Plant Hormones and Responses

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Plant Hormones

...referred to as plant growth regulators - they coordinate responses and move via active transport, diffusion and mass flow 

Auxins - promotes cell elongation, inhibits growth of side shoots, inhibits leaf abscission

Cytokinins - promtoe cell division

Gibberellins - promote seed germination and growth of the stems

Abscisic Acid - inhibits seed germination and growth, causes stomatal closure in response to low water avaliability

Ethene - promotes fruit ripening

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...the directional growth response of plants

Phototropisms - shoots grow towards the light

Geotropism - roots grow towards the pull of gravity

Chemotropism - pollen tubes grow down the style to enable fertilisation

Thigmotropism - shoots of climbing plants wind around other plants for support

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...immature cells (stem cells) capable of cell division...

Apical Meristems - Apex of roots/shotos -> responsible for the roots and shoot getting longer

Lateral Bub Meristems - Buds -> give rise to side shoots

Lateral Meristems - in a cylinder in the roots and shoots -> responsible for the roots and shoots getting wider

Intercalary Meristems - between the nodes -> responsible for the shoot getting longer

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Auxins - Examples

  • High concentrations prevent leaf and fruit drop and promote flowering in commercial flower productions

Taking Cuttings: - dipping the cutting in root powder encourages root growth

Seedless Fruit: - treating unpollinated flowers with auxins promotes growth of seedless fruit and triggers automatic auxin production in ovules

Herbicides for weeds: - Auxins promote shoot growth so much that the stem cannot support itself and it buckles to the plant dies

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Gibberellins - Examples

Fruit Productions:- delay senescence, extending the time citrus fruit can be left unpicked and so is avaliable in shops for longer.

  • In apples Giberrelins elongate the fruit to improve shape
  • Grape Stalks elongate so they are less compacted and can grow bigger

Brewing: - speeds up the process of aleurone layer producing anylase enzyme to digest starch into maltose

Sugar Production: - Stimulates growth between nodes making stems longer, increasing yeild

Plant Breeding: - speed up prcoess by inducing seed formation in young trees

Stopping plants producing gibberellins is also useful

  • Gibberellin inhibitors keep flowers short - prevents lodging 
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Ethene - Examples

  • speeds fruit ripening
  • promotes fruit drop in cherries, cotton and walnuts
  • promotes lateral growth

Restricting ethene's effect is also useful

  • fruit in low temperatures with little oxygen and high carbon dioxide concentration prevents ethene synthesis and prevents ripening - fruit can be stored for longer
  • silver salts increase the shelf life of cut flowers
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Cyytokinins - Examples

Used in tissue culture and mass plant productions - produces a lot of side shoots that grow into lots of small plants

Prevents yellowing of lettuce leaves once they have been picked

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