ANS fast (12th Biology)............
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Explanation:
Characteristics
Plant Growth Regulators can be of a diverse chemical composition such as gases (ethylene), terpenes (gibberellic acid) or carotenoid derivates (abscisic acid). They are also referred to as plant growth substances, phytohormones or plant hormones. Based on their action, they are broadly classified as follows:
Plant Growth Promoters – They promote cell division, cell enlargement, flowering, fruiting and seed formation. Examples are auxins, gibberellins and cytokinins.
Plant Growth Inhibitors – These chemicals inhibit growth and promote dormancy and abscission in plants. An example is an abscisic acid.
Note: Ethylene can be a promoter or an inhibitor, but is largely a Plant Growth Inhibitor.
Browse more about Plant Growth and Development
Plant Growth and Development
Growth and its Phases
Vernalisation
Photoperiodism
All plant growth regulators were discovered accidentally. Let’s take a detailed look at each regulator and learn about it more closely:
Auxins
Discovery
Auxins were the first growth hormone to be discovered. They were discovered due to the observations of Charles Darwin and his son, Francis Darwin. The Darwins observed that the coleoptile (protective sheath) in canary grass grows and bends towards the source of light. This phenomenon is ‘phototropism’. In addition, their experiments showed that the coleoptile tip was the site responsible for the bending. Finally, this led to the isolation of the first auxin by F. W. Went from the coleoptile tip of oat seedlings.
(Source: Wikimedia Commons)
Types
First isolated from human urine, auxin is a term applied to natural and synthetic compounds that have growth regulating properties. Plants produce natural auxins such as Indole-3-acetic acid (IAA) and Indole butyric acid (IBA). Natural auxins are found in growing stems and roots from where they migrate to their site of action. Naphthalene acetic acid (NAA) and 2, 4-dichlorophenoxyacetic (2, 4-D) are examples of synthetic auxins.
Effects
Promote flowering in plants like pineapple.
Help to initiate rooting in stem cuttings.
Prevent dropping of fruits and leaves too early.
Promote natural detachment (abscission) of older leaves and fruits.
Control xylem differentiation and help in cell division.
Applications
Used for plant propagation.
To induce parthenocarpy i.e. the production of fruit without prior fertilization.
2, 4-D is widely used as a herbicide to kill dicotyledonous weeds.
Used by gardeners to keep lawns weed-free.
Note: The growing apical bud in higher plants inhibits the growth of the lateral buds. This phenomenon is ‘Apical Dominance‘. Removal of the apical bud allows the lateral buds to grow. This technique is commonly used in tea plantations and hedge-making.
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