The plant growth regulators (PGRs) are small, simple molecules of diverse chemical composition. They could be indole compounds (indole-3-acetic acid, IAA, IBA); adenine derivatives (N-6-furfurylamino purine, kinetin), derivatives of carotenoids (abscisic acid, ABA); terpenes (gibberellic acid, $GA_3$) or gases (ethylene, $C_2H_4$). Plant growth regulators are variously described as plant growth substances, plant hormones or phytohormones in literature.