Q11.Explain plant hormones with their functions.
Answers
Answer:
Plant hormones are natural substances which control many aspects of plant development. They control everything from the length between nodes on the branches to the programmed death, or senescence seen in many annual plants.
Answer:
What is Plant Hormones?
Plants need sunlight, water, oxygen, minerals for their growth and development. These are external factors. Apart from these, there are some intrinsic factors that regulate the growth and development of plants. These are called plant hormones or “Phytohormones”.
Plant hormones are chemical compounds present in very low concentration in plants. They are derivatives of indole (auxins), terpenes (Gibberellins), adenine (Cytokinins), carotenoids (Abscisic acid) and gases (Ethylene).
These hormones are produced in almost all parts of the plant and are transmitted to various parts of the plant.
They may act synergistically or individually. Roles of different hormones can be complementary or antagonistic.
Hormones play an important role in the processes like vernalisation, phototropism, seed germination, dormancy etc. along with extrinsic factors.
Synthetic plant hormones are exogenously applied for controlled crop production
Charles Darwin first observed the phototropism in the coleoptiles of canary grass and F.W. Went first isolated auxin from the coleoptiles of oat seedlings.
What are the main functions of plant hormones?
Plant hormones control all the growth and development activities like cell division, enlargement, flowering, seed formation, dormancy and abscission.
Based on their action, plant hormones are categorised into two categories:
Plant Growth Promoters
Plant Growth Inhibitors
Types of plant hormones
Auxin Hormone
Auxin means “to grow”. They are widely used in agricultural and horticultural practices. They are found in growing apices of roots and stems and then migrate to other parts to act.
Natural: Indole-3-acetic acid (IAA), Indole butyric acid (IBA)
Synthetic: 2,4-D (2,4-Dichlorophenoxyacetic acid), NAA (Naphthalene acetic acid)
Auxin Indole acetic acid
Functions:
Cell elongation of stems and roots
Apical dominance, IAA in apical bud suppresses the growth of lateral buds
Induces parthenocarpy i.e. development of fruit without fertilisation e.g. in tomatoes
Prevents premature fall of leaves, flowers, fruits
Useful in stem cuttings and grafting where it initiates rooting
Promotes flowering e.g. in pineapple
2,4-D is widely used as a herbicide to kill undesirable weeds of dicot plants without affecting monocot plants
Helps in cell division and xylem differentiation
Gibberellins Hormone
There are more than 100 gibberellins (GA1, GA2, GA3…..) that are known. They are acidic in nature. These are found in higher plants and fungi.
Gibberellins, Gibberellic acid (GA)
Functions:
Promotes bolting, i.e. sudden elongation of internodes just before flowering in rosette plants like cabbage, beet
Delays senescence
Induces parthenocarpy
Elongation of the stem and reverses dwarfism
Induces maleness in certain plants like cannabis
Induces the formation of hydrolytic enzymes such as lipase, amylase in the endosperm of germinating cereal grains and barley seeds
Breaks seed dormancy
Cytokinins Hormone
Cytokinins play an important role in cytokinesis process. Cytokinins are naturally synthesised in the plants where rapid cell division occurs e.g. root apices, shoot buds, young fruits, etc. Movement of cytokinins is basipetal and polar.
Natural: Zeatin (corn kernels, coconut milk), isopentenyladenine
Synthetic: Kinetin, benzyladenine, diphenylurea, thidiazuron
Cytokinins, Zeatin
Functions:
It promotes lateral and adventitious shoot growth and used to initiate shoot growth in culture
Helps in overcoming apical dominance induced by auxins
Stimulate the formation of chloroplast in leaves
Promotes nutrient mobilisation and delay leaf senescence
Abscisic Acid Function
It is a growth-inhibiting hormone. ABAs act as an antagonist to GAs. It inhibits plant metabolism and regulates abscission and dormancy. It is also called “stress hormone” as it increases tolerance of plants.
Abscisic acid (ABA)
Functions:
Induces abscission of leaves and fruits
Inhibits seed germination
Induces senescence in leaves
Accelerates dormancy in seeds that is useful for storage purpose
Stimulates closure of stomata to prevent transpiration under water stress
Ethylene Plant Hormone
It acts as a growth promoter as well as an inhibitor. Occurs in gaseous form. It is synthesised in the ripening fruits and tissues undergoing senescence. It regulates many physiological processes and one of the most widely used hormones in agriculture.
Ethylene
Functions:
It hastens the ripening of fruits
Controls epinasty of leaves
Breaks seed and bud dormancy
Stimulates rapid elongation of petioles and internodes
Promotes senescence and abscission of leaves and flowers
Induces root growth and root hair formation thereby increasing the absorption surface
Stimulates femaleness in monoecious plants
Apical hook formation in dicot seedlings
Other than the main 5 hormones, there are other hormones too that affect the plant’s physiological processes, e.g. brassinosteroids, salicylates, jasmonates, strigolactones, etc.