Plant Physiology - Phyto Hormones by UK Sir

 

Phyto Hormones

PGRs are small organic compound with different chemical composition, can influence the physiological activity of plant at very low conc.

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There are some intrinsic factors that regulate the growth and development of plants. These are called plant hormones or “Phytohormones”.

Plant Responce

-       Plant hormones are chemical compounds present in very low concentration in plants.

-       They are derivatives of indole compounds (auxins), terpenes (Gibberellins), adenine derivatives (Cytokinins), carotenoids derivatives (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.

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                Hormones play an important role in the plant life along with extrinsic factors.

-                Synthetic plant hormones are exogenously applied for controlled crop production

Based on their action, plant hormones are categorised into two categories:

 (Plant Growth Promoters and Plant Growth Inhibitors)

Want to know about Mineral Nutrients:

https://uksirnotes.blogspot.com/2021/06/Mineral-nutrition-plants-UKSir.html

Want to know about Respiration:

https://uksirnotes.blogspot.com/2021/05/plant-physiology-respiration-by-uk-sir.html

Auxins

-     Auxin means “to grow”.

-     1^st phyto hormone to be discovered

-     Charles Darwin and his son Francis Darwin observed that the tip of some grasses sestive towards light.

-     F.W.Went (1928) 1^st isolated auxin from the tip of Avena sativa (Coleoptile of oat)

-     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)

-     IAA resembles Tryptophan from which it is synthesized.

 

-     Synthetic: Indole butyric acid (IBA),  2,4-D (2,4-Dichlorophenoxyacetic acid), NAA (Naphthalene acetic acid) 2,4, 5 –T (2,4,5, trichlorophenoxyacetic acid)

Physiological effect: (Use/ Application)

-     It helps in Cell elongation of stems and roots.

-     Also used in Apical dominance, IAA in apical bud suppresses the growth of lateral buds.

-      It induces parthenocarpy i.e. development of fruit without fertilisation e.g. in tomatoes, Banana etc

-     It prevents premature fall of leaves, flowers, fruits.

-     Useful in stem cuttings and grafting where it initiates rooting

-     It promotes flowering e.g. in pineapple and Litchi

-     2,4-D is widely used as a herbicide to kill undesirable weeds of dicot plants without affecting monocot plants

-     It helps in cell division and xylem differentiation.

 

Gibberellins

-     GA s 1^st isolated from fungus – Gibberella fujikuroi which cause foolish seeding disease of Rice.

-     There are more than 110 gibberellins (GA1, GA2, GA3…..) are known. They are acidic in nature.

-     The most common GA is Gibberelic Acid (GA₃)

-     These are found in higher plants and fungi.

-     GA synthesis occurs in Young leaf and buds, developing seed fruits etc.

-     These are tetra diterpenoids made up of 4 isoprene units.

Gibberelic Acid:

 

Physiological effect: (Use/ Application)

-     It promotes bolting, i.e. sudden elongation of internodes just before flowering in rosette plants like cabbage, beet

-     It can delays senescence

-     Also induces parthenocarpy , improve the shape of apple.

-     Help in elongation of the stem and reverses dwarfism

-     It can induces maleness in certain plants like cannabis, cucumber etc.

-     It is a substitute of vernalization(low temp treatment for flowering)

-     It Induces the formation of hydrolytic enzymes such as lipase, amylase in the endosperm of germinating cereals.

-     Some time act as pheromone.

-     It can increase the height of Sugarcane and sugar yield.

 

Cytokinins

-     Skoog and Miller identified and crystalized cytokinesis promoting substance called Kinetin

-     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.

-     Chemically these are purine derivatives similar to adenine.

-      

Natural: Zeatin (corn kernels, coconut milk), isopentenyladenine(IPA)

Synthetic: Kinetin, benzyladenine, diphenylurea, thidiazuron, BAP (Benzybaminopuririe)

Zeatin

Physiological effect: (Use/ Application)

-     It stimulates Cytokinesis (Cell division)

-     Cause enlargement of stem cell

-     It promotes lateral and adventitious shoot growth and used to initiate shoot growth in culture

-     Cause delay of leaf senescence.

-     Remove dormancy of seed and bud.

-     Induce flowering in SDP.

-     Helps in overcoming apical dominance induced by auxins.

-     Stimulate the formation of chloroplast in leaves

-     High conc. Induce epinasty (Downward curvature of leaves)

 

Abscisic Acid  (ABA)

-     It is a growth-inhibiting hormone.

-     Chemically these are 15- C terpinoid compound.

-     ABAs act as opposite to GA s (Anti Gibberellin).

-     It inhibits plant metabolism and regulates abscission and dormancy.

-

     It is also called “stress hormone” as it increases tolerance of plants.

 

 

 

Physiological effect: (Use/ Application)

-     It promotes dormancy of Buds and seeds.

-     Induces abscission of leaves, flowers and fruits

-     It causes leaf senescence.

-     Inhibits seed germination

-     Accelerates dormancy in seeds that is useful for storage purpose

-     Stimulates closure of stomata to prevent transpiration under water stress

 

Ethylene

-     R. Gane (1934) – told ethylene is a natural plant hormone responsible for fruit ripening.

-     It acts as a growth promoter as well as an inhibitor.

-    

It is present in  a volatile gaseous form.

-     The precursor is amino acid Methionine.

-     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.

      Ehylene - 

Physiological effect: (Use/ Application)

-     It promote the ripening of fruits and senescence.

-                Breaks seed and bud dormancy

-                Break dormancy of potato tuber.

-                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

-                Induces flowering in Mango, Pineapple etc.

 

 

 

 

 

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

Want to know about Mineral Nutrients:

https://uksirnotes.blogspot.com/2021/06/Mineral-nutrition-plants-UKSir.html

Want to know about Respiration:

https://uksirnotes.blogspot.com/2021/05/plant-physiology-respiration-by-uk-sir.html

" Feel Free to ask in Comment session, I will answer.  ... UK Sir "