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Plant hormones (phytohormones)

The phytohormones are a chemically heterogeneous group of organic compounds that influence a number of important plant processes. Like animal hormones, phytohormones also act in very small quantities and at a certain distance from their place of synthesis. The spectrum of activity of the phytohormones is, however, much broader than that of the animal hormones; and most of the development processes in plants are jointly regulated by several phytohormones.

According to their structure, the classic phytohormones are divided into five groups:

  1. Auxins: among other things, stimulation of stretch growth, formation of adventitious and lateral roots, inhibition of leaf fall.
  2. Cytokinins: among other things, stimulation of cell division, promotion of the expulsion of side buds, delay of senescence.
  3. Gibberellins: Increase in sprout growth, initiation of flower formation.
  4. Abscisic acid: including promoting fruit and leaf fall, inhibiting seed germination.
  5. Ethylene: among other things, promoting fruit ripeness, promoting leaf and fruit fall.

In addition, other substances with hormonal effects have been discovered in plants, e.g. jasmonic acid, brassinosteroids, oligosaccharins and signal peptides. Systemin, for example, is an 18 amino acid peptide that is involved in defense against pathogens.