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Plant growth regulators are biological molecules which are produced by the plants and have an impact on the physiological features of plants. Their function is to adjust the progress of plants to guarantee agrarian production. They are:
ü    Auxins
ü    Cytokinins
ü    Ethylene
ü    Abscisic acid
ü    Gibberellins    
Discovery of Auxins Among the aforementioned, the inspections regarding the properties of Auxins was created by Charles Darwin and Francis Darwin, who noticed that  the first leaves of canary grass were very much perceptive to light and credited this property to the Auxins.    
Physiological functions

i.  Regulate development of plant cells
ii. Originate the occurrence of apical dominance
iii.  Regulates the splitting up of the plant tissue (vascular cambium) as well as distinction of vascular tissues to the growing trails to create xylem and phloem iv. Encourages the natural or artificial production of fruit without fertilization of ovules (parthenocarpy)
v. Inhibits detaching of fruit and leaves
Horticultural applications
i. Utilized as the rooting hormones in sculpting of stems
ii.2-4 D----a herbicide used for  eradication of large leaf dicot weeds iii.Encourage parthenocarpy in tomatoes
iv.Stimulate flowering in pineapple and litchi