Plant growth regulators
Plant growth regulators :
-Plant hormones (phytohormones) are physiological intercellular messengers that are
needed to control the complete plant lifecycle, including germination, rooting, growth,
flowering, fruit ripening, foliage, and death.
-In addition, plant hormones are secreted in response to environmental factors such as abundance of nutrients, drought conditions, light, temperature, chemical or physical stress. Hence, levels of hormones will change over the lifespan of a plant and are dependent upon season and environment.
-The term “plant growth factor” is usually employed for plant hormones or substances of
similar effect that are administered to plants. Growth factors are widely used in
industrialized agriculture to improve productivity.
-The application of growth factors allows
synchronization of plant development to occur. For instance, ripening tomatoes can be
controlled by setting desired atmospheric ethylene levels.
Traditionally five major classes of plant hormones are listed:
- abscisic acid, and
1) Auxins :
-Auxin is the active ingredient in most rooting mixtures.
-These products help the vegetative propagation of plants. On a cellular level auxins influence cell elongation, cell division, and the formation of adventitious roots.
-Some auxins are active at extremely low concentrations.
-Typical auxin concentration range from 0.01 to 10 mg/L.
-Cytokinins promote cell division, stimulate shoot proliferation, activate gene expression
and metabolic activity in general. At the same time, cytokinins inhibit root formation.
-This makes cytokinins useful in culturing plant cell tissue where strong growth without root
formation is desirable.
-Concentrations of cytokinin used for horticulture vary between 0.1 to 10 mg/L
–Gibberellins are derivatives of gibberellic acid. They are natural plant hormones and
promote flowering, stem elongation, and break the dormancy of seeds.
-Gibberellins are particularly effective at breaking seed dormancy and at speeding
-Abscisic acid (ABA) is a plant growth inhibitor and an antagonist of gibberellins: it
induces dormancy, prevents seeds from germinating, and causes abscission of leaves,
fruits, and flowers.
-High concentrations of abscisic acid can be induced by environmental stress such as drought.
-Ethylene is unique in that it is found only in gaseous form. It induces ripening, causes
leaves to abscess, and promotes senescence.
-Plants often increase ethylene production in response to stress and before death.
-Ethylene concentrations fluctuate with the seasons while playing a role in inducing foliage and ripening of fruit.