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Literature review
BLANK and YOUNG (1998) reported that smoke increases the permeability to
solutes of a subdermal seed membrane for some species of California chaparral.
They also stated that these specific mechanisms of fire cue stimulation may be
species dependent. This response involves triggering via elevated nutrient content or
via the presence of stimulating gases in the smoke or triggering chemicals that
permeate the embryo and induce enzymatic changes that trigger germination. In an
experiment done by BROWN and VAN STADEN (1997) the dormancy of celery
seeds was broken by a combination of plant-derived smoke, benzyladenine and
gibberellins in the dark at temperatures between 18 and 26°C. From these results it
could be argued that smoke extracts act in a similar way to cytokinins in the celery
seed as it enhances gibberellin activity.
In a study conducted at the ultrastructure level by EGERTON-WARBURTON (1998),
the causal factor(s) associated with seed dormancy and the stimulation of
germination were investigated for Emmenanthe penduliflora (Hydrophyllaceae)
seeds. It was found that a short exposure to smoke resulted in two major
morphological changes. These changes are closely associated with the stimulation
and acceleration of germination. The first major and most visible smoke induced
morphological change observed was an intense chemical scarification of the external
cuticle (EGERTON-WARBURTON, 1998). This has a direct and destabilising effect
on the external cuticle and is manifested as the formation of oil-like spheres or
micelles. These micelles increase the surface area of the seed for the exchange of
water and solutes, as well as altering the hydrophobicity of the seed surface
(EGERTON-WARBURTON, 1998).
The second, more important smoke induced morphological change occurred at the
internal cuticle. Here the exposure to smoke stimulus caused a significant increase in
the number and diameter of permeate channels in the cuticle of Emmenanthe
penduliflora seeds (EGERTON-WARBURTON, 1998). The creation of such channels
within the cuticle increases the permeability of this layer. It was shown through the
use of the fluorescent apoplastic tracer dye, lucifer yellow (carbohydrazide), that
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