THE FULL REPORT
This is a summary of the RIRDC full Smoke Germination of Australian Plants (98/108, KPW-1A) by Dr Kingsley Dixon, of the Kings Park and Botanic Garden Plant Science Division (phone 08 9480 3640, fax 08 9480 3641).  The report is available from RIRDC on 02 6272 4819 on online from RIRDC's catalogue.

 
Smoke - not heat and ash from a fire as previously thought - germinates many Australian native plants, according to a report published by the Rural Industries Research and Development Corporation (RIRDC).

The discovery has enabled scientists to develop methods of using smoke which are now being widely used for nursery propagation, and land rehabilitation and management.

A report to RIRDC, "Smoke germination of Australian plants", details research and applications developed by a team at Kings Park and Botanic Garden Plant Science Division in Western Australia, following a lead from South African botanists.

The team, led by Dr Kingsley Dixon, reveals that more than 400 Australian native species respond to smoke treatment.

Smoke can be applied by treating water to create "smoke water", or in the aerosol form, to seed trays, bushland soil, or directly applied to seeds.

The paper summarises methods that can be used to smoke-germinate native plants at home, on the farm, or in the nursery.

The study

The goal was to develop smoke-germination technology for commercial horticulture of Australian species less responsive to conventional seed propagation methods.

The key objectives were to:
 


To meet these objectives, the researchers first conducted a series of tests on native species where the inhibitory germination mechanism appeared to be the tough seed coat and then species in which the problem was located in the seed endosperm and/or embryo.

The next part of the research involved methods used to treat seeds with smoke. The work refined techniques and established the benefits and disadvantages of each of the following methods:

 
  • application of aerosol smoke to seed trays;
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  • direct application of aerosol smoke to seed;
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  • application of smoke water to seed trays; and
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  • imbibing seeds in smoke–treated water.
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    The results

    Examples of the results obtained from two species are shown below.

    Smoke-responsive native species occur throughout temperate southern and arid Australia. Even species from habitats not fire prone such as the alpine herb fields of Tasmania may germinate well following application of smoke.

    Tropical species may require research to determine the extent to which smoke may be important for their germination.

    Where heat and ash are of limited value for breaking dormancy in many species, smoke can promote:
     

  • germination of species which are difficult to germinate by conventional means;
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  • more uniform germination;
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  • earlier germination; and
    •


    Trees, shrubs, herbs and annuals all respond to smoke treatment.

    Proteaceae, Myrtaceae and most other dominant Australian plant families contain smoke-responsive species.

    The researchers warn that experimentation is essential to determine if an untested species might be smoke responsive. It is not possible to rely only on evidence from related species to predict whether a plant is smoke responsive.

    For regions in southern Australia, smoke is best applied from autumn to early winter. For tropical or arid zone species, some experimentation may be required to determine the best time to apply smoke. As a general rule, sowing and smoking should be done when germination is most likely to occur in nature.

    Excessive watering of smoke-treated seed trays should be avoided. Smoke is highly water soluble and over-watering can leach the active agents from the soil before seed dormancy has been broken.

    Seeds can be smoked directly in a smoke tent or soaked in a diluted solution of smoke water for 6–24 hours. Treated seeds are then dried and sown when required. Alternatively, trays containing sown seeds can be smoked for 60 minutes and then carefully watered for the first 6–10 days to ensure adequate penetration of the smoke chemicals.

    Broadcast seed which has been smoke treated is an effective way to germinate a wide variety of species. Smoke-treated seeds used in broadcasting often germinate better, including seeds of species which do not normally require smoke for germination under nursery conditions, e.g. eucalypts and banksias. Broadacre seed which has been smoke treated may take up to two years to emerge in some plant families.

    If a seed bank is present in soil, then good germination is possible following the addition of smoke either using aerosol smoke (area for treatment is limited using this method) or smoke water (using automated sprayers).

    A smoke tent with smoke generator, cooling pipe and inlet fan (see illustration below) can be used to produce aerosol smoke for treating seed. The tent can contain up to three levels of shelving. Leave approximately 30 cm between each shelf to ensure adequate flow of smoke between the shelves.

    Figure 1. Germination levels for Lysinema ciliatum (left) and Stylidium affine (right) seed with dilutions of smoke water (control is distilled water). Germination was scored at 6 weeks.
     

     
     

    Smoke water is produced by drawing smoke from the smoke generator through drums (20–30 L) containing water for up to 60 minutes.

    Most types of fresh or dried plant or woody materials will produce potentially useful smoke. Good germination results are obtained when a mixture of dry and green foliage and twigs are burnt. Plants with white sap (e.g. Euphorbiaceae) or oleander should not be combusted as the smoke produced may be noxious.
     

    Figure 2. Apparatus for aerosol smoke generation

    Summary

    The report says that once a species has been found to respond to smoke, it can be developed for nursery production or field sowing.

    Aerosol smoke has proved the only effective treatment for some species. Others respond equally well to aqueous smoke. These species are more adaptable for nursery production as they can be either treated directly with aqueous smoke and sown or treated and then dried for later use.

    For direct use, the seed should generally be imbibed in 10–20% full strength aqueous smoke for 3–6 hours or overnight. A small trial may be required to determine the optimum treatment. The seed should then be sown in punnets and germinated.

    To prime seed prior to storage, the seed should be imbibed in 10–20% full strength aqueous smoke for 12–48 hours. The seed must be completely dried for storage.

    Treated seeds should be sown in punnets of potting mix to a depth equivalent to the width of the seed. Kings Park and Botanic Garden uses a mix composed of coarse sand, brown river sand and composted sawdust in a ratio of 1:3:6 with the addition of osmocote, ferrous sulphate, dolomite lime and lime.

    The trays should be germinated at about 18–22 ± 1°C with day/night fluctuations and regular watering.

    Seed can be treated for direct sowing by either pre-imbibing with aqueous smoke or the seed can be aerosol smoked to coat it with a smoke residue. The seed can be directly sown or broadcast in the field. Pre-imbibed seed is probably better if the seed is to be mechanically sown as the sticky residue on aerosol-smoked seed may cause the seed to bind together.
     

    Figure 3. Various methods for smoke application.


     
    The protocol for developing  species for nursery production
    • Step 1. Ensure the seed batch is mature and free of any granivorous insects. Some seeds may require 12 months storage before they are ripe for germination.
    • Step 2. Ensure the seed is viable (not dead). There are two effective means by which this may be done:
      • (i) An initial test for viability is to cut open three replicates of 30–50 seed to inspect the endosperm and embryo. If a large proportion of the batch does not have a full endosperm of healthy appearance (white colour, not floury) the batch may be of little use. Also look for insect damage.

      (ii) To determine if the seed is capable of germination it is necessary to extract the embryos of a subsample of seed and grow them in vitro with growth hormones (gibberellic acid and zeatin) using the method described in section 3.2.1 of the Full Report. This will ensure that germination difficulties experienced with whole seed is due to dormancy issues rather than a lack of a viable embryo.
       
    • Step 3. Germination test. If a large proportion of the seed is whole with viable embryos yet germination is still low, the next step is to try smoke stimulation using the aerosol smoke tent and aqueous smoke in vitro (as described in section 3.2.1 of the Full Report). It is important to determine if the seed responds to aqueous smoke as effectively as aerosol smoke. A range of dosages of aqueous smoke should be trialed to maximize germination (10–20% for 3, 6 and 12 hours).
    • Step 4. If smoke is unable to improve germination it may be necessary to determine the site of the dormancy. This involves an in vitro study as described in section 3.2.2 of the Full Report. A variety of treatments, including nicking the seed coat, removing the coat, and extracting the embryo are employed.
    • Step 5. Other seed pretreatments may need to be carried out in combination with smoke to break dormancy in species unresponsive to smoke alone. Seed treatments can include the following:
     Storing the seed in the soil for a year, however this often results in a simultaneous viability decline.
    • Removal of the fruit may overcome dormancy in fleshy-fruited species as inhibitors are sometimes contained in the fruit (e.g. epacrids such as Astroloma foliosum and Nitraria billardieri).
    • If the seed coat is a barrier to the diffusion of inhibitors from the seed or to the uptake of water, nicking the coat can alleviate this (e.g. Emmenanthe penduliflora).
      Acid treatment of seeds helps to degrade the seed coat, removing inhibitors and making the coat more permeable to water (e.g. Scaevola crassifolia).