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(typically 4 weeks) and weighing the callus after this time using aseptic techniques
(STEPHAN-SARKISSIAN, 1990).
2.9.7 Organogenesis
Organogenesis involves the de novo production of organs directly from an explant or
through initial callus culture (SCHWARTZ et al., 2004). In the Iridaceae
organogenesis involves the regeneration of unipolar meristems (ZIV, 1997).
Organogenesis is regulated by altering the components of the culture medium
(BROWN & CHARLWOOD, 1990). Most important of these components is the auxin
to cytokinin ratio, which determines the developmental pathway the regenerating
tissue will take (BROWN & CHARLWOOD, 1990). Shoots are usually induced to form
first by increasing the cytokinin to auxin ratio of the culture medium (BROWN &
CHARLWOOD, 1990). These shoots can then be easily rooted (SLATER et al.,
2003).
De novo organ formation via indirect organogenesis, which involves intermediate
callus formation and a differentiation phase, may increase the possibility for
somaclonal variation (SCHWARTZ et al., 2004). Any stage in the process of
organogenesis that involves callus growth should be minimized.
After dedifferentiation the explant acquires a state of competence, defined as its
ability to respond to organogenic stimuli (SCHWARTZ et al., 2004). The attainment of
competence can not always be achieved with a single step. The induction phase
occurs between the time of competence and determination (SCHWARTZ et al.,
2004). During induction, processes resulting from the expression of genes guides
developmental processes and precede morphological differentiation. It has been
suggested that such a genetically determined developmental process can be
interrupted by certain physical and chemical stimuli (PIERIK, 1997). At the end of the
induction phase, the cells are fully committed to the production of shoots or roots. At
this point the tissue can be removed from the root or shoot producing medium and
placed on a basal medium without plant growth regulators (PGR’s), containing
mineral salts, vitamins and a carbon source (SCHWARTZ et al., 2004). The desired
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