The following points highlight the five main types of plant growth regulators. The types are: 1. Auxins 2. Gibberellins 3. Cytokinins 4. Ethylene 5. Abscissic Acid.
Plant Growth Regulators: Type # 1. Auxins:
Auxins (GK Auxein = to grow), isolated initially from human urine were the first plant PGR to be discovered.
However, IAA also occurs in human urine, especially in persons suffering from pellagra (niacin or nicotinic acid deficiency).
The term auxin is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth regulating properties. They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action.
Auxins, like IAA and indole butyric acid, IBA, have been isolated from plants.
2, 4-D (2, 4 dichlorophenoxyacetic), NAA (naphthalene acetic acid) are the synthetic auxins.
All above mentioned auxins have been used extensively, particularly in agricultural practices.
Applications of auxins have a wide range. They have been used for more than 50 years in horticulture and agriculture. They help to initiate rooting in stem cuttings, an application widely used for plant propagation. Auxins promote flowering, e.g., in pineapples. They help to prevent fruit and leaf drops at early stages but promote the abscission older/mature leaves and fruits.
In most higher plants, the growing apical bad inhibits the growth of lateral (axillary) buds, a phenomenon called apical dominance.
Removeal of shoot tips (decapitation) usually results in the growth of lateral buds. It is widely applied in tea plucking, hedge-making.
Auxins also induce parthenocarpy, e.g., in tomatoes. They are widely used as herbicides. Auxin 2, 4-D, is widely does not affect mature monocotyledonous plants. It is also spread to kill dandelions/daisies in lawns by gardeners. Auxin also control xylem differentiation and help cell-division.
Plant Growth Regulators: Type # 2. Gibberellins:
This is another kind of promotery PGR. There are more than 100 gibberellins reported from different organism such as fungi and higher plants, and the number is still increasing.
They are denoted as GA1, GA2, GA3 and so on. However, GA3 was one of the first gibberellins to be discovered and remains the most intensively studied form.
All GA5 are acidic. They produce a wide range of physiological responses in the plants. They specifically make genetic dwarfs, e.g., in peas, grow tall. Their ability to cause increase in length of axis is used to make the stalks of graphs lengthy, thereby allowing the grapes to grow larger.
Gibberellins GA4 and GA7 cause apple fruits to elongate and improve shape. They also delay senescence, thus the fruits can be left on the tree longer to extend market period.
GA3 is used to speed up malting process in brewing industry.
Spraying sugarcane crop with gibberellins increases the length of the stem. It helps increase yield an much as by 20 tons per acre.
Spraying juvenile conifers with GA4 and GA7 hastens the maturity period. Thus leading to early seed production. Gibberellins also promote bolting (i.e., internode elongation prior to flowering) in beet, cabbages and many plants with rosette habit.
Plant Growth Regulators: Type # 3. Cytokinins:
These PGRs have specific effects on cytokinesis, and were discovered as kinetin (a modified form of adenine, a purine) from the herring sperm DNA. Kinetin does not occur naturally in plants.
The searches for natural substances with cytokinin like activities led to the isolation of Zeatin from corn-kernels and coconut milk, etc. Since the discovery of zeatin, several naturally occurring cytokinins, and some synthetic compounds, with cell division promoting activity, have been identified from plants.
Natural cytokinins are synthesised in regions where rapid cell division is occurring, e.g., root apices, developing shoot buds, young fruits, etc.
They help in production of new leaves, chloroplasts in leaves, lateral shoot growth as well as adventitious shoot formation. Cytokinins overcome apical dominance. They promote nutrient mobilization that helps in delay of leaf senescence.
Plant Growth Regulators: Type # 4. Ethylene:
This is a simple gaseous PGR. It is synthesised in large amounts by tissues undergoing senescence and ripening of fruits. Ethylene influences of plants include horizontal growth of seedlings, swelling of the axis, tighter apical look formation in dicot seedlings.
Ethylene promotes senescence and abscission of plant organs (especially leaves, flowers etc.) Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of fruit. This rise in rate of respirsation is called respiratory climacteric.
Exposure of plants to ethylen causes drooping of leaves and flowers, a phenomenon known as epinasty.
Ethylene is a natural PGR and product of metabolism in plants.
Ethylene break seed and bud dormancy, initiate germination in peanut seeds, sprouting of potato tubers.
Ethylene promotes rapid internode/petiole elongation in deep water rice plants. It also helps leaves/upper parts of shoot to remain above water.
This also promotes root growth and root hair formation. Thus helping plant to have more absorption surface.
Ethylene promotes in pineapple family (Bromeliaceae), and is used for synchronizing fruit set.
Ethylene also induces flowering in mango. Ethylene regulates so many physiological processes.
It is one of the most widely used PGR in agriculture. The most widely used compound is ethepon, 2-chloroethyl phosphonic acid (Ether). Ethepon in aqueous solution is readily absorbed and transported within the plant and release ethylene slowly.
Ethepon hastens fruit ripening in tomatoes and apples. It accelerates abscission in flowers and fruits(thinning of cotton, cherry, walnut, etc.). It promotes female flowers in cucumbers, and therefore, increases the yield.
Plant Growth Regulators: Type # 5. Abscissic Acid:
Abscissic acid regulates abscission and dormancy. However, like other PGRs it also has wide range of effects on plant growth and development. It acts as general plant growth inhibitor and also inhibits plant metabolism. Abscissic acid (ABA) inhibits closure of stomata in epidermis, and increases tolerance of plant to various kinds of stresses, and therefore, it is also called stress hormone.
ABA plays an important role in seed development, maturation and dormancy. By inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth. In most situations ABA acts as an antagonist to gibberellins.
However, for any and every phase of growth, differentiation and development of plants, one or the other PGR has to play some role. Such roles may be complimentary or antagonistic. They could be individualistic or synergistic.
Abscission:
The shedding of leaves, flowers and fruits from the living plant is known as abscission. These parts abscise in a region called the abscission zone, which is located near the base of petioles of leaves. In most plants leaf abscission is preceded by the differentiation of a distinct layer of cells, the abscission layer. During abscission the walls of the cells abscission layers are digested, which causes them to become soft and weak.
Auxins delay the onset of early stages of leaf abscission but promote the later stages.
Auxin level within leaves is very high in young leaves and progressively decreases in maturing leaves and in very low in senescent old leaves.
When applied externally to young leaves, IAA (indole acetic acid) inhibits leaf drop. However in later stages of leaf growth, if applied externally, it hastens the process.
It is also known that high levels of auxin trigger biosynthesis of ethylene within cells. Ethylene strongly hastens the formation of abscission zone.
There is an interaction between auxin and ethylene for an event, i.e., the abscission zone formation.
In the similar way, there are so many events in the life of a plant where more than one PGRs interact to affect that event, e.g., dormancy in seeds/buds, senescence, apical dominance, etc.