Plant Hormones

Plant growth and development are controlled by plant hormones (phytohormones)

  • There are 5 main plant hormones that coordinate plant growth and development


  • Promote primary growth (lengthening) by promoting cell elongation and increasing the rate of cell division
  • Promote apical dominance – whereby the apex / tip of a plant grows while the lateral buds remain undeveloped
  • Auxin concentrations may change in response to directional stimuli (i.e. play a key role in tropisms)


  • Promote cell division (cytokinesis) and ensure roots and shoots grow at equal rates
  • Promotes secondary growth (thickening) and help to control the rate of branching by a plant
  • Cytokinins are also involved in stimulating the growth of fruit


  • Triggers germination in dormant seeds (initiates plant growth)
  • Gibberellin also causes stem elongation by promoting cell elongation and cell division


  • A gas which acts as a plant hormone and stimulates maturation and ageing (senescence)
  • It is responsible for the ripening of certain fruit (auxins and gibberellins promote fruit growth but inhibit ripening)
  • It also contributes to the loss of leaves (abscission) and the death of flowers

Abscisic Acid:

  • Abscisic acid (ABA) principally functions to inhibit plant growth and development
  • It promotes the death of leaves (abscission) and is responsible for seed dormancy
  • It generally initiates stress responses in plants (like winter dormancy in deciduous plants)
  • Abscisic acid controls the closing of stomata and hence regulates water loss in plants

Summary of the Main Classes of Plant Hormones

plant hormones