View/Open - ResearchSpace - University of KwaZulu-Natal

Literature review
environmental stresses of the post-propagation environment (HUYLENBROECK et
al., 2000).
During acclimatization there is a switch to autotrophy and changes in stomatal
functioning and cuticular composition (HUYLENBROECK et al., 2000). Water is
rapidly lost from the in vitro cultured plantlet because of the failure of the stomata to
respond to stimuli that would normally induce their closure (ROBERTS et al., 1990).
The poorly developed cuticle results in a rapid loss of water (ROBERTS et al., 1990).
Vitrified plants do not acclimatise well to in vivo conditions. Vitrified plants are
common where liquid media and low agar concentrations are used (PIERIK, 1997).
In vitrified plantlets there is a reduced deposition of cellulose and lignin, leading to an
increase in water uptake by the cells and resulting in glassy swollen leaves and
stems (ROBERTS et al., 1990). Because of this, and the low rates of photosynthesis
sustained by the in vitro cultured plants, they easily suffer from photoinhibition and
water stress; leading to the production of reactive oxygen species
(HUYLENBROECK et al., 2000). It has been demonstrated that micropropagated
plants develop antioxidant mechanisms during acclimatization (HUYLENBROECK et
al., 1998).
In vitro grown leaves are the only source of nutrition to cover metabolic demands and
to sustain plant adaptation and regrowth during the first days after transplanting
micropropagated plants to greenhouse conditions (HUYLENBROECK et al., 1998).
The good and sustainable health of leaves is therefore essential to the
acclimatization and survival of the plant (HUYLENBROECK et al., 1998).
Plant hardening is usually carried out under greenhouse conditions to increase the
chance of survival (AHLOOWALIA & PRAKASH, 2002). A commonly used
greenhouse is the Quonset type. This consists of movable or fixed benches with
hardening tunnels on them (AHLOOWALIA & PRAKASH, 2002). It is also
advantageous to acclimatise plants to lower humidities while they are still under in
vitro conditions (AHLOOWALIA et al., 2002). In this way, plants grown in strongly
82