Auxins and gibberellins are two hormones out of five major hormones of plants. Auxins are present in higher plants while gibberellins are found in a few plants and fungi.
What is Auxin?
Auxin is discovered in 1926 and is first group of plant hormones. Auxins are known as plant hormone regulator earlier. Auxin is present in the form of indole acetic acid in plants. However, some other chemical compounds also exhibit the functions of auxins. One of the important function is to stimulate cell elongation of the young shoots. Auxins are synthesized in apical meristem and young leaves. Developing seeds and fruits also consist of high levels of auxins. It is transported through parenchyma cells and translocate through tracheary elements of xylem and sieve elements of phloem. Transportation is known as unidirectional and occurs from tip to base. Auxins are used commercially in plant nurseries and crop production. Its IAA form is used as a hormone to promote the growth of roots on cuttings and detached leaves. Synthetic auxins are also used in greenhouses to promote normal development of fruits of tomato plants. Unfertilized plants flowers are treated with auxins to set fruits on plants. Synthetic auxins are also used as herbicides.
What is Gibberellin?
In 1926, a Japanese botanist, Kurosawa succeeded in obtaining a filtered extract (gibberellin) from fungus while investigating rice foolish seedling disease. In 1938, two scientists, Yabuta and Sumiki discovered gibberellin in crystalline form. Biological activity and functions of these hormones were studied and reported by Japanese workers first time. Gibberellins are a group of plant hormones (about 125 closely related plant hormones) that promotes plant growth mainly through cell elongation. Gibberellins are primarily produced at meristems of apical buds and roots, young leaves, and developing seeds. Translocation of gibberellin is the acropetal i.e. base to top. Gibberelins are made in the stem and root apical meristem, seed embryos and young leaves. These hormones perform a diverse range of functions in the plants for example cell elongation, internodes elongation, increase of fruit size, breaking of bud and seed dormancy, gender expression, modification of flower sex expression, effect on pollen development and growth and enhancement of growth in cereal seedling by stimulation the digestive enzymes (amylase). Gibberellins also increase internode elongation of genetically dwarf plants. Gibberellins break dormancy in the seeds of plants which need exposure light and cold for germination. Abscisic acid is used as a strong antagonist of gibberellin action. It is applied to trees under power lines to control growth and decease the frequency of pruning in urban areas.
Auxin vs. Gibberellin
- Auxin accelerates growth in shoot segments while gibberellin promotes growth in intact shoots.
- Auxin shows little effect on leaf growth while gibberellin enhances the leaf growth.
- Auxin has an effect on apical dominance while gibberellin has no effect on apical dominance.
- Auxin does not cause bolting in root plants and rosette plant while gibberellin causes elongation of stem or bolting in rosette plants and root crops.
- Auxin has no influence on the requirement of vernalisation while gibberellin can replace the need of vernalisation in most plants.
- Auxin does not break dormancy in seeds and buds while gibberellin helps in breaking dormancy.
- Auxin is necessary for the growth and formation of callus while gibberellin does not have any effect on the growth of callus.
- Auxin has no effect on the flowering of long day plants while gibberellin can replace the need of long photoperiod in long
- Auxin shows the effect on root growth in little concentration while gibberellin has no effect on root growth.
- Auxin does not produce hydrolyzing enzymes for mobilizing food reserve during seed germination while gibberellin produces hydrolyzing enzymes.
- Auxin has feminising effect in some plants while gibberellin has masculinizing effect in plants.
- Auxin does not cause cell division in plants while gibberellin promotes cell division in plants.