Liquid Culture Systems for in vitro Plant Propagation

100 K. Y. Paek et al.
3.1 Dissolved oxygen
One of the main functions of the bioreactor is to promote the mass
transfer of oxygen from the gaseous to the liquid phase. Since oxygen is only
sparingly soluble in water (0.25 mmol l -1 at 25�C. 1 atm., 21% (v/v) O2 in
the air ), it is necessary to drive the diffusion of oxygen into the aqueous
phase to meet the demand of actively growing tissues or cells (Leathers et
al., 1995). This is accomplished by modifying operational parameters such
as aeration rate, agitation speed, impeller design, gas mixing and bioreactor
configuration. Culture mixing is also important because dissolved oxygen
(DO) must be transported rapidly to the culture tissues or cells. In general, it
is essential that the dissolved oxygen concentration remains above the
critical DO2 level at all the times for optimal cell growth (Leather et al.,
1995; Sajc et al., 2000). The critical dissolved oxygen concentration, DO2crit,
can be described as the dissolved oxygen concentration above which no
further increase in specific oxygen uptake rate can be measured. At DO2
levels below the DO2crit, cells have reduced energy (ATP) levels, which may
have direct effects on cellular metabolism and morphology (Leathers et al.,
1995). Practically, this is important because the DO2crit value is used for
designing appropriate bioreactor operating systems to ensure that an oxygenlimiting
condition does not suppress the metabolic activity of the culture.
Therefore, supplying adequate amounts of oxygen (above the DO2crit) is a
major concern in bioreactor scale-up.
3.2 Mixing
The other key parameter is mixing, which is necessary to distribute
equally cells or tissues, and nutrients throughout the liquid phase (Leathers
et al., 1995, Sajc et al., 2000; Honda et al., 2001). Mixing is normally carried
out by sparging, mechanical agitation or a combination of these two, but the
magnitude of hydrodynamic forces associated with mixing should be small
enough not to cause cell or tissue damage, but sufficient to stimulate selected
cell functions. However, there has been little quantitative work on the effect
of hydrodynamic forces on plant tissue engineering. Previous studies have
focused mostly on the kinetics of cell growth and product formation, and the
effect of hydrodynamic conditions on the structure and composition of plant
tissue is not well understood.