Friday, 27 October 2017

CBSE Class 10 - Biology - Study Notes about Plant Growth Hormones (Phytohormones) (#cbseNotes)

Plant Hormones (Phytohormones)

CBSE Class 10 - Biology - Study Notes about Plant Growth Hormones (Phytohormones) (#cbseNotes)


Plant hormones or phytohormones are small, simple chemical molecules, which are present in low concentration in plants and regulate growth, differentiation and development.


Two Categories of Phytohormones:

Plant growth promoters: Examples are auxins, gibberellins and cytokinins.
Plant growth inhibitors: Examples are abscisic acid, ethylene.


All about Auxins
Auxins are weakly acidic growth hormones.

Darwin (1880) was first to find about Auxins in the tip of canary grass.

These hormones are found in shoot apex, leaf Primordia and developing seeds.



Promote cell enlargement and division and initiate root formation on stem cuttings.

Produce tropic plant responses like phototropism and geotropism.

Commercially used in tissue and organ culture to form callus and initiate root formation.

Used to induce flowering in litchi and pineapple



Gibberellins
Weakly acidic growth hormones.

Kurosawa discovered these chemicals in fungi.

Commonly found in seed embryos, roots and young leaves near the shoot tip.

They stimulate stem elongation and leaf expansion.

These hormones also promote seed germination, fruit growth and induce parthenocarpy.

Widely used to increase the number and size of fruits, e.g., apple, grapes, tomato.

Also used to increase length of stem and yield of sugarcane.


Cytokinins
These are basic hormones.

Discovered by Miller et al in 1955.

It is mainly found in roots. Also synthesised in seed and young fruits.

Promote cell division and cell elongation.

Overcome seed dormancy of various types.

Essential nutrient medium used in tissue culture as required for morphogenesis.

Cytokinins help in opening of stomata while abscisic acid causes its closure.

Ethylene
Growth inhibiting gaseous hormone.

These are found in almost all parts of plants in minimal amount but maximum production occurs during ripening of fruits

Induces abscission and senescence of various parts, i.e., leaves, flowers, fruits, etc.

Ethylene lamps are used for ripening of fleshy fruits, e.g., banana, mango, apple, tomato, etc.



Abscisic acid
Growth inhibiting acidic hormone.

First discovered by Addicott et al (1963) from cotton bolls

Found in many parts of the plant but is more abundant in chloroplast of green cells.

Induces dormancy of buds, seeds and underground stems, hence also called as dormin.

Promotes abscission of flowers and fruits

Used in prolonging dormancy of buds, storage organs and seeds.

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