Phytohormone is an organic substance other than nutrients active in minute quantity which is formed in certain parts of a plant and is then usually translocated to some other parts or sites where it evokes physiological, biochemical and /or morphological responses.
Phytohormones are divided into five groups: Auxin, Cytokininis, Gibberllins, Abscisic acid (ABA), and ethylene.
Auxin is earliest known plant growth regulator. It promotes cell elongation especially in shoots. It obtained from plant are called natural auxin.
Gibberellin: Kurosawa first time noted the existence of gibbellins in a fungus Gibberella fuzikuroi of ascomycetes infected rice plant. This is popularly known as Bakanne disease of rice.
Auxins vs Gibberellin
| Auxins | Gibberellin |
| They are mostly found in higher plants. | They are mostly found in fungi and few higher palnts. |
| Structure: It has a double or single unsaturated ring structure. A Side chain is present. | It has tetracyclic gibbane structure. Unsaturation is comparatively less. A side chain is absent. |
| Auxin promotes growth in shoot segments. | Gibberllin promotes growth in intact shoot. |
| There is little effect on leaf growth. | Leaf growth is enhanced. |
| Auxin has no effect on genetically dwarf plants. | It enhances internode growth in genetically dwarf plants. |
| It causes apical dominance. | Apical dominance is not affected. |
| It does not cause bolting in rosette plants and root crops. | It bring about elongation of stem or bolting in rosette plants and root crops. |
| It has no influence on the requirement of vernalisation. | Gibberllin can replace the requirement of vernalisation in most plants. |
| It has no effect on the flowering of long day plants. | It can replace the requirement of long photoperiods in long day plants. |
| It is essential for the growth and formation of callus. | It does not influence formation and growth of callus. |
| It promotes rooting on cuttings. | It has no effect on rooting of cuttings. |
| It does not break dormancy. | It helps in breaking dormancy. |
| Auxin transport is polar. | It shows channel transport in different directions. |
Root growth is promoted by low concentration of auxin and inhabited by its normal concentration. | Gibbellin has no specific effect on root growth. |
| It does not produce hydrolysing enzymes for mobilizing food reserve during seed germination. | Gibbellin produces hydrolysing enzymes for solubilisation of food reserve during seed germination. |
