Abstract

According to Environmental Protection Agency, a plant growth regulator (PGR) is defined as “any substance or mixtures of substances intended, through physiological action, to accelerate or retard the rate of growth or maturation or otherwise alter the behavior of plants Yau [1] & Mahajan [2]. As reported in the current literature, five principal categories of PGRs are recognized in plants: Auxins (Auxs), Gibberellins (GAs), Cytokinins (CKs), Ethylene (ET), and Abscisic acid (ABA). They are known to be produced in higher plants. In addition to these categories, two other categories appear to be active, in some cases, for regulating the plant growth: the brassinosteroids (BRs) and polyamines (POA) Naqvi [3]. Over 70 years, Aux, GAs, CKs, ABA and ET were considered as the main groups of phytohormones (for a review, see Kende & Zeevaart [4] and Hopkins & Hüner [5] classified the plant hormones (or PGRs) into five classes: GAs, Auxs, ET, CKs, ABA and BRs. In their review, Ogunyale et al. [6] classified PGRs in five major classes: Auxs, CKs, GAs, ET and ABA.