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

Plant Hormones (Phytohormones)

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:

① Plant growth promoters: Examples are auxins, gibberellins and cytokinins.
② Plant growth inhibitors: Examples are abscisic acid, ethylene.


All about Auxins
⒈ Auxins are weakly acidic growth hormones.

⒉ Darwin (1880) was first to find about Auxins in the tip of canary grass.

⒊ These hormones are found in shoot apex, leaf Primordia and developing seeds.



⒋ Promote cell enlargement and division and initiate root formation on stem cuttings.

⒌ Produce tropic plant responses like phototropism and geotropism.

⒍ Commercially used in tissue and organ culture to form callus and initiate root formation.

⒎ Used to induce flowering in litchi and pineapple



Gibberellins
⒈ Weakly acidic growth hormones.

⒉ Kurosawa discovered these chemicals in fungi.

⒊ Commonly found in seed embryos, roots and young leaves near the shoot tip.

⒋ They stimulate stem elongation and leaf expansion.

⒌ These hormones also promote seed germination, fruit growth and induce parthenocarpy.

⒍ Widely used to increase the number and size of fruits, e.g., apple, grapes, tomato.

⒎ Also used to increase length of stem and yield of sugarcane.


Cytokinins
⒈ These are basic hormones.

⒉ Discovered by Miller et al in 1955.

⒊ It is mainly found in roots. Also synthesised in seed and young fruits.

⒋ Promote cell division and cell elongation.

⒌ Overcome seed dormancy of various types.

⒍ Essential nutrient medium used in tissue culture as required for morphogenesis.

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

Ethylene
⒈ Growth inhibiting gaseous hormone.

⒉ These are found in almost all parts of plants in minimal amount but maximum production occurs during ripening of fruits

⒊ Induces abscission and senescence of various parts, i.e., leaves, flowers, fruits, etc.

⒋ Ethylene lamps are used for ripening of fleshy fruits, e.g., banana, mango, apple, tomato, etc.



Abscisic acid
⒈ Growth inhibiting acidic hormone.

⒉ First discovered by Addicott et al (1963) from cotton bolls

⒊ Found in many parts of the plant but is more abundant in chloroplast of green cells.

⒋ Induces dormancy of buds, seeds and underground stems, hence also called as dormin.

⒌ Promotes abscission of flowers and fruits

⒍ Used in prolonging dormancy of buds, storage organs and seeds.