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Literature review
and the addition of compounds such as vitamins and a carbon source is necessary
(SLATER et al., 2003).
Callus culture is often performed in the dark as light can result in differentiation of the
callus (SLATER et al., 2003). The culture often loses its requirement for auxin and/or
cytokinin during long-term culture (SLATER et al., 2003). This process is known as
habituation and is common in callus cultures from some species such as sugar beet
(SLATER et al., 2003).
By manipulating the auxin to cytokinin ratio whole plants can subsequently be
produced from callus cultures (PHILLIPS et al., 1995). Callus culture can also be
used to initiate cell-suspension cultures (PHILLIPS et al., 1995).
Endogenous levels of plant growth regulators and polar growth regulator transport
can drastically influence callus induction (PIERIK, 1997; SMITH, 2000b). Explant
orientation and different sectioning methods affect callus induction (SMITH, 2000b).
Callus cultures subcultured regularly on agar media exhibit a sigmoidal growth curve
(PHILLIPS et al., 1995). PHILLIPS et al. (1995) describe five phases of callus growth.
Phase I is a lag phase, where cells prepare to divide. Phase II is an exponential
phase, where the rate of cell division is the highest. Phase III is a linear phase, where
cell division slows, but the rate of cell expansion increases. Phase IV is a
deceleration phase, where the rates of both cell division and expansion decrease and
phase V is a stationary phase, where the number and size of cells remain constant.
Callus growth can be monitored in a non-destructive manner using fresh weight
measurements (STEPHAN-SARKISSIAN, 1990; PHILLIPS et al., 1995). Dry weight
measurements are more accurate, but involve the destruction of the sample
(PHILLIPS et al., 1995). Mitotic index measurements of cell division rates are not
easy to perform as they require numerous measurements to be made at various time
intervals with very small amounts of tissue (PHILLIPS et al., 1995). Fresh weight
measurements are performed by culturing a known weight of callus for a given time
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