question_answer 4)

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. 

Answer:

Five main groups of natural plant growth regulators are auxins, gibberellins, ethylene, cytokinins and ABA. Auxins (i) Discovery Auxins was first isolated from human urine. They are generally produced by the growing apices of the stems and roots. Auxins like IAA and indole butyric acid (IBA) have been isolated from plants. NAA (naphthalene acetic acid) and 2,4-D (2, 4-dichlorophenoxyacetic) are synthetic auxins. (ii) Physiological function 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. (iii) Agricultural horticultural applications Auxins also induce parthenocarpy, e.g., in tomatoes. They are widely used as herbicides 2, 4-D, widely used to kill dicotyledonous seeds, does not affect mature monocotyledonous plants. It is used to prepare seed-free lawns by gardeners. Auxin also controls xylem differentiation and helps in cell division.