Plant growth regulators are the chemical molecules secreted by plants affecting the physiological attributes of a plant. There are five main plant growth regulators. These are:

(i) Auxins

(ii) Gibberellic acid

(iii) Cytokinins

(iv) Ethylene

(v) Abscisic acid

(i) Auxins

Discovery:

The first observations regarding the effects of auxins were made by Charles Darwin and Francis Darwin wherein they saw the coleoptiles of canary gross bending toward a unilateral source of light.

It was concluded after a series of experiments that some substance produced at the tip of coleoptiles was responsible for the bending. Finally, this substance was extracted as auxins from the tips of coleoptiles in oat seedlings.

Physiological functions:

a. They control plant cell-growth.

b. They cause the phenomenon of apical dominance.

c. They control division in the vascular cambium and xylem differentiation.

d. They induce parthenocarpy and prevent abscission of leaves and fruits.

Horticultural applications:

a. They are used as the rooting hormones in stem cuttings.

b. 2-4 D is used weedicide to kill broadleaf, dicotyledonous weeds.

c. They induce parthenocarpy in tomatoes.

d. They promote flowering in pineapple and litchi.

(ii) Gibberellic acid

Discovery:

Bakane or the “foolish rice seedling” disease was first observed by Japanese farmers. In this disease, rice seedlings appear to grow taller than natural plants, and become slender and pale green. Later, after several experiments, it was found that this condition was caused by the infection from a certain fungus Gibberella fujikuroi. The active substance was isolated and identified as gibberellic acid.

Physiological functions:

a. It causes elongation of internodes.

b. It promotes bolting in rosette plants.

c. It helps in inducing seed germination by breaking seed dormancy and initiating the synthesis of hydrolases enzymes for digesting reserve food.

Horticultural applications:

a. It helps in increasing the sugar content in sugarcane by increasing the length of the internodes.

b. It increases the length of grape stalks.

c. It improves the shape of apple.

d. It delays senescence.

e. It hastens maturity and induces seed-production in juvenile conifers.

(iii) Cytokinins

Discovery:

Through their experimental observations, F. Skoog and his co-workers found that the tobacco callus differentiated when extracts of vascular tissues, yeast extract, coconut milk, or DNA were added to the culture medium. This led to the discovery of cytokinins. Physiological functions:

a. They promote the growth of lateral branches by inhibiting apical dominance.

b. They help in the production of new leaves, chloroplasts, and adventitious shoots.

c. They help in delaying senescence by promoting nutrient mobilisation.

Horticultural applications:

a. They are used for preventing apical dominance.

b. They are used for delaying senescence in leaves.

(iv) Ethylene

Discovery:

It was observed that unripe bananas ripened faster when stored with ripe bananas. Later, the substance promoting the ripening was found to be ethylene.

Physiological functions:

a. It helps in breaking seed and bud dormancy.

b. It promotes rapid internode-elongation in deep-water rice plants.

c. It promotes root-growth and formation of root hairs.

d. It promotes senescence and abscission of leaves and flowers.

e. It hastens the respiration rate in fruits and enhances fruit ripening.

Horticultural applications:

a. It is used to initiate flowering and synchronising the fruit set in pineapples.

b. It induces flowering in mango.

c. Ethephon is used to ripen the fruits in tomatoes and apples, and accelerate the abscission of flowers and leaves in cotton, cherry, d.nd walnut.

d. It promotes the number of female flowers in cucumbers.

(v) Abscisic acid

Discovery:

During the mid-1960s, inhibitor-B, abscission II, and dormin were discovered by three independent researchers. These were later on found to be chemically similar and were thereafter called ABA (Abscisic acid).

Physiological functions:

a. It acts as an inhibitor to plant metabolism.

b. It stimulates stomatal closure during water stress.

c. It induces seed dormancy.

d. It induces abscission of leaves, fruits, and flowers.

Horticultural application:

a. It induces seed dormancy in stored seeds.