Agriculture Reference
In-Depth Information
(Pausas & Keeley
2009
). However, considering that fires have been well docu-
mented since the Paleozoic, we see little reason to not expect traits such as smoke-
stimulated germination to have had a very early origin.
Smoke-stimulated seed germination of species that regularly recruit seedlings
after fire is reported for diverse angiosperms and from MTC regions of South
Africa, Australia, California and the Mediterranean Basin, and has been invoked
as an example of convergent evolution that has arisen multiple times in different
lineages (Keeley & Bond
1997
). On the basis of other examples of physiological
convergence such as C
4
photosynthesis with a variety of different biochemical
pathways in different lineages (Roalson
2007
), this model would predict that unre-
lated species would evolve mechanisms that are triggered by different components
in smoke, and there is some support for that idea (Keeley & Fotheringham
2000
).
The mechanism of smoke overcoming dormancy does not appear to be the same in
all smoke-stimulated taxa. In some taxa it works directly on physiological barriers
to germination and in others it is associated with embryo development (Keeley &
Fotheringham
1997
,
1998
) and in others it overcomes external environmental
allelopathic inhibitors (Krock
et al.
2002
).
In contrast, Flematti
et al.
(
2004
) contend that smoke-stimulated germination in
postfire-recruiting species represents a universal response to a single organic
molecule present in smoke. This compound, karrikinolide, a type of butenolide,
draws such a claim because it turns out to be an effective germination stimulant in
not just species that recruit after fire but a vast array of plants where fire is not
involved in their life history, for example many agricultural species (van Staden
et al.
2000
). It has been proposed that this is a universal seed germination trigger
that can be produced by other forms of disturbance, thus has likely been part of
plant evolution for much of the Tertiary (Chiwocha
et al.
2009
).
Although this is clearly an intriguing finding, the conclusion in papers by
Flematti and Chiwocha that this is the universal trigger for postfire seed germin-
ation is premature. Smoke-stimulated fire ephemerals are known that fail to
respond to this butenolide (Downes
et al.
2010
). Other chemicals in smoke
are also known to trigger germination (Keeley & Fotheringham
1997
). The
primary question about karrikinolide and other butenolides is how can they
cue germination to precisely the immediate postfire year when they appear to be
widespread in nature and can trigger germination at extraordinarily low concen-
trations (10
7
M or lower)? We have no field studies to date showing whether
even these levels are present in soils after fire, but more importantly that they
disappear after the first couple of years when germination generally ceases.
These butenolides are apparently produced by other disturbances in the environ-
ment, yet a great many postfire species will not germinate after disturbances
other than fire. Additionally, smoke also produces a butenolide that inhibits the
stimulatory effect of smoke and it is unknown what balance of different bute-
nolides is needed to trigger germination (Light
et al.
2010
). Thus, there is the
need for research on the physiological and ecological role of this and other
compounds in smoke.
