Plant Growth And Development

The physiological effects of plant growth regulators are described in NCERT Class 11, Chapter 15 (Plant Growth and Development). Gibberellins are growth promoters best known for causing pronounced elongation of stems and internodes. When gibberellic acid is applied to genetically dwarf varieties of pea or maize, the plants grow as tall as their normal counterparts, demonstrating that the dwarfism is due to a deficiency in gibberellin-mediated internode elongation. Abscisic acid is a growth inhibitor that promotes dormancy and stomatal closure, so it would suppress rather than promote elongation. Ethylene is a gaseous regulator chiefly associated with fruit ripening, senescence and horizontal growth, not the lengthening of internodes in dwarfs. Cytokinins primarily promote cell division and delay senescence but do not characteristically reverse genetic dwarfism through internode elongation. A biological consistency check supports the answer: since gibberellins specifically stimulate the elongation of stem internodes, supplying them to a dwarf that lacks adequate internode growth restores normal height, exactly the observed effect.