Plant hormones are internal chemicals involved in the control of growth and development of plant cells. there are five recognised major groups of plant hormones:
Auxin (such as IAA)
Stimulates stem elongation, root growth, differentiation and branching, apical dominance, development of fruit; instrumental in Phototropism and gravitropism. Auxins stimulates females. Found in endosperm and embryo of seed; meristems of apical buds and young leaves.
Cytokinins (such as kinetin)
Affect root growth and differentiation; stimulate cell division and growth, germination, and flowering; delay senescence. They are synthesised in the roots and transported to other organs.
Gibberellins (such as GA1)
Promote seed and bud germination, stem elongation, leaf growth; stimulate flowering (G.A. enhance male) and development of fruit; affect root growth and differentiation.
Abscisic acid (ABA)
Inhibits growth; closes stomata during water stress; counteracts breaking of dormancy. Found in leaves, stems, green fruit.
Promotes fruit ripening; opposes or reduces some effects of auxin; promotes or inhibits growth and development of roots, leaves, flowers, depending on species. Produced in tissues of ripening fruits, nodes of stems, senescent leaves.
The competence of a target cell to respond to hormones is influenced by receptor proteins for the hormone and can be affected by age, other cells, and the timing of the signal. It may be true that plant hormones are necessary, but do not control the response in question. New groups of plant hormones have been reported.