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

The five main groups of natural plant growth regulators are:AuxinsGibberellinsCytokininsEthyleneAbscisic acid Discovery, physiological functions and applications of gibberellins is as follows:- Discovery of Gibberellins:Gibberellins are kind of promotery PGR. There are more than 100 gibberellins reported from different organisms such as fungi and higher plants. They are denoted as GA₁, GA₂, GA₃ and so on. However GA₃ was one of the first gibberellins to be discovered and remains the most intensively studied form. The ‘bakane’, (foolish seedling) a disease of rice seedlings was caused by a fungal pathogen Gibberalla fujikuroi E. Kurosawa reported the appearance of symptoms of the disease in uninfected rice seedlings when they were treated with sterile filterate of the fungus. The active substances were later identified as gibberellic acid. Physiological effects of gibberellins are: 1. Elongation of stem and expansion of leaf. 2. Reversal of dwarfism, particularly in corn. 3. Parthenocarpy. 4. Substituting cold treatment. 5. Seed germination as gibberellins stimulate production of digestive enzymes. 6. Breaking of seed and bud dormancy. 7. Bolting occurs at start of the reproductive phase. It can be artificially induced in cabbage by application of gibberellins 8. Cell division of cambium. Practical uses of application of Gibberellins. 1. To break seed dormancy. 2. To induce parthenocarpy. Commercial application such as spraying of Thompson seedless grapes results in larger fruits with longer stalks. 3. To increase yields of melons and useful curcurbits. 4. It causes elongation of sugarcane and also increases sugarcane contents.