There are five main groups of natural plant growth regulators. They are:
(1) Auxins
(2) Gibberellins
(3) Cytokinins
(4) Abscisic acid
(5) Ethylene
Auxins
A) Discovery of Auxin
In 1881, Charles Darwin and Francis Darwin performed experiments on coleoptiles of canary grass and observed that they bend towards the light, when exposed to light from a unidirectional source. They concluded that the tip of the coleoptile was responsible for sensing light, and proposed that a messenger is transmitted in a downward direction from the tip of the coleoptile, causing it to bend.
In 1913, Peter Boysen-Jensen concluded that the substance secreted at the tip is soluble in water (gelatin).
In 1928, F.W. Went showed that a chemical messenger diffuses from coleoptile tips and causes it to grow towards light. He later proposed that the messenger substance is a growth-promoting hormone, which he named auxin.
In 1931, Kogl and Haagen Smith isolated crystalline compounds from human urine and named them as auxin-a, auxin-b and heteroauxin.
B) Physiological functions of Auxins
Apical dominance
Control of abscission
Root formation in stem cuttings
Parthenocarpy
Differentiation of xylem and phloem
C) Agricultural/Horticultural Applications
Application of auxin promotes flowering in case of orchards.Examples: pineapple, litchi, etc.
Auxins also induce parthenocarpy in some plants like tomato, cucumber, pepper, etc.
Auxins like IAA, IBA, NAA induce rooting in stem cuttings of various plants.
Auxin like 2,4-D is widely used as a herbicide to kill dicotyledonous weeds and to prepare weed free lawns by gardeners.
Auxins regulate the maturation of fruits on trees of apples, oranges and grape fruit.