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.
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