List five main groups of natural plant growth regulators. Write a note on physiological functions and agricultural/horticultural/application of any one of them.

The main five groups of  plant growth regulators (PGRs) are : 

1. Auxins 
2. Cytokinins
3. Gibberellins
4. Ethylene
5. Abscisic acid. 

Auxin - 

Physiological functions of auxins are:

1. They  help to initiate and enhance rootings in the stem cuttings.

2. They promote flowering in certain plants like pineapple.

3. They prevent the leaf and fruit drop at early stages. They promote abscission of older or mature leaf and fruit. 

4. They induce apical dominance.

5. They are responsible for tropic movements.

Horticultural/Agricultural importance:

1. They induce parthenocarpy in plants like tomatoes..

2. They are used as weedicides that kill dicotyledenous weeds.