Hint: Growth hormones are defined as organic substances which are synthesized in minute quantities in one part of the plant body and transported to another part where they influence specific physiological processes. A group of plant hormones including auxins, gibberellins, cytokinins, ethylene, and abscisic acids are presently known to regulate growth.
Step by step answer:
Cytokinins are plant growth substances which act primarily on cell division and have little or no effect on extension growth. Physiological effects of cytokinins – cell division, cell enlargement, initiation of interfascicular cambium, morphogenesis, counteraction of apical dominance, breaking of dormancy, delay of senescence, accumulation, and translocation of solutes.
Hence, the correct answer is option A, i.e., Cytokinins
Plant growth hormones:
-Auxin includes all chemical substances which promote the growth of the stem or coleoptiles section. Physiological effects of auxins -Cell elongation and longitudinal growth, cell division in the cambium, cell division, and tissue culture, root growth and root initiation, apical dominance, prevention of abscission layer, parthenocarpy. They are also used as herbicides.
-Gibberellins are the plant growth hormones that enhance the longitudinal growth of stem when applied to intact plants. The presence of meristematic cells is necessary for the elongation growth caused by them. Physiological effects of gibberellins- elongation of the stem, the reversal of dwarfism, bolting and flowering, substituting the cold treatments, cell division in the cambium, etc
Plant growth inhibitors.
-Ethylene prevents elongation of stem and roots in a longitudinal direction however, the inhibition of growth in length is associated with radial enlargement of the tissue. Ethylene also causes suppression of bud growth and causes apical dominance. The growth of fruit is stimulated by ethylene. Ethylene stimulates the formation of a separation layer or abscission zone in leaves, flowers, and fruits. Ethylene stimulates rooting of cuttings, initiation of lateral roots, and growth of root hair
-Abscisic acid which is also a growth hormone which inhibits growth and induces bud dormancy, seed development and germination, abscission, geotropism, stomatal closing, helps to regulate water stress.
Note: Ethylene is a gaseous hormone involved in the ripening of fruit. Instead of acting separately, the plant hormones generally interact with each other in causing various growth and morphogenic effects.