Fire is an important natural element of many ecosystems ranging from forests to grasslands, and is recognized as an important ecosystem management tool. Several aspects of fire, such as heat, quick release of nutrients from burnt plant tissue, and compounds in ash, have been studied for their influence on seed germination and/or plant growth. Since 1990, focus has been directed on the importance of smoke compounds in breaking seed dormancy and enhancing germination for some species. DeLange and Boucher (1990) were the first to report that compounds found in plant smoke stimulated seed germination. Since their initial work, many other studies have shown that specific water-soluble compounds found in the smoke from burning plant tissue play an important role in breaking seed dormancy (Dixon et al., 1995; Brown and van Staden, 1997; Blank and Young, 1998; Keeley and Fotheringham, 1998; Tieu et al., 1999; Landis, 2000; Pennacchio et al., 2007; Jefferson et al., 2008; Lindon and Menges, 2008). Investigations of smoke compounds focused on plant species from shrubland in South Africa, Florida, and Western Australia; chaparral in California; and tallgrass prairie in the midwestern United States (DeLange and Boucher, 1990; Brown, 1993a, 1993b; Dixon et al., 1995; Keeley and Fotheringham, 1998; Tieu et al., 1999; Cochrane et al., 2002; Pennacchio et al., 2007; Jefferson et al., 2008; Lindon and Menges, 2008). Reports show that some of the most widely described stimulants in smoke are karrikins (a class of butenolide derivatives), water-soluble, naturally occurring compounds that are created from burning plant tissue and are involved with breaking seed dormancy (Flematti et al., 2004, 2008, 2009; Pennacchio et al., 2007). Recently, cyanohydrins, also found in smoke, have been identified as influencing seed germination and seedling growth (Chiwocha et al., 2009; Nelson et al., 2012).
For those interested in investigating the influence of smoke on seed germination, various methods have been reported for exposing seeds to smoke compounds. These methods include the application of commercial smoke preparations, direct fumigation of seeds, and the application of smoke solutions. Three currently available commercial preparations are: “Seed Starter– Australian Smoky Water” from the Friends of Kings Park and Botanic Garden in Perth, Western Australia; “Kirstenbosch Instant Smoke Plus” seed primer from the National Botanical Institute in Cape Town, South Africa; and Cape “Super Smoke Plus” Seed Germinator Primer from Seedman.com. Commercial preparations are convenient, but have disadvantages. For instance, they may contain additional seed germination enhancers such as gibberellic acid, which is known to break plant embryo dormancy and stimulate germination in some species (Brown and van Staden, 1997; Baskin and Baskin, 1998; Keeley and Fotheringham, 1998; Cochrane et al., 2002). Separating the effects of smoke compounds from other additives would not be possible when using commercial preparations. In addition, commercial preparations do not allow researchers to control the species of plant materials burned to produce smoke solutions, and chemicals are known to vary within different assemblages of plants. Although butenolides are formed when cellulose is burned and all plants contain cellulose, other compounds yet to be discovered may be involved in breaking seed dormancy and these may depend on the species of plants burned (Landis, 2000; Flematti et al., 2004, 2009). Therefore, producing smoke—whether for fumigation of seeds or for smoke solutions—using plant species from the ecosystem being studied may be the most appropriate method to produce smoke compounds that would occur naturally (Baxter et al., 1995). Standardized smoke solutions also could be produced from burning cellulose filter paper (Flematti et al., 2004). Many studies use direct fumigation of seeds to enhance seed germination (DeLange and Boucher, 1990; Brown et al., 1993; Baxter et al., 1995; Dixon et al., 1995; Blank and Young, 1998; Keeley and Fotheringham, 1998; Pennacchio et al., 2007; Jefferson et al., 2008; Lindon and Menges, 2008). Direct fumigation of seeds is effective for promotion of germination of some species, but because the identified active compounds are known to be water soluble, they could naturally be carried into the soil by rainwater or snowmelt. Hence, the use of smoke solutions also is effective and may be more convenient due to ease of storage and application. Smoke solutions have therefore become a common method to investigate the influence of smoke compounds on seed germination (Brown, 1993a; Brown and van Staden, 1997; Keeley and Fotheringham, 1998; Tieu et al., 1999; Landis, 2000; Flematti et al., 2004). Previous reports have described systems used to prepare smoke solutions, although descriptions often lack details of construction and/or have limitations such as being expensive to build or not being suitable for indoor use (Brown, 1993a; Baxter et al., 1994; Brown and van Staden, 1997; Keeley and Fotheringham, 1998; Tieu et al., 1999; Landis, 2000; Flematti et al., 2004).
Because of these limitations, we developed an effective system to prepare small volumes of smoke solutions from burning plant tissues to investigate the effects of smoke compounds on seed germination. Our system is easy to assemble and use, compact, effective, and inexpensive. In addition, the amount of plant tissue added to the smoker, the length of time for combustion, and the amount of ash remaining after combustion can be quantified.
METHODS AND RESULTS
Smoke solution system—The smoke system (Fig. 1) was constructed in a fume hood so that any excess smoke could be evacuated safely from the laboratory. The sample plant material is burned inside a stainless steel bee smoker with attached bellows (10 cm diameter × 23.5 cm high; model 15239, GloryBee Foods, Eugene. Oregon, USA) (Fig. 1-1). The smoke flows from the smoker through a heat-resistant rubber heater hose (80 cm long, 2.7 cm inner diameter [ID], and 3.4 cm outer diameter [OD]: HBD/Thermoid, Bellefontaine, Ohio, USA) clamped to the spout of the smoker (Fig. 1-2,3). The other end of the rubber hose is placed below the surface of 300 mL of water in a 1000-mL sidearm flask with a 3.6-cm ID mouth (Corning Pyrex Vista filter flask; Corning Life Sciences. Tewksbury, Massachusetts, USA) (Fig. 1-4). The dimensions of the hose and flask are chosen so that the outer diameter of the tubing will fit snugly into the inner diameter of the mouth of the flask. The flask is clamped to a ring stand for stability (Fig. 1-5). A water aspirator is attached to the side arm of the flask (Fig. 1-6,7). The vacuum generated by the aspirator is used to help draw the smoke from the smoker into the water.
Plant tissue preparation—After collection, material from each plant species or a mixture of species is placed into individual paper bags. The woody species are chipped; the forbs and grasses are cut into small pieces. Each sample is mixed thoroughly to maximize homogeneity and allowed to air dry at room temperature. The drying time will vary depending on moisture content of original material, size of pieces, and ambient temperature and humidity. Dried samples are stored at room temperature in paper bags.
Smoke solution preparation—Burns are initiated by placing 50 g of plant material into the bee smoker and igniting the material with a refillable Bernz-Omatic butane micro torch (Worthington Cylinder Corporation. Columbus, Ohio, USA). The material is allowed to burn with the smoker open for 10–20 s, after which time the smoker lid is closed and the tubing is clamped to the opening of the smoker. The smoke is drawn through the water in the flask, dissolving the water-soluble compounds. Bellows on the bee smoker are pumped for the duration of the process to keep the plant material burning and to increase the amount of smoke entering the water in the flask. As the plant material burns, more is added to the bee smoker until the total sample is burned. During the burning process, caution must be exercised because the smoker gets very hot. Once the plant material has finished burning, the system is allowed to cool for approximately 15 min. After the cooling process, the ash contents in the smoker are removed and weighed. The smoke-water solution in the side-arm flask then is available to use in its concentrated form or diluted as needed for analysis or seed treatments. This apparatus could be used repeatedly, but because considerable residue remains in the smoker, heater hose, and flask, these components may need to be replaced for each plant species. The cost for each of these items is minimal. The smoker, heater hose, and flask are labeled so that they can be reused when the same species is burned repeatedly.
Results—The smoke solution system described offers several advantages compared to systems described previously, including ease of construction and use. The materials used to construct the system are readily available (Table 1) and the system is easy to assemble, requiring no special tools. The system is compact, being able to fit inside a fume hood (1.3 m width × 0.7 m depth × 1.1m height). With caution, our system can be used safely, presenting minimal fire hazard or exposure to smoke fumes, because it is located in a fume hood that drafts excess smoke from the building. The bee smoker does become very hot so caution is required, including use of heat-resistant gloves. Regardless of weather conditions, the system can be used year round. The system allows smoke solutions to be quantified because the mass of plant material is weighed initially, the mass of ash is weighed after burning is completed, and the volume of water in the flask also can be measured. These measurements would allow others to create similar solutions and replicate the trials. In addition, the ash that is collected could be used to test whether it affects germination or other parameters. In contrast to using commercial smoke solutions where species and other components may be unknown, this system also allows the investigator to select which species to use for creation of customized smoke solutions.
This smoke system also saves money and time, being economical, quick to assemble, and easy to use. The materials to build the system are inexpensive, totaling only US$270 (2013) to assemble initially, provided one has access to a fume hood (Table 1). Of the materials listed, several are available in most laboratories. Another advantage for this system is its efficient use of time. The system is very easy to operate with minimal training (usually less than an hour) required for staff, saving time as well as money. The system also requires minimal time to create a smoke solution. Depending on the original starting plant material (i.e., what species, how coarse, how dry, how much), burning required 1–5 h for 100–200 g plant material and resulted in 5–87 g ash (Tables 2 and 3). More research is needed to understand the variation in burn times and quantity of ash produced for different samples.
Suppliers and costs for items needed to create the smoke solution system.a
Burning information for tallgrass prairie, woody, and other species used in this study.
This system has been used to support a variety of different projects in our research program. Smoke solutions were created from burning 13 individual species (Table 2), including both herbaceous and woody. In addition, we created smoke solutions using mixtures of species from four longleaf pine savanna sites in Florida (Table 3). Smoke solutions created using this system were used for germination studies showing the effects of smoke solutions from buffelgrass (an invasive species in Arizona) on native species of the Sonoran Desert (Maruszak and Coons, 2013). Smoke solutions from longleaf pine savannas were also tested on two federally threatened species (Pinguicula ionantha R. K. Godfrey and Scutellaria floridana Chapm.) (Annis, 2013; O'Brien, 2013).
A need existed to develop a system to create plant smoke solutions for which more variables could be controlled (Landis, 2000). Many earlier reports were unclear about how the smoke solution was created, and thus procedures are not easily reproducible. Others have used smoke solutions created from commercially prepared products (Dixon et al., 1995; Cochrane et al., 2002), but this method does not allow the investigator to select specific plant species for creation of a smoke solution. Two important components of our system are a bee smoker and a heater hose, which were used in earlier studies to create aerosol smoke but not smoke solutions (Baxter et al., 1995; Lindon and Menges, 2008). We believe that our system provides greater control of variables while offering several additional advantages over previous systems. Our system is more compact, allowing for creation of a smaller volume (300–750 mL) of liquid in a more confined space than some systems (Tieu et al., 1999; Flematti et al., 2004). Unlike systems where water was simply exposed to smoke, our system increases the opportunity for water-soluble compounds to dissolve by bubbling the smoke through water (Keeley and Fotheringham, 1998). In our system, a vacuum is created by an inexpensive and easy-to-use water aspirator to pull smoke through water, in contrast with some systems that are described as using suction or a pump (Tieu et al., 1999; Flematti et al., 2004). Among our system's numerous advantages are ease of obtaining materials, assembling the system, and training staff in its use. In addition, the system is safe to use indoors as it is small enough to fit into a fume hood. Some systems are quite large (DeLange and Boucher, 1990; Brown, 1993b), and are used outdoors where heat can be troublesome, making it best to create smoke on cooler days (Brown et al., 1993). The system is inexpensive to construct and to operate, and requires a short amount of time for its creation and use. Perhaps most important of all, this system offers a potential advance in technology for creation of smoke solutions because one can quantify how the smoke solution was created so studies can be described in detail and reliably repeated. One improvement for the described system would be to develop an automated way to pump the bellows on the bee smoker. Overall, this smoke-water production system is very effective for preparation of smoke solutions to use for seed germination studies.
Burning information for mixtures of species from longleaf pine savannas in Florida.