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

(N/A) The five main groups of natural plant growth regulators are:
$(i)$ Auxins
$(ii)$ Gibberellins
$(iii)$ Cytokinins
$(iv)$ Ethylene
$(v)$ Abscisic acid
Note on Auxins:
Discovery: Charles Darwin and Francis Darwin observed that canary grass coleoptiles bent toward light. Later,$F$.$W$. Went isolated auxins from the tips of oat seedlings.
Physiological functions: They promote cell elongation,apical dominance,and vascular tissue differentiation. They also induce parthenocarpy.
Agricultural/Horticultural applications: Used as rooting hormones in stem cuttings,as weedicides (e.g.,$2,4-D$),and to induce fruit set in tomatoes.