Part II

Plant Hormones

Plant hormones (also known as phytohormones) are chemicals that regulate plant growth, which, in the UK, are termed 'plant growth substances'. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations.

Functions of Plant Hormones

  • Effect depends on size of action, stage of plant growth and hormone concentration
  • Control plant growth and development by affecting division, elongation, and cell differentiation
  • Hormonal signal is amplified by gene expression, enzyme activity, or membrane properties

Five types of plant hormones:

  • Auxin
  • Cytokinins
  • Gibberellins
  • Abscisic Acid
  • Ethylene

Auxin acts to :

Auxin is a plant hormone that causes the elongation of cells in shoots and is involved in regulating plant growth.

Promote root initiation

  • Promote cell elongation
  • Trigger the growth of prop roots that grow from aboveground stems
  • Holds down the growth of lateral branches
  • Keeps in leaf drops before leaf ages; promote leaf drop afterward
  • Elongation, that travels one way
  • Pattern Formation, controls where leaves will grow, whether the stems are going to lengthen, etc.

Cytokinins Acts To :

Cytokinins is any of a class of plant hormones, produced by the roots and traveling upward through the xylem, that promote tissue growth and budding and, on application, retard plant senescence.

Induce growth of shoot

  • Promote cell division (recall that cytokinesis is the process of cell division after mitosis)
  • Stimulate formation of adventitious buds
  • Reverse suppression by auxin
  • Delay senescene (ageing) of leaves by maintaining chlorophyll content
  • Cell division
  • Anti-ageing

Gibberellins Acts To :

Gibberellins are growth hormones that stimulate cell elongation and cause plants to grow taller. Gibberellins also have a role in other plant processes, such as stem elongation, germination, flowering, and fruit ripening.

Stimulate cell clongation

  • Promote cell enlargement
  • Promote uptake of starch tissue by the embryos in germinating seeds
  • Reverse genetic dwarfism
  • Stimulate the vascular cambium to produce secondary phloem in woody plants
  • Stem Elongation
  • Fruit Growth
  • Seed Germination

Abscisic Acid Acts To :

Abscisic Acid slows plant growth and directs leaf primordia to develop scales to protect the dormant buds during the cold season.
  • Slows Growth
  • Seed Dormancy, what keeps the seeds from germinating the fruits
  • Drought Tolerance

Ethylene Acts To :

Ethylene is a small hydrocarbon gas. It is naturally occurring, but it can also occur as a result of combustion and other processes. You can't see or smell it. Some fruit will produce ethylene as ripening begins.
  • Triple Response
  • Senescence
  • Leaf Abscission
  • Fruit Ripening
Plant Hormones