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SMOKE-INDUCED GERMINATION |
Few images capture the
resiliency and tenacity of life more than the emergence of young, green
seedlings through black, charred soil. Recurring fires are, of course,
an integral part of many ecosystems, and when such areas are protected
from conflagration, their local ecology becomes severely disturbed. But
how does the passage of a fire stimulate the seeds of certain species
to germinate so quickly? There is no simple answer; indeed, there is a
wealth of potential germination-inducing factors that change in the
post-fire environment, ranging from heat scarification to altered light levels to increased nitrate levels in the soil, to mention just a few.
In the 1990s, however, it became apparent that one of the most
important inducers of germination in post-fire environments is smoke
itself (Delange and Boucher, 1990
; Brown, 1993; Baldwin and Morse,
1994). This discovery raised a number of questions. For example, what
is the factor in smoke that induces germination? Is there more than one
factor? Will a given species respond differently to smoke from
different sources? How does smoke interact with other dormancy-breaking
cues, and what is the physiological mechanism by which smoke acts? The
answers to these questions are by no means clear
researchers are still
wrestling with phenomenological descriptions of this process. Only a
few pioneering papers have addressed the physiological mechanisms of
smoke-induced germinationthe focus of this article.
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Smoke Is a Powerful and Widespread Germination Cue |
The slow combustion of dry or green plant material from
many sources produces compounds that are water-soluble and that
stimulate the germination of many seeds. The active principals are
apparently produced around 160°C to 200°C and are volatilized at
higher temperatures. Although the identities of the active molecules
are unknown, their remarkable effects on seed germination have already
found wide application. Smoke extracts have already been used as seed
pretreatments for enhancing the conservation of threatened or rare
species, the horticultural exploitation of desirable plants, and in the reclamation of mine spoils and disturbed land (see review by Brown and
van Staden, 1997).
How widespread a phenomenon is smoke-induced germination? Surveys, most
of which have focused on species from fire-prone areas, have revealed
that more species respond favorably to smoke than do not (Brown et al.,
1993; Roche et al., 1997). The positive effect of smoke on seed
germination, however, is by no means limited to species native to
fire-prone habitats (Pierce et al., 1995). In many species, the effects
of smoke are astounding. Smoke, for example, has been reported to
enhance the germination of the South African plants Erica
clavisepala and Restio festuciformis by more than 7,000% and 25,000%, respectively (Brown et al., 1993,
1994).
There can be complex, species-specific interactions between smoke and
other environmental factors. In some cases, smoke is a better enhancer
of germination than heat, or heat and smoke act synergistically to
enhance germination. Factors such as seed age, light levels,
temperature, and hydration levels can also influence the extent of
smoke-induced germination. Whatever its mechanism, smoke's ability in
enabling seeds to rapidly overcome dormancy is long lasting. Seeds
treated with smoke retain an enhanced ability to germinate even after 1 year of storage (Brown et al., 1998).
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Smoke's Site of Action |
Anatomical studies of seeds of smoke-stimulated species have
revealed that such seeds have many analogous characteristics that
separate them from most heat shock-stimulated seeds. These features
include outer seed coats that are highly textured, a poorly developed
outer cuticle, the absence of a dense palisade tissue in the seed coat,
and a subdermal membrane that is semipermeable. Some evidence suggests
that the permeability characteristics of this subdermal layer may be
altered by smoke (Keeley and Fotheringham, 1998; Egerton-Warburton,
1998). The observed cuticular changes are consistent with the
hypothesis that volatiles in smoke exert a surfactant-like reaction to
break seed dormancy in the California chaparral annual
Emmenanthe penduliflora.
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Search for the Active Factor(s) in Smoke |
Aqueous extracts of plant-derived smoke are a complex mixture of
thousands of components (Adriansz et al., 2000). Thus, identifying the
active factor or factors is a daunting task. It is not surprising, therefore, that early studies have yielded divergent findings as well
as several potential candidates.
An interesting question is whether the burning or heating of all plant
materials generates the same active compound(s). van Staden et al.
(1995a) concluded that similar types of compounds are present in smoke
extracts derived from different plant material. A comparison of the
active smoke extracts from Passerina vulgaris and
Themeda triandra allowed for the identification of 12 active compounds, of which seven were found in both extracts (van
Staden et al., 1995b). Not all smoke derived from individual species is
equally effective in promoting seed germination. The strong, smoke-induced germination of T. triandra seed was not
seen in response to the smoke of all of the 27 species that Baxter et al. (1995) tested.
The finding that the combustion of cellulose alone has a stimulatory
effect on germination has raised the possibility that one of the
bioactive components of plant-derived smoke may originate from a
thermal breakdown product of hemicellulose or cellulose (Preston and
Baldwin, 1999). Aqueous smoke extracts prepared from a range of plants,
and extracts prepared by heating agar and cellulose, contained
compounds that stimulated the germination of light-sensitive lettuce (Latuca sativa) seeds (Jager et al., 1996a).
Chromatographic evidence suggested that the same active
compound(s) is produced from T. triandra leaves, agar,
and cellulose. Adriansz et al. (2000) identified 1,8-cineole as an
active germination enhancer in smoke. Different conclusions have been
reached about a possible role for octanoic acid being an active factor
in smoke (Sutcliffe and Whitehead, 1995; Jager et al., 1996b).
Baldwin et al. (1994) estimated that smoke-derived germination cues for
Nicotiana attenuata are active at concentrations of less
than 1 pg/seed and, due to their chromatographic behavior, inferred
that a number of different chemical structures are active. Hence,
although the chemical nature of the germination cue remains elusive, it
is known that the germination cues are extremely stable, water-soluble,
and active at low concentrations.
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A Role for Nitric Oxide? |
Keeley and Fotheringham (1997) found that the dormant
seeds of E. penduliflora were induced to germinate by
smoke or vapors emitted from smoke-treated sand or paper. Nitrogen
oxides induced 100% germination in a manner similar to smoke. The
authors estimated that chaparral wildfires generate sufficient nitrogen
oxides from combustion of organic matter or from post-fire biogenic
nitrification to trigger germination of E. pendulifera.
They also found that nitrogen oxide-triggered germination is not the
result of changes in imbibition, as is the case with heat-stimulated seeds.
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SMOKE-INDUCED GERMINATION
Smoke Is a Powerful...