term phytohormones is implied to those chemical substances which are synthesized by plants and thus, naturally occurring. On the other hand, there are several manufactured chemicals which often resemble the hormones in physiological action and even in molecular structure. Recently, another two groups, the brassinosteroids and polyamines were also known to behave like hormones. .
Plant growth regulators – classification Plant Growth Regulators are classified as natural and synthetic based on their source and a detailed flow diagram is given in Figure . . Plant Growth Regulators (PGRs) Natural (Phytohormones) Synthetic Plant Growth Promoters Growth inhibitors Auxin Gibberellin Cytokinin Ethylene Abscisic acid NAA , -D , , - T Figure . : Classification of Plant Growth Regulators .
Characteristics of phytohormones i. Usually produced in tips of roots, stems and leaves. ii. Transfer of hormones from one place to another takes part through conductive systems.
iii. They are required in trace quantities. iv. All hormones are organic in nature.
v. There are no specialized cells or organs for their secretion. vi. They are capable of influencing physiological activities leading to promotion, inhibition and modification of growth.
. Synergistic and Antagonistic effects i. Synergistic effects : The effect of one or more substance in such a way that both promote each others activity. Example: Activity of auxin and gibberellins or cytokinins.
ii. Antagonistic effects : The effect of two substances in such a way that they have opposite effects on the same process. One accelerates and other inhibits. Example: ABA and gibberellins during seed or bud dormancy.
ABA induces dormancy and gibberellins break it. . . Auxins .
Discovery During , Charles Darwin noted the unilateral growth and curvature of Canary grass ( Phalaris canariensis ) coleoptile to light. The term auxin ( Greek : Auxin – to Grow) was first used by F. W. Went in using Oats ( Avena ) coleoptile and isolated the auxin.
F. W. Went in collected auxin in agar jelly. Kogl and Haugen Smith ( ) isolated Auxin from human urine, and called it as Auxin A .
Later on in , similar active substances was isolated from