Agriculture Reference
In-Depth Information
Box 3.1 (
cont.
)
As a general rule species with smoke-stimulated germination have very diffe-
rent seeds from those with heat-stimulated germination (Keeley & Fotheringham
2000
). The latter are characterized by having a water-impermeable seed coat that
is ruptured by heat shock. Following such a heat treatment, and given the
appropriate thermal, light and moisture regime, these seeds germinate readily.
Dormant seeds of smoke-stimulated species have water-permeable seed coats
and heat has no detectable effect on permeability. Outer seed coats are highly
sculptured and in all cases so far examined they lack a dense palisade layer.
Commonly the outer coat comprises loosely packed tissues with a subdermal
semi-permeable cuticle. Most of the smoke-stimulated species fully imbibe water
during dormancy, indicating smoke-stimulated germination is overcoming
endogenous dormancy (Keeley & Fotheringham
1998
). However, some smoke-
stimulated species can be germinated by mechanical seed coat scarification
(Keeley & Fotheringham
1997
). In most smoke-stimulated species, dormancy
appears to be innate but in some cases it apparently overcomes secondarily
induced dormancy (Krock
et al.
2002
).
The transfer of smoke or other combustion chemicals to the seed appears to take
several pathways. The exposure of dried seeds to smoke prior tomoist incubation is
one mode, but aqueous and vapor transfers from smoke-treated substrates to seeds
during moist incubation also are potential modes (
Fig. B3.1.1
).
A large number of experimental studies have been conducted searching for
smoke-stimulated germination. A significant number of these have failed to
appreciate that even in smoke-stimulated seeds, some concentrations of smoke
are lethal and the lethal dose is species specific (Keeley & Fotheringham
1998
).
Studies that report a lack of smoke-stimulated germination, without conduct-
ing the experiment over a large concentration gradient, or demonstrating that
the seeds are still viable after smoke treatment, are inconclusive.
Smoke has been demonstrated to stimulate germination in many species that
do not recruit seedlings after fire. This includes plants from arid land commu-
nities where fire is currently rare as well as a great many agricultural species
(Pierce
et al.
1995
; Drewes
et al.
1995
; Taylor & van Staden
1998
). It has been
suggested that this observation raises questions about the adaptive significance
of smoke-stimulated germination in species from fire-prone communities
(Pierce
et al.
1995
). We agree that all observed cases of smoke-stimulated
germination should not be interpreted as evidence of an adaptive trait. How-
ever, for species with seedling recruitment restricted to postfire environments,
and with seeds that remain dormant except when exposed to smoke (i.e. smoke-
dependent
germination), it seems inescapable that this is truly an adaptive trait.
Whether or not it is a true adaptation that evolved in response to fire is a rather
different question (see
Chapter 9
). It is advisable to distinguish between smoke-
stimulated germination and smoke-dependent germination in sorting out
explanations of trait selection.
Continued
