Plant hormones are specialized chemical substances produced by plants. They are the main internal factors controlling growth and development. Hormones are produced in one part of a plant and transported to others, where they are effective in very small amounts. Depending on the target tissue, a given hormone may have different effects.
Plant hormones play an integral role in controlling the growth and development of plants. A plant hormone is generally described as an organic compound synthesized in one part of the plant and translocated to another part, where in low concentrations elicits a physiological response.
There are five generally recognized classes of plant hormones; some of the classes are represented by only one compound, others by several different compounds. They are all organic compounds, they may resemble molecules which turn up elsewhere in plant structure or function, but they are not directly involved as nutrients or metabolites.
Hormone Source Action
Auxins apical meristem (only moves down),
embryo of seed,
young leaves • Control of cell elongation
• apical dominance (prevents lateral buds)
• prevents abscission
• continued growth of fruit
• cell division in vascular and cork cambium
--formation of lateral roots from pericycle
--formation of adventitious roots from cuttings
Gibberellins Roots and young leaves • Cell (stem) elongation (works in stems and leaves, but not roots)
• breaking seed/bud dormancy
• stimulating fruit set
Cytokinins roots, embryos, fruits
actively growing • Promote cell division
--signal axillary/lateral bud growth
--prevent leaf abscission
• chloroplast development
• breaking dormancy in some seeds
• enhance flowering
• promote fruit development
Abscissic Acid leaves, stems, green fruit • Reduces cell division (helps maintain dormancy of seeds and buds)
• prepare plants for winter
1. decreasing cell division
2. developing protective scales
3. deposition of waterproofing substances
• closes stomata
Ethylene tissues of ripening fruit,
nodes of stems,
senescent leaves, flowers Growth...