Plant Growth Hormones
Auxin was first isolated from human urine. The term ‘auxin’ is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds which have certain growth regulating properties. Auxins are usually produced by the growing apices. IAA and IBA (Indole Butyric Acid) have been isolated from plants. Naphthalene Acetic Acid (NAA) and 2, 4 – D (2, 4-dichlorophenoxyacetic) are synthetic auxins.
Functions of Auxins:
- Auxins help to initiate rooting in stem cuttings. This property is widely used for plant propagation by stem cuttings.
- Auxins promote flowering. Auxins help to prevent fruit and leaf drop at early stages but promote abscission of older and mature leaves and fruits.
In most of the higher plants, the growing apical bud inhibits the growth of lateral buds. This phenomenon is called apical dominance. Farmers remove shoot tips to ensure the growth of lateral buds. This practice is widely used in tea plantations and in hedge-making.
Auxins induce parthenocarpy, e.g. in tomatoes. Auxins are widely used as herbicides, e.g. 2, 4-D is widely used to kill dicotyledonous weeds. It is also used to prepare seed-free lawns by gardeners. Auxins also control xylem differentiation and help in cell division.
There are more than 100 gibberellins. They are denoted as GA1, GA2, GA3 and so on. Giberellic Acid (GA3) was one of the first gibberellins to be discovered. All gibberellins are acidic.
Functions of Gibberellins:
- Gibberellins cause an increase in length of axis. They cause fruit elongation and also delay senescence. Thus, gibberellins can be helpful in keeping the fruits for a longer duration on tree. In brewing industry, GA3 is used to speed up the malting process.
- Spraying sugarcane crop with gibberellins increases the length of stem. This helps in increasing the yield by as much as 20 tonnes per acre.
- Gibberellins are sprayed on juvenile conifers to hasten the maturity period. This leads to early seed production. Gibberellins also promote bolting in beet, cabbages and many plants with rosette habit. Internode elongation just prior to flowering is called bolting.
Cytokinins have specific effects on cytokinesis. Kinetin was discovered from autoclaves of herring sperm DNA. Kinetin does not occur naturally in plants. Zeatin is a naturally occurring cytokinin which was isolated from corn-kernels and coconut milk.
Cytokinins are synthesized in the regions of rapid cell division. It helps to produce new leaves, chloroplast in leaves, lateral shoot growth and adventitious shoot formation. Cytokinins help in overcoming the apical dominance. Cytokinins promote nutrient mobilization which helps in the delay of leaf senescence.
Ethylene is a simple gaseous PGR. It is synthesised in large amounts by tissues undergoing senescence and ripening fruits.
- Horizontal growth of seedlings, swelling of axis and apical hook formation (in dicot seedlings) are some of the examples of activities of ethylene.
- Ethylene promotes senescence and abscission; especially of leaves and flowers. It is highly effective in fruit ripening.
- Ethylene breaks seed and bud dormancy. It initiates germination in peanut seeds. It initiates sprouting of potato tubers.
- Ethylene promotes rapid internode/petiole elongation in deep water rice plants.
- Ethylene also promotes root growth and root hair formation.
- Ethylene is one of the most widely used PGR in agriculture. It is used to initiate flowering and for synchronizing fruit-set in pineapples. It also induces flowering in mango. It is used for hastening fruit ripening in tomatoes and apples. It accelerates abscission in flower and fruits.
ABA is a plant growth inhibitor. It plays a major role in seed development, maturation and dormancy. ABA stimulates closure of stomata and increases the tolerance of plants to various types of stresses. Hence, it is also called the stress hormone.
Flowering in certain plants depends on a combination of light and dark exposures and also on the relative duration of light and dark periods. This response of plants to variable duration of sunlight is called photoperiodism.
Flowering is an important step towards seed formation. Hence, phtoperiodism plays an important role in plant evolution.
In some plants, flowering is quantitatively or qualitatively dependent on exposure to low temperature. This phenomenon is called vernalisation. Flowering is promoted during the period of low temperature because of vernalisation.
Many important crops; like wheat, barley, rye, etc. also have spring varieties. The spring variety is normally planted in the spring and come to flower and produce grain before the end of the growing season. But winter varieties of these plants would normally fail to bear flower if they are planted in spring. Such plants are planted in autumn so that they can flower during winter and bear seeds during spring. Such plants are usually harvested during mid-summer. Biennial plants also show vernalisation.