View/Open - ResearchSpace - University of KwaZulu-Natal

Literature review
(DEBERGH, 1994). In vitro techniques such as embryo rescue, protoplast fusion and
genetic transformation enable plant breeders to accomplish wider crosses
(DEBERGH, 1994).
2.9.1 Explant selection
There are numerous variables that should be considered when selecting an explant
for in vitro culture. These are mostly caused by the physiology of the plant and its
environment.
Physiologically younger tissues are generally much more responsive to tissue culture
(SMITH, 2000b). In many cases, older tissues will not form callus that is capable of
regeneration. Younger tissue is usually the newest formed and therefore easier to
surface disinfect (KYTE & KLEYN, 1996; SMITH, 2000b). Plant material at the base
of a plant may, however be more suitable than explants higher up (KYTE & KLEYN,
1996). The smaller the explant, the harder it is to culture (SMITH, 2000b). Larger
explants have more nutrient and plant growth regulator reserves to sustain the
culture. A large explant is often more difficult to decontaminate (KYTE & KLEYN,
1996; SMITH, 2000b). Explants should be obtained from plants that are healthy as
opposed to plants under nutritional or water stress or plants exhibiting disease
symptoms (SMITH, 2000b). Plant material in a state of active growth is cleaner, and
more suitable for aseptic culture, compared to dormant tissue. To control
contamination, donor plants should be pre-screened for diseases (SMITH, 2000b;
AHLOOWALIA et al., 2002).
The season of the year can have an effect on contamination and the response of the
explant in culture (SMITH, 2000b). Contamination tends to increase as summer
progresses. Plant material obtained from the field is often more contaminated than
material obtained from greenhouses or growth chambers (KYTE & KLEYN, 1996;
SMITH, 2000a). Mother plants should ideally be maintained under dust, insect and
disease free conditions (AHLOOWALIA et al., 2002). These plants should also not
be stressed and they should preferably be grown under controlled conditions that
promotes active growth (KANE, 2004). Such conditions should preferably include
conditions of low relative humidity. Drip irrigation should preferably be used as the
64