Plant growth hormones

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Auxins

Auxins 

  • Auxins promote cell elongation, inhibits the growth of side shoots and abscission (shedding of leaves)
  • It is produced in the apex (the shoot tips) and move by active transport, diffusion or by mass flow in the phloem sap/xylem vessel
  • The elongation is proportional to the concentration of the auxin
  • Auxin works by promoting the active transport of the hydrogen ions through the cell wall by using ATPase enzymes, this means the pH is lowered and encourages the activity of the enzymes that break down the cell wall making it less rigid and allowing more water to move into the cells thus making them expand
  • It can inhibit abscission rates by acting on the abscission zone. However if the auxin production decreases (or the apex is chopped off) then the abscission zone becomes more sensitive and the ethene production increases, the ethene means the activity of the enzyme cellulase increases and the petriole breaks away from the stem.
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The commercial uses of auxin

1) It prevents leaf & fruit drop and promotes flowering

2) However very high auxin levels can promote fruits to drop

3) Cuttings can be dipped into growth promoting powders that contain auxins and fungicides to encourage the growth of roots 

4) Seedless fruit can be grown as a result of auxin application - it allows unpollinated fruits to grow without ovule growth 

5) They can be used as herbicides - they can be added to plants that dont have the enzymes to break down the auxin so the auxin just remains in the plant causing excessive elongation causing the plant to buckle and die 

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Cytokinins

Cytokinins

  • They promote cell division 
  • It can promote the growth of buds - if directly applied onto buds then it can override apical dominance (the tip of the shoots inhibit lateral bud growth - (presence of auxin))
  • The high concentration of auxin makes the shoots a sink for cytokinins so the lateral buds do not recieve much if any at all. When the apex is removed the cyokinins are able to move more evenly around the plant which then allows the growth of side shoots 
  • They prevent senescing (aging) as they make the shoots act as a sink for the contents of the phloem which means they are well supplied with nutrients
  • If the production of cyokinins stop then the shoots are no longer supplied with sufficient quantities of nutrients so therefore the plant begins the process of senescence 
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The commercial uses of cytokinins

  • They can delay leaf senescing so can stop lettuce leaves turning yellow 
  • They can be used on tissue culture to mass produce plants as it encourages the growth of side buds which can then be removed and grown into individual plants 
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Gibberellins

Gibberellins

  • They promote seed germination and growth of stems 
  • The gibberellin family has been found to promote growth of plants, even dwarf plants. In order to get the plants to grow though gibberelic acid needs to go through a number of stages in order to get the correct compounds to encourage the growth.
  • A plant that had no GA20 but had GA1 was grafted onto a plant with GA20 but no GA1 - seperately they wouldnt grow but together they could 
  • It also can cause growth at internodes of plants as it loosens the cell wall and encourages protein production (for elongation and cell division)
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The commercial uses of gibberellins

1) They are used in fruit production, they can elongate the lives of citrus fruits as they prevent senescence, they work with cytokinins to elongate apples, they increase the length of the stems of grapes to make them less compact 

2) It is used in the brewing industry, it switches on the genes for amalyase production which breaks down the strach into maltose which then is used to make the beer

3) Sugar production benefits from gibberellin application as it increases the length of the internodes which is where the sugar is stored and this increases the yield 

4) Planting breeding - gibberellin can induce the seed formation in confier tree's earlier than expected as the confier tree's take a long time to reach adulthood. Also flowers that only flower in the second year can be treated with gibberellin to induce flowering in the first year 

5) Gibberellin synthesis inhibitors can be sprayed onto plants if they are wanted to be kept short and stubby, this means that the stems dont bend and damage the plants due to the weight of the water in the seed heads. 

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Abscisic acid

Abscisic acid 

  • It inhibits seed germination and growth as well as causing stomatal closure/opening when the plant is stressed 
  • If there are high levels of auxin then there will be high levels of abscisic acid, the abscisic acid is stored in the lateral buds which prohibits the growth of the buds and it is only when the auxin concentration falls that the abscisic acid levels do too
  • 90% of water is lost via transpiration and the abscisic acid plays a role in the maintenance of this 
  • It promots abscission and prohibits cell elongation
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Ethene

Ethene 

  • Promotes fruit ripening 
  • A low level of auxin means that the ethene concentration will increase and the sensitivity of the abscission zone increases and the enzyme cellulase digests the cells of the abscission zone and breaks the petiole away from the stem
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The commercial use of ethene

1) Ethene is a gas so needs to be converted into a soltuon that can be sprayed onto the crops - it is easily absorbed and the ethene is released slowly 

2) It can be used to speed up the ripening of apples 

3) Promotes the fruit drop of cherries and walnuts 

4) Promote the female sex expression in cucumbers to prevent them self pollinating which would lead to a bitter tasting product and would decrease the yield 

5) It can be used to promote the lateral growth in plants 

6) Low temperatures, low oxygen levels and high levels of carbon dioxide can prohibit the synthesis of ethene and therefore prevent the ripening of fruits which can be useful when shipping 

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