Smoke in the
water
There are plant
species in many parts of the world whose seeds germinate after fire. The
mechanisms for this are varied but in the two fire ephemerals, Actinotus
leucocephalus and Tersonia cyathiflora, studied by Katherine
Baker and colleagues in Australia
(pp. 1225–1236), organic chemicals in smoke appear to be the key signal.
However, these species also exhibit other facets of seed dormancy. To show
this, the authors buried seeds in either recently burnt or unburnt soil and
exhumed them at intervals over a period of 2 years to study the factors that
affected germination. Here we focus on the seasonal variation in response to
dormancy-breaking chemicals supplied as smoke-water. The data for T.
cyathiflora are very clear: at no time did the seeds germinate in plain
water; smoke-water was an absolute requirement, whether the seeds had been
buried in burnt or unburnt soil. However, imposed on this response to
smoke-water was a very clear seasonal pattern. Seeds
exhumed in spring did not germinate; seeds exhumed in autumn did. Thus there
was a seasonal cycling between total dormancy and germinability. Seeds stored
dry in the laboratory did not germinate in any season; burial in soil was
essential. In A. leucocephalus, the picture was more complex. Firstly,
seeds stored in the laboratory showed an increasing ability over 24 months to
germinate in smoke-water (burial was not necessary to break dormancy) with no
seasonal variation. Secondly, seeds buried in non-burnt soils germinated in
plain water. However, these seeds and those buried in burnt soils (which
required smoke-water) showed the same seasonal pattern of germinability as seen
in T. cyathiflora. In A. leucocephalus, the seasonal variation
could be obtained by exposing seeds in the laboratory to cycles of moisture and
temperature variation. This was not successful in T. cyathiflora, in
which other factors, as yet unknown, must play a major role.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
