Where There’s Smoke, There’s Germination
The role that plant-derived smoke has played in our history is far greater than most people realize. Ever since our early cave-dwelling ancestors started experimenting with fires more than 1.6 million years ago, we have employed smoke for a variety of purposes. More than 1,300 different species of plants have since been smoked or smudged for medicinal, cultural, and self-indulgent reasons. Uses range from treating asthma attacks and other respiratory-related illnesses, communicating with gods, getting “high,” keeping insects and agents of evil at bay, and even for stopping earthquakes! So profound has been the use of plant-derived smoke in the past that smokable plants and their substances have diverted the course of history, have spawned powerful organizations, and have even been fought over in wars.
While many of the uses for smoke may seem obvious to most people, there is one that the vast majority may not have considered. Remarkable as it may seem, smoke has been successfully employed to promote germination in the seeds of a variety of plant species. The first bona fide scientific report on this phenomenon appeared less than 30 years ago, but smoke may have been used for this purpose for centuries prior to that. The French Récollet missionary, Gabriel Sagard, reported the use of smoke for seed germination in the New World as early as 1632.
In 1624, Sagard, like so many of his Franciscan brothers, was sent to study the customs and mores of the tribes of New France. This was an area of North America that was originally colonized by the French and extended from Newfoundland to the Gulf of Mexico. During his time with the tribes of the Huron people, Sagard noticed that they employed an interesting method for promoting germination of pumpkin seeds. To improve the overall number of sprouts, they reportedly suspended special germination boxes, lined with multiple layers of soil and seed, above fires where the smoke formed. It is not yet clear, however, whether or not it was the heat of the fire, the smoke it produced, or a combination of the two that induced the effect. Contemporary studies suggest that both can individually, or synergistically, promote germination.
Farther to the south, in the highlands of Guatemala, the Mam Maya people used the copal resins of Bursera trees, which are related to the plants that produce frankincense and myrrh, to improve germination in maize seeds. During a ritual called pomixi, or “copal of maize,” resins from the bark were combined with a drop of sacrificial blood and were then burned to fumigate and prepare seeds. Other smoke-farming practices were reported elsewhere in the world. These included those of early peasant farmers of the Eifel Mountains of Germany, farmers of Great Britain’s Isle of Man, and the Ndebele tribesmen (also called the Matabele) of southern Africa. It is doubtful that they realized the effects of smoke on their crops and probably used it more as a fertility charm rather than a farming tool.
Smoke and Seed Dormancy
The first real scientific evaluation of the smoke germination phenomenon was made 27 years ago by researchers in the Republic of South Africa. Working at the National Botanical Institute and the Botany Department of the University of Stellenbosch, the team of de Lange and Boucher discovered that seeds of a local South African plant species, false heath (Audouinia capitata), sprouted after they had been treated with plant-derived smoke. This species occurs naturally in the fire-prone shrublands of the Cape Floristic Region of South Africa. The researchers concluded that smoke had somehow broken dormancy in the seed.
Not all the fire-prone species that undergo dormancy require smoke to act as a germination cue, however. A variety of dormancy mechanisms exist, many of which require other environmental cues to induce germination. Renowned seed biologists Carol and Jerry Baskin of the University of Kentucky have devised a system of classification for categorizing the five known types of dormancy. These include morphological dormancy, morphophysiological dormancy, physiological dormancy, physical dormancy, and combinational dormancy (physiological dormancy and physical dormancy). Smoke is most likely to act on seeds that display some form of physical dormancy.
Exactly how smoke affects seed germination is not well understood. Researchers in Western Australia and South Africa recently discovered that one of the active substances in smoke was butenolide. This substance is produced when cellulose, a naturally occurring plant fiber, is burned. How butenolide and other related compounds actually affect seed germination is not yet known, but is being actively investigated.
Tallgrass Prairie Species
The effects of smoke were recently reported for a number of species in other fire-prone ecosystems, including the Southwest Botanical Province of Western Australia and the chaparrals of the Californian Floristic area. Many arid, temperate, and subtropical regions of the world also possess smoke-responsive species, as do the Mediterranean regions of Europe. As part of our research at the Chicago Botanic Garden, we have discovered that several midwestern tallgrass prairie species similarly produce seed that respond in one way or another to smoke. Until then, it was thought that a period of cold treatment, called stratification, was the only requirement to promote germination, even though not all plants need it. In studies we conducted in our laboratory, the ex situ germination of 37 prairie species was measured in response to aerosol smoke that was produced by burning commercially available straw. Approximately one-third of the species tested reacted positively to smoke, while others were either inhibited, or exhibited no effect at all.
Six different species of Echinacea were included in our study, with germination in all but one being promoted. The only exception was Topeka purple coneflower (Echinacea atrorubens). Germination of Tennessee coneflower (E. tennesseensis) seed, in contrast, improved considerably following smoke application. One minute of smoke treatment was sufficient to increase germination from approximately 50% of viable seed to over 90%.
This finding is significant in lieu of the fact that the species is federally listed as endangered in the United States. It is in cases such as these that the use of smoke, and some of its natural products, may prove useful in the future. Other threatened and endangered species successfully treated in this manner include pale purple coneflower (E. pallida) and purple coneflower (E. purpurea), both of which occur in Illinois but are not currently imperiled in the state.
Germination in 9 of the 11 other members of the daisy family (Asteraceae) tested thus far, and which are also common in midwestern tallgrass prairies, have tested positive to smoke treatments. One that occurs locally in our state is devil’s bite (Liatris scariosa).
This species is listed as threatened in Illinois and threatened or endangered elsewhere. Up to 60 minutes of exposure to smoke was required to improve its germination by almost 30%. Other plants that are imperiled elsewhere, but commonly occur in Illinois’ tallgrass prairies and responded to smoke, include stiff goldenrod (Oligoneuron rigidum var. rigidum, formerly Solidago rigida) and American feverfew (Parthenium integrifolium).
We have also tested species that are not imperiled, such as tickseed coreopsis (Coreopsis lanceolata). Different exposures to smoke revealed that 10 minutes doubled the number of germinants in this species. Similar responses were observed for seed of members of the Fabaceae, Poaceae, Rhamnaceae, and Lamiaceae families. Of interest to Illinois are Canadian milk vetch (Astragalus canadensis; Fabaceae), sideoats grama (Bouteloua curtipendula; Poaceae), and New Jersey tea (Ceanothus americanus; Rhamnaceae). Germination in these species increased proportionally to smoke treatments ranging from 1 to 60 minutes of duration. Hairy mountain mint (Pycnanthemum pilosum; Lamiaceae), a species considered very rare in the Chicago region and elsewhere, required exposures of approximately 32 minutes to induce a significant response. Germination was increased by almost 33% following smoke treatments of that duration. Doses beyond that inhibited germination. Exposure times greater than 60 minutes significantly decreased germination, or inhibited it completely in most of the species screened.
One species whose germination was inhibited by short exposures to smoke was wild bergamot (Monarda fistulosa; Lamiaceae). This is a species with a wide distribution in Illinois and elsewhere in the United States. Its germination was decreased by approximately 30% after only 1 minute of smoke treatment. Many other species throughout the world have reacted to smoke in this way. This comes as no surprise, given the number of potentially harmful compounds and oxidatively abrasive chemicals that can occur in smoke. Many plants release volatile chemicals, such as camphor, alpha-pinene, alpha-phelandrene, and eugenol, during fires. Some of these and other potentially toxic substances may also be harmful to humans in high doses, so appropriate measures should be taken to avoid inhaling the smoke during work of this type. Compounds such as polycyclic aromatic hydrocarbons and gases like carbon monoxide are common smoke products that can seriously affect the health of humans. The inhibitory effects of smoke on seeds can, at least, be counteracted if they are immediately rinsed with water for 2 to 5 minutes after exposure. This will not diminish the effects of the promotory agents.
Two of the tallgrass species that did not respond to any smoke treatments were blue wild indigo (Baptisia australis; Fabaceae) and Indian grass (Sorghastrum nutans; Poaceae). It is not yet clear whether or not they simply lack the receptors or mechanisms to respond to the chemicals in smoke, or if there are other factors at play. Results of other studies conducted in our laboratory with Echinacea species suggest that the age of their seeds and prolonged periods of cold stratification can significantly alter the effects of smoke. More research is obviously required to resolve this issue.
The results of our studies have provided us with more insight into the fire ecology story of midwestern tallgrass prairies, along with many more unanswered questions. What is certain is that smoke may play a greater role in maintaining tallgrass prairie composition and structure than was previously thought. In a book due to be published later this year, we list more than 600 species of plants—including the ones mentioned here—whose seeds respond to smoke treatment. The book also lists over 2,000 ethnobotanical uses for smoke derived from over 1,300 plant species.
Other Uses for Smoke
In addition to helping break seed dormancy, smoke and its products may have other useful applications. They could potentially be used in the fight against invasive plant species. By making the seeds of an invasive species germinate all at once, the seedlings can be treated with a single dose of herbicide, saving time and money for land managers. Smoke also could be used in environments that require fire-related cues, but cannot be burned due to their locations. These include remnant prairies in urban environments and roadsides. Smoke water—water through which aerosol smoke has been bubbled—can be used in cases such as these. Of course, butenolide, which will soon be available commercially, should help with work of this type.
Conclusions
It is not at all surprising that smoke has been used to promote and even inhibit germination. Plants produce hundreds of natural products, many of which are liberated and carried aloft with other smoke particles when plants are burned. We are only now starting to discover how these products are generated and, more importantly, how to apply them. That the seed of many species respond to smoke is also not surprising. Various ecosystems are prone to fire, especially in Mediterranean climatic regions, and have resulted in plants that have adapted to its effects and products. Our studies have shown, however, that other ecosystem types, including temperate, midwestern tallgrass prairies, may also produce smoke-responsive plant species. More research into this phenomenon will be required before we have all the answers.
Marcello Pennacchio is an ethnobotanist and conservation scientist with the Division of Plant Science and Conservation at the Chicago Botanic Garden in Glencoe. Lara V. Jefferson is a restoration ecologist and manager of training programs, also with the Division of Plant Science and Conservation at the Chicago Botanic Garden. Kayri Havens is the Medard and Elizabeth Welch Director of the Division of Plant Science and Conservation at the Chicago Botanic Garden.
Effects of Smoke on the Seed Germination of Native Plants
Promotes Germination
American feverfew (Parthenium integrifolium L.)
Bush’s purple coneflower (Echinacea paradoxa [J.B.S. Norton])
Canadian milk vetch (Astragalus canadensis L.)
Hairy mountain mint (Pycnanthemum pilosum Nutt.)
New Jersey tea (Ceanothus americanus L.)
Pale purple coneflower (Echinacea pallida [Nutt.] Nutt.)
Purple coneflower (Echinacea purpurea [L.] Moench)
Round-headed bush clover (Lespedeza capitata Michx.)
Sideoats grama (Bouteloua curtipendula [Michx.] Torr.)
Stiff goldenrod (Oligoneuron rigidum var. rigidum)
Tennessee purple coneflower (Echinacea tennesseensis [Beadle] Small)
Tickseed coreopsis (Coreopsis lanceolata L.)
No Effect
Big bluestem (Andropogon gerardii Vitman)
Black-eyed Susan (Rudbeckia hirta L.)
Blue wild indigo (Baptisia australis [L.] R. Br. ex Ait.f.)
Bottlebrush grass (Elymus hystrix L. var. hystrix)
Butterfly weed (Asclepias tuberosa L.)
Compass plant (Silphium laciniatum L.)
Decurrent false aster (Boltonia decurrens [Torr. & Gray] Wood)
Indian grass (Sorghastrum nutans [L.] Nash)
Kankakee mallow (Iliamna remota Greene)
Leadplant (Amorpha canescens Pursh)
Little bluestem (Schizachyrium scoparium [Michx.]
Nash var. scoparium)
Ohio spiderwort (Tradescantia ohiensis Raf.)
Pale Indian plaintain (Arnoglossum atriplicifolium [L.] H.E. Robins)
Prairie dropseed (Sporobolus heterolepis [Gray] Gray)
Purple prairie clover (Dalea purpurea Vent.)
Rough blazing-star (Liatris aspera Michx.)
Sawtooth sunflower (Helianthus grosseserratus Martens)
Sea oats (Chasmanthium latifolium [Michx.] Yates)
Smooth blue aster (Symphyotrichum laeve [L.] A.& D.
Love var. laeve)
Hinders Germination
Fringed willow herb (Epilobium glandulosum Lehm)
Royal catchfly (Silene regia Sims)
Wild bergamot (Monarda fistulosa L.)
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Topic: Smoke and Germination some new Evidence (Read 3490 times)