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Literature review
tolerable level (LIGHT et al., 2002). This effect would be a favourable adaptation to a
post-fire environment as the inhibitory compounds will only leach with sufficient
rainfall (LIGHT et al., 2002). Smoke thus enables seeds to germinate at the right
time, grow faster and have a more robust root system and so have a major
competitive advantage in their natural environment (BLANK & YOUNG, 1998).
In early experiments by DE LANGE and BOUCHER (1990), smoke was generated by
burning a mixture of fresh and dry plant material in a metal drum. This smoke was
then fed into a polythene tent and allowed to settle on the soil where the seeds were
stimulated to germinate. One disadvantage is that the germination cue of smoke is
easily confused with the effect of temperature on germination. This is because
temperatures slightly higher than ambient temperature significantly increase seedling
emergence in many species (BASKIN & BASKIN, 1998). Due to the complications of
separating smoke from high temperatures, direct exposure to smoke is not
recommended.
Aqueous smoke extracts were pioneered by DE LANGE and BOUCHER in 1990 and
since then many authors have demonstrated that the active component of airborne
smoke is soluble in water (BROWN & VAN STADEN, 1997; TAYLOR & VAN
STADEN, 1998; SPARG et al., 2005). The method for preparing such a solution
usually involves forcing smoke that has been generated in a drum to bubble through
water (BROWN & VAN STADEN, 1997). Combustion usually proceeds slowly and
the burning material is made to smoulder, thus releasing relatively large quantities of
smoke (BROWN & VAN STADEN, 1997).
In an experiment conducted by JÄGER et al. (1996) it was found that aqueous
smoke extracts prepared from a range of plants, as well as extracts prepared by
heating agar and cellulose contained compounds which stimulated the germination of
Grand Rapids lettuce seed. They also demonstrated that the same active compound
is produced by burning Themeda triandra leaves, agar and cellulose by providing
evidence obtained by thin-layer chromatography and high-performance liquid
chromatography. This was also demonstrated with the same methods by BROWN
and VAN STADEN (1997).
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