On the basis of the function there are five types of plant hormones:

  1. Auxin
  2. Gibberellin
  3. Cytokinin
  4. Ethylene
  5. Abscisic acid (ABA)

Functions of auxin:

1. Cell enlargement: Auxin causes cell enlargement by solubilisation of carbohydrates, loosening of cell wall micro-fibrils, synthesis of new wall material and increase in respiration. 

2. Prevention of lodging: Lower inter-nodes of the stem of cereals are long and weak. AS a result the plant bends down or droops. Application of NAAM prevents lodging.

3. Apical dominance: In many plants, apical bud suppresses the growth of lateral buds. This condition is known as apical dominance. A plant with strong apical dominance has little or no branching like in sunflower.

4. It is responsible for the photo-tropism and geotropism.

5. It promotes root initiation in callus and stem cutting.

6. Delay of abscission of leaves by preventing formation of abscission layer. But promotes this in older leaves and fruits.

7. It induces parthenocarpy (production of fruit without fertilization).

8. Increases the number of female flowers.

9. Activity of cambium is promoted by auxin.

10. Healing of injury in plants.

11. Promotes xylem differentiation.

Functions of Gibberellins:

1. In bolting: In rosette forming plants, inter-node growth is poor but large leaves appear to rise arise in tufts. The inter-nodes suddenly elongate and the stem becomes normal just before flowering. This is called bolting.

2. Parthenocarpy: Gibberellins have been found to be effective in inducing pathenocarpy in tomatoes, apple etc.

3. Breaking of dormancy: These can effectively break the dormacy of potato tuber, winter buds and seeds of many trees.

4. Increase in fruit size: These increases the number and fruit size of grapes.

5. Production of male flowers: These induces production of male flowers on genetically female plants.

6. Inter-nodal elongation: It helps in the elongation of the stem but not in roots. 

7. It helps in the germination of the seed.

8. It is used to delay the ripening in plants like citrus.

Functions of cytokinin:

1. Cell division: These are found in a higher amount where rapid division is going on.

2. Morphogenesis: Cytokinins promotes cell division and in the presence of auxin, it promote cell division even in the meristematic tissues. In tissue culture, mitotic division are accelerated when both auxin and cytokinin are present. The ratio of high cytokinin and low auxin promote shoot buds in tissue culture.

3. Apical dominance: Cytokinin and auxin acts antagonistically in the control of apical dominance.

4. Delay in senescence: It delay the senescence of plant organs by controlling protein synthesis and mobilization of resources. This phenomenon is called the Richmond Lang effect. They helps to produce chloroplast in leaves. These are also called anti-ageing hormones.

5. Flowering: It helps in inducing flowering in certain species of plants like Lemna and Wolffia. 

6. It promotes phloem transport. 

7. It also promotes accumulation of salts in the cells.

8. It promotes production of female flowers.

9. It increases the resistance to low and high temperature and diseases.

Functions of ethylene:

1. It helps in ripening of fruits.

2. Inhibition of stem elongation and stimulation of transverse growth by causing increase in the girth of the plant and promotes horizontal growth.

3. It promotes apogeotropism in roots.

4. It increases the speed of senescence.

5. It also induces abscission of leaves.

6. It helps in breaking the dormancy of storage organs and initiates germination in peanut seeds.

7. Root initiation: Low concentration of ethylene induces rooting and growth of lateral roots and root hairs.

8. Flowering: It is used to initiate flowering and synchronizing fruit set in pineapples.

9. It helps in production of female flowers in a plant which is genetically male.

Function of abscisic acid:

1. It hastens the formation of abscission layer and senescence.

2. Transpiration: It helps in closing of stomata by causing potassium ions to leave the guard cells during periods of water shortage or drought and hence is also known as stress hormone.

3. It promotes bud dormancy in seeds during winters.

4. Seed dormancy: It induces seeds dormancy hence is named dormin. Thus it helps the seed to withstand desiccation and other unfavorable factors.

5. It inhibits cambial activity.

6. Flowering: in induces flowering in some short day plants like strawberry.

7. It plays an important role in seed development, maturation and dormancy.

8. It induces synthesis of carotenoids in green oranges making them yellow.