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Literature review
Plant growth and developmental processes are controlled by plant growth regulators
(GABA, 2004). The study of plant growth regulator function is complex because
several plant growth regulators usually work in concert with each other and their
concentration within plant tissues changes with time, season and developmental
stage (GABA, 2004). The effect of plant growth regulators on plant growth and
development depend on the chemical structure of the plant growth regulators used,
the plant tissue used and the genotype of the plant (GABA, 2004). The type and
concentration of the plant growth regulators used will vary according to the culture
purpose (PIERIK, 1997).
2.9.3.1 Auxins
Auxins (for example: IAA, NAA, 2,4-D, or IBA) is required by most plants for cell
division and root initiation (PIERIK, 1997; SMITH, 2000b). IAA or indole-3-acetic acid,
was the first plant growth regulator to be isolated (GABA, 2004). IAA is rapidly
degraded in growth media and inside the plant (GABA, 2004). For this reason
chemical analogues of IAA with similar biological activity are often substituted
(GABA, 2004). These more stable synthetic auxins include 2,4-D, IBA and NAA
(GABA, 2004). IAA is added at a concentration of 0.01 to 10 mg l -1 , while synthetic
auxins, such as IBA, NAA and 2,4-D, are used at concentrations of 0.001 to 10 mg l -1
(PIERIK, 1997). IAA can be considered to be a weak auxin (PIERIK, 1997). Cultures
in which a large quantity of IAA has been added are often less successful than
cultures to which low concentration of a stronger auxin, such as NAA have been
added (PIERIK, 1997).
At high concentrations auxin can suppress morphogenesis (SMITH, 2000b). Auxins
have numerous effects on plant growth and differentiation, depending on their
chemical structure, their concentration and the affected plant tissue (GABA, 2004).
Auxins generally stimulate cell elongation, cell division in cambium tissue and,
together with cytokinins, the differentiation of phloem and xylem and the formation of
adventitious roots (PIERIK, 1997). High concentrations of auxins can induce somatic
embryogenesis (GABA, 2004).
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