A plant hormone or phytohormones is a chemical substance produced naturally in plants which is translocated to another region for regulating one or more physiological reactions when present in low concentration.
The term 'hormone' was first of all used by Starling (1906) in animal physiology for specific organ forming substances, which are secreated by cell in one part of body and produce their effect in other part of body.
Circulatory system of plants is not so developed, but still behaviour of certain substances in plants was found to be similiar to hormones, which are called plant growth hormones or phytohormones.
Thimann defined phytohormones as,'an organic substance, naturally produced in plant, affecting growth and other functions remote from its place of production and is active in very minute amount.'
So all phytohormones are growth regulators but all growth regulators are not phytohormones.
Phytohormones are two types :- Growth promotors :- Auxins, Gibberellins and Cytokinins
Growth inihibitors :- Abscisic acid and Ethylene.
Auxins
The very existence of growth substances was proposed by Charles Darwin (1880) in his book The Power of Movements in Plants.
While working on Canary grass (Phalaris canariensis), he demonstrated that apical tips of seedlings are sensitive to unilateral illumination and show positive phototropism.
The term 'hormone' was first of all used by Starling (1906) in animal physiology for specific organ forming substances, which are secreated by cell in one part of body and produce their effect in other part of body.
Circulatory system of plants is not so developed, but still behaviour of certain substances in plants was found to be similiar to hormones, which are called plant growth hormones or phytohormones.
Thimann defined phytohormones as,'an organic substance, naturally produced in plant, affecting growth and other functions remote from its place of production and is active in very minute amount.'
So all phytohormones are growth regulators but all growth regulators are not phytohormones.
Phytohormones are two types :- Growth promotors :- Auxins, Gibberellins and Cytokinins
Growth inihibitors :- Abscisic acid and Ethylene.
Auxins
The very existence of growth substances was proposed by Charles Darwin (1880) in his book The Power of Movements in Plants.
While working on Canary grass (Phalaris canariensis), he demonstrated that apical tips of seedlings are sensitive to unilateral illumination and show positive phototropism.
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