Positive role of ethylene are as follows:
1. Breaking of dormancy : It breaks dormancy of storage organs.
3. Root initiation : In low concentration ethylene helps in root initiation, growth of lateral roots and root hairs.
4. Fruit ripening : It aids in ripening of fruits. C Ethylene is used to induce artificial ripening of these fruits e.g. Citrus, Apple, Mango, Banana etc.
5. Flowering : It stimulates flowering in pineapple and related plants though in other cases the gaseous hormone causes fading of flowers.
Negative role of Ethylene :
1. Growth : It inhibits longitudinal growth and stimulates transverse growth.
2. Senescence : It hastens the senescence of leaves and flowers.
3. Abscission : Abscission of various parts is stimulated by ethylene which induces the formation of hydrolases.
4. Apical dominance : Ethylene promotes apical domiance and prolongs dormancy of lateral buds.
Auxins are of two types :
(1) Natural auxins : These are naturally occuring auxins, isolated from plants and are regarded as phytohormones e.g. IAA. NAA etc.
(2) Synthetic auxins : These are synthesized by artificially e.g. IBA 2,4-D etc.
Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedling.