually active if the apical meritstem is active - apical dominance
  • Auxin is made in the tip of the shoot - it travels down to the lateral buds and inhibits their activity
  • This can be seen in experiments - if the apical meristem is cut off two different shoots but synthetic auxin is added to one then this one with show apical dominence and no shoots will grow but the lateral buds on the other shoot will grow 
  • Auxin doesnt travel upwards against gravity - if a growing shoot is turned up side down no auxin will travel to the lateral buds and they will begin to grow - apical dominence is prevented


  • If low concentraions of gibberelin is applied to the stems of dwarf beans they begin to grow rapidly to the size of non-dwarf beans - dwarf beans did not have the gene to produce GA and so their stalks didnt grow as much
  • Gibberellin moves through the plasma membrane of a cell and binds to a receptor protein - this causes the breakdown of DELLA protein, allowing transcription factors to be released and the transcription of the gene can begin
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Leaf Abscission

Leaf Abscission - when plants and trees drop their leaves

  • helps plants reduce water loss through their leaf surfaces, avoid frost famage, or reduce energy consumption when leaves cannot photosynthesise
  • hormones involved are auxin, ethene and abscisic acid
  • Auxin inhibits abscission, ethene is a gas hormone, and absicisc acid is a plants stress hormone - eg produced when theres a lack of water or low temperature
  • As leaves age they produce less auxin, making them more sensitive to ethene - ethene inhibits auxin production
  • this causes an abscission layer to form at the base of the leaf stalk - made of thin walled cells which are weakened by enzymes until the leaf falls off
  • before this the tree/plant will create a protective layer of cells containing suberin, which is waterproff
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Commercial uses of plant hormones


  • can be sprayed onto developing fruits to prevent abscission - fruits stay on the plants for londer so can be harvested when fully ripe
  • can be sprayed onto flowers where it initiates fruit production before pollination so to create seedless fruit
  • can be applied to lower cut ends of shoot to stimulate root production
  • can act as herbicides - can promote root growth to such an extend that the stem cannot support itself and the plant buckles and dies


  • allows fruit to stay longer on the trees, allowing it to grow to maximum size
  • allows the stems of sugar cane to grow longer, therefore storing more sucrose
  • can cause fruit to grow even when not fertillised, leading to seedless fruit
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Commercial uses of plant hormones - 2


  • ethene causes ripening - fruit can be picked before it is ripe and transported long distances - it can then be exposed to ethene to make it ripe ready for sale
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