Identify the plant growth regulator, which is not a growth-promoting hormone from the options given below.
Based on their role in a living plant body, the plant growth regulators (PGR) can be classified into growth promoting hormones and growth inhibiting hormones.
Growth promoting hormones are involved in several activities like cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, seed formation, etc. Example: Auxins, gibberellins, and cytokinins.
Growth inhibiting hormones induce dormancy and abscission (dropping of leaves, flowers, and fruits). They also perform an essential function in plant responses to wounds and stresses of biotic and abiotic origin. Abscisic acid belongs to this group.
Gaseous PGR, ethylene, could fit either of the groups. But since it predominantly inhibits growth activities, they can be categorized as growth-inhibiting hormones.