View/Open - ResearchSpace - University of KwaZulu-Natal
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2.9.3.4 Abscisic acid<br />
Literature review<br />
Abscisic acid is rarely used in tissue culture protocols and has a negative effect on<br />
growth in most cases (PIERIK, 1997). It is mainly used in plant tissue culture to<br />
facilitate somatic embryo maturation (GABA, 2004) but may also be used in some<br />
regeneration processes and rarely used to produce somatic embryos (GABA, 2004).<br />
ABA induces the formation <strong>of</strong> essential LEA (late embryogenesis abundant) proteins<br />
found at late stages <strong>of</strong> embryogenesis in somatic and sexual embryos (GABA, 2004).<br />
LEA proteins are associated with tolerance to water stress resulting from desiccation<br />
and cold shock (GOYAL et al., 2005).<br />
2.9.3.5 Ethylene<br />
Ethylene or physiological reactions similar to that caused by ethylene, is produced by<br />
certain plastic containers, plant tissue and as a result <strong>of</strong> fire (PIERIK, 1997). This is<br />
the only gaseous natural plant growth regulator and it is naturally produced by all<br />
plant tissues in a controlled fashion (GABA, 2004). Endogenously produced ethylene<br />
can accumulate in a closed vessel to levels that negatively affect plant growth and<br />
development (PIERIK, 1997). The biological effect <strong>of</strong> ethylene depends on how air-<br />
tight the vessel is and the sensitivity <strong>of</strong> the plant material (GABA, 2004).<br />
Ethylene is primarily known for its effects on fruit ripening (GABA, 2004). Exposure to<br />
ethylene also results in reduced stem length, restricted leaf growth, premature leaf<br />
senescence and may cause increased growth <strong>of</strong> axillary buds (GABA, 2004). An<br />
enhanced ethylene concentration can induce callus formation, while inhibiting bud<br />
and shoot regeneration (GABA, 2004). Low concentrations <strong>of</strong> ethylene stimulate<br />
somatic embryogenesis, while high concentrations <strong>of</strong> ethylene inhibit somatic<br />
embryogenesis (GABA, 2004). Explants need a low level <strong>of</strong> ethylene for correct<br />
biological functioning, but too high an ethylene concentration leads to symptoms <strong>of</strong><br />
excess (GABA, 2004).<br />
Such symptoms <strong>of</strong> excess include stunted growth, a reduction in leaf size and leaf<br />
drop (PIERIK, 1997; NOWAK & PRUSKI, 2002). These plants do not acclimatise well<br />
to the in vivo environment and <strong>of</strong>ten desiccate shortly after being transferred to soil<br />
(NOWAK & PRUSKI, 2002).<br />
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