Smoke as a germination cue

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Abstract
Fire is a major environmental selective force that influences plant communities in many parts of the world. Reproductive strategies have evolved as adaptations to the various factors generated by, and/or associated with, fire. This is particularly true for seeds, in which strategies have evolved that respond to both the physical and chemical germination cues that may be associated with fires. Some of these cues are clearly of a primary nature, while others may play a secondary and/or highly specific, or even permissive, role in the germination process. Clearly, there must be considerable interaction between the physical and chemical cues generated by wild fires. Smoke is the most striking chemical factor. Smoke released from burning vegetation contains a chemical signal that triggers germination of both fire climax and non‐fire climax species from different parts of the world. It is used in horticulture to stimulate seed germination of wildflower species and can break dormancy and improve germination of vegetable crops, such as lettuce and celery. Smoke can be applied to seeds immediately before sowing, or the seeds may be pretreated and stored until conditions are appropriate for sowing. Both smoke and aqueous smoke‐water are active in this respect. The active constituent(s) is volatile, thermostable, water soluble and long‐lasting in aqueous solution and in the soil. Attempts to identify the active compound(s) and to determine the mechanism(s) of action have been unsuccessful. Smoke treatment is effective on many seeds that have a light requirement for germination. A very clear concentration effect, resembling that of hormonal responses has been established with aqueous smoke solutions. Smoke extracts interact with gibberellins, cytokinins, abscisic acid and ethylene in photoblastic and in thermodormant seeds. However, despite these interactions it remains questionable whether smoke acts via hormones in stimulating seed germination. There is a clear connection between smoke and life history traits. Smoke may well be the overriding trigger for germination in relation to specific growth habits, regeneration strategies, seed storage, seed sizes, dispersal modes and structures for a large number of species growing in fire‐prone habitats. It is becoming increasingly clear that smoke as a germination (or growth regulating) cue must have evolved as part, or as a consequence of fire, as an evolutionary factor. As such, it is probably a very old development and serves as an additional protection mechanism to ensure germination at optimal times for seedling survival.