🌸 Plant Growth Regulators
Hormone groups, functions, and agricultural uses of plant growth regulators in crop management.
This lesson covers plant growth regulators, their hormone groups, and their role in regulating plant development under field conditions.
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. Using this method, fruits that
are separated from their parent plant will still respond to growth factors; allowing
commercial plants to be ripened in storage during and after transportation. This way the
process of harvesting can be run much more efficiently. Other applications include
rooting of seedlings or the suppression of rooting with the simultaneous promotion of cell
division as required by plant cell cultures. Just like with animal hormones, plant growth
factors come in a wide variety, producing different and often antagonistic effects. In
short, the right combination of hormones is vital to achieve the desired behavioral
characteristics of cells and the productive development of plants as a whole.
Traditionally five major classes of plant hormones are listed: auxins, cytokinins,
gibberellins, abscisic acid and ethylene. However as research progresses, more active
molecules are being found and new families of regulators are emerging; one example
being polyamines such as putrescine or spermidine.
Note that this classification is based partially on the chemical structure and partially on
the commonalities of plant physiological effects that certain substances exhibit.
Members of one class may not relate from a structural point of view to another. Auxins
for instance include not only many indole 3-carboxylic acid derivatives but numerous
phenylacetic acids as well. Most cytokinins (such as zeatins) are derivatives from
adenine but still differ widely in their chemical structure. Hence, the mechanism driving
action may be different in each case and likewise each specific activity will differ also.
This is demonstrated by the range of optimal concentrations required for different factors
which spans many decimals (0.001 – 100 mg/L).
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
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. Natural cytokinin hormone levels are high during maximum
growth periods of mature plants. In addition, cytokinins slow the aging process in plants.
Concentrations of cytokinin used for horticulture vary between 0.1 to 10 mg/L
Gibberellins
Gibberellins are derivatives of gibberellic acid. They are natural plant hormones and
promote flowering, stem elongation and break dormancy of seeds. There are about 100
different gibberellins, but gibberellic acid (GA3) is the most commonly used form.
Gibberellins are fundamental to plant development especially with respect to the growth
of stems. Low levels of gibberellins will prevent plants from reaching their natural height.
Gibberellin synthesis inhibitors are extensively used in grain production to keep stems
artificially short: shorter and thicker stems provide better support and resist weather
conditions better too.
Gibberellins are particularly effective at breaking seed dormancy and at speeding
up germination. Seeds that are difficult to germinate are frequently treated with gibberillic
acid solutions
Abscisic Acid
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. Elevated levels of abscisic acid will eventually induce dormancy,
when all non-essential processes are shut down and only the essential metabolism is
maintained in guard cells
Ethylene
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.
Polyamines
Polyamines are unique as they are effective (and are applied) in relatively high
concentrations. Typical concentrations range from 5 to 500 mg/L. Polyamines
influence flowering and promote plant regeneration
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Summary Cheat Sheet
| Topic | Key exam point |
|---|---|
| Main topic | Plant growth regulators (PGRs) |
| Major natural groups | Auxins, gibberellins, cytokinins, ABA, and ethylene |
| Other group in lesson | Polyamines are also included as growth-regulating compounds |
| Auxin role | Root initiation, cell elongation, apical dominance, and fruit set-related uses |
| Gibberellin role | Stem elongation, bolting, and seed/germination-related responses |
| Cytokinin role | Cell division, shoot initiation, and delayed senescence |
| ABA role | Dormancy and stress-related physiological regulation |
| Ethylene role | Ripening, senescence, and abscission responses |
| Exam distinction | Hormone name, physiological role, and agricultural use are often asked separately |
| Trap | Do not confuse plant nutrients with plant growth regulators; PGRs act in minute quantities as signaling compounds |
References
3 sources • [1] [2] [3]
References
Principles of Soil Science and Agricultural Chemistry — Standard BSc Agriculture Textbook
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