The term phytoregulator is used for a chemical substance which
is an integral part of plant metabolism and in small concentrations
can activate or depress any developmental process in that plant.
Phytoregulators can be natural or synthetic. Higher plants generate
physiological active substances termed phytohormones. These natural
phytoregulators are synthesized by the plant itself and act in a
location distant from the source of production. There are several
phytohormone groups, the best known is the auxin group.
In plants, phytohormones control the intensity of metabolism, the
production of other phytohormones, production and decomposition
of pigments, displacement of certain substances into certain organs
or storage in them, elongation of the cell wall, the development
of flowers, and also growth and growth rate (with absence of individual
hormones leading to dwarfing and inhibition of development).
Auxins occur in all plants, most importantly indole-3-acetic acid.
Many plants also contain indole-3-aceticaldehyde, indole-3-butyric
acid, indole-3-acetonitrile and indole metabolic derivatives such
as 2-hydroxyindole-3-acetic acid and ethyl indole-3-acetate with
similar cell elongation action.
The principle auxin in higher plants is Indole-3-Acetic Acid (IAA).
This natural auxin is synthesized by the plant itself from the amino
acity tryptophan. There are several other naturally occurring auxins
in higher plants, although IAA is by far the most important.
Physiological Effects of IAA
- In high concentrations inhibition, in small concentrations promotion
of longitudinal cell growth and the related water absorption of
the cells. IAA causes a fairly rapid increase in the cell wall
extensibility in coleoptiles and young stems.
- Promotion of cell division of the meristematic tissue.
- Promotion of root formation even on tissues which do normally
form roots, but inhibition of growth, excpet in smallest concentrations.
- Promotion of callus formation and callus.
- Inhibition of budding and growth of auxiliary buds and bud formation.
- Influencing of leaf, flower and fruit shedding.
- Influencing of flower formation
- Formation of parthenocarpic fruit (seedless fruit).
- Promotion of respiration and protein formation.
The optimal tissue auxin concentration:
- For elongation growth is typically 10-5 to 10-6
parts per million
- For root growth 10-10 parts per million
- For bud growth 10-8 parts per million
- For flower promotion 10-4 parts per million
There are other classes of synthetic auxin analogs. These compounds,
such as alpha-(p-chlorophenoxy) isobutyric acid or PCIB, have little
or no auxin activity but specifically inhibit the effects of auxin,
i.e., anti-auxins. When applied to plants, anti-auxins may compete
with IAA for specific receptors, thus inhibiting normal auxin action.
This inhibition can be overcome by adding excess IAA.