Five main groups of natural plant growth regulators are auxins, gibberellins, ethylene, cytokinins and ABA.
(i) Discovery Auxins was first isolated from human urine. They are generally produced by the growing
apices of the stems and roots. Auxins like LAA and IBA have been isolated from plants. NAA (Naphthalene Acetic acid) and 2, 4-D (2,4-dichlorophenoxyacetic) are synthetic auxins.
(ii) Physiological functions They help to initiate rooting in stem cuttings, an application widely used for plant propagation. Auxins promote flowering, i.e., in pineapples. They help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits.
(in) Agricultural/horticultural applications Auxins also induce parthenocarpy, e.g., in tomatoes. They are widely used as herbicides 2, 4-D, and are used to kill dicotyledonous seeds. They do not affect mature monocotyledonous plants, instead is used to prepare seed-free lawns by gardeners. It also controls xylem different and helps in cell division.