What are Plant Hormones – Class 10

Most multicellular organisms use another means of communication between different cells, called then Chemical Communication. In this, instead of an electric impulse, a chemical compound is released, which would diffuse all around the original cells. Other cells around will detect the compound using special molecules. They can recognise and transmit the information carried by it. These compounds are called Hormones.

Hormones

Hormones are released by the stimulated cells and diffuse all around the original cell. Synthesis occurs at places away from the site of action from where they simply diffuse. Target cells detect this compound by the special molecules present on their surface.

Their processing and transmittance are slower than electrical impulses. They reach all the cells of the body regardless of nervous connections and it is done steadily and persistently.

Also Check- Tropic Movement in Plants

What are Plant Hormones ?

  • Plants respond to stimuli by producing chemical compounds called Hormones that work as Messengers. These chemical substances are naturally produced in plants.
  • They are capable of regulating their important processes.
  • Different plant hormones help to coordinate growth, development and responses to the environment.
  • They are produced in one area of the plant, transported around the body of the plant and have their effects at a location far away from the site of their production. 
  • The main way in which plants respond to these hormones is by growth.
  •  In some cases growth is stimulated while in others it is inhibited. Sometimes one side of a plant grows more than the other which results in the bending of shoots or roots in response to a particular stimulus.
  • The main areas of growth (cell-division) in plants are the meristems. These areas occur just behind the tip of a shoot or root.
  • These meristems are sensitive to hormones. The hormones help to stretch the cellulose walls of the meristematic cells to facilitate division.
  • Plants respond to a variety of stimuli. They are sensitive to light. Plants respond to the direction from which light comes, the intensity of the light and the length of daily exposure to it. They are also sensitive to water, temperature, gravity, etc. Different parts of the same plant may react differently to the same stimulus, for example, shoots grow towards light but roots grow away from it.

Types of Plant Hormones

  1. Auxins
  2. Gibberellins
  3. Cytokinins
  4. Ethylene
  5. Abscisic acid (ABA)

Major Classes of Plant Hormones and their effects are as follows

Auxins

Auxin

These are usually synthesized in the tip of shoots. It helps them to grow longer. When plants areplaced facing a light source, they show bending towards it. This is because the auxin diffuses towards the shady side of the shoot stimulating the cells to grow longer.

5 Important Functions of Auxins :

1. Auxins promote the growth of stem, roots and fruits by cell elongation. During the process, cell wall is extensively

increased. This hormone induces cell wall loosening or relaxation. Plant cell wall gets stretched like a rubber when put under tension. Rate of cell-elongation is proportional to auxin concentration. Stem cell elongation requires more auxin in comparison to roots.

2. Auxins delay leaf senescence (aging or falling of leaves).

3. Auxins promote the growth of apical buds and inhibit the growth of lateral buds. In the majority of vascular plants, apical buds dominate over lateral buds. The growth of lateral buds occurs only after the removal of apical buds. This phenomenon of the suppression of growth of lateral buds by apical buds is called apical dominance.

4. Auxins induce rooting in the cutting of some plants like rose, bougainvillea etc.

5. Auxins can induce fruit formation without fertilization in fruits like apples, tomatoes, bananas, etc. Such development of fruits without fertilization is called parthenocarpy and the fruits are called parthenocarpic fruits.

Also Check – Parts of Plant cell

Gibberellins

These are the hormones that help in the growth of the stem and flower. In higher plants, gibberellins are mainly distributed in meristematic regions like stem-apex, root-apex, buds, seeds, etc.

5 Important Functions of Gibberellins :

1. The main function of gibberellins is to promote the growth of internodes by cell elongation.

2. Gibberellins break seed dormancy and initiate germination.

3. They promote fruit growth and are capable of inducing parthenocarpy.

4. They also delay senescence (ageing).

5. Gibberellins are also widely used in horticulture and food industries. These hormones enhance longitudinal growth of internodes in dwarf plants. They are used commercially to increase the length of grapes, elongate apples and improve their shape. Gibberellic acid is used to speed up the malting process in the brewing industry.

 Cytokinins

These are the hormones,which promote cell division. Highest concentration of cytokinins occurs in germinating seeds, developing fruits, embryo, etc.,i.e.areas of rapid cell division. Cytokinins are widely distributed in plants. They are produced in root tips and are transported through xylem cells.

5 Important Functions of Cytokinins :

1. Cytokinins stimulate plant growth by cell division as against auxins and gibberellins which stimulate growth by cell elongation. They promote cell division even in non- meristematic tissues.

2. In seeds, cytokinins cause expansion of cotyledons.

3. They break seed dormancy and promote germination.

4. They promote chlorophyll synthesis in chloroplasts and delay leaf senescence.

5. Cytokinins inhibit apical dominance.

Ethylene

Ethylene is the only hormone which is a gas at ordinary temperature. It is produced in fruits and remain in the same fruit. Therefore unlike other hormones, its site of synthesis and site of action are not different. A gas causing ripening of fruits was first found to be emanating from oranges which helped in the ripening of bananas when stored together. Ethylene is produced in higher plants, and fungi. All living cells are capable of producing ethylene, but more ethylene is produced in meristematic tissues.

5 Important Functions of Ethylene :

Ethylene causes two processes in plants: reduction in stem elongation and acceleration of senescence. Besides, it also helps in:

1. Ripening of fruits.

2. Initiating germination in peanut seeds.

3. Sprouting of potato tuber.

4. Promoting root growth and root-hair formation.

5. Inducing flowering in mango.

Thus, ethylene is the most widely used plant growth hormone in agriculture.

 Abscisic acid (ABA)

Abscisic acid is a growth-retarding hormone. It is found in angiosperms, gymnosperms, pteridophytes and some mosses. It is found in the chloroplasts of leaves. Fruits and seeds contain the highest amount of ABA.

4 Important Functions of Abscisic acid (ABA)

1. Abscisic acid acts as a general plant growth inhibitor by slowing down plant metabolism.

2. ABA inhibits seed germination and development. It induces seed dormancy and thereby helps seeds to withstand desiccation (extreme dryness) and other factors unfavorable for growth (as opposed to gibberellins).

3. It accelerates senescence (aging) and abscission (falling) of leaves, buds, flowers and fruits.

4. ABA stimulates the closure of stomata in the epidermis and increases the tolerance of plants To various kinds of stresses. Therefore it is also called as Stress Hormone .

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