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Review of the Promotive Effects of Smoke on Seed Dormancy

Peter Olde

Until very recently there was a view that Grevillea seed was like wattle seed in that the seed coat delayed water penetrating to the embryo. All you needed to do to achieve germination was to remove the seed coat and let the water into the embryo and bingo! the radicle would spring forth and soon thereafter the seedling leaves or cotyledons would emerge to feed the new roots. Right ! and Wrong!! Studies recently published by E. Charles Morris (E.C. Morris 2000) from the University of Western Sydney have yielded some surprising results about seed germination of Grevillea.

It has been generally known for many years that fire plays a very important role in the germination of seed and in the regeneration of bushland. Indeed, as early as 1935, Levyns reported germination of seed of a wide variety of heath species after fire. Keeley et al. (1985) showed a similar effect in the Californian chaparral.

Defining which aspect of the fire was the most important or even crucial to germination was somewhat problematic. Was it heat, or the ash-bed, or the minerals leached into the soil, the simple lack of competition, ethylene? Was it the age of the seed or something to do with the seed coat? Well, perhaps in some measure all of these and more. For many plants that maintain a soil seed bank in fire-prone environments, the greatest germination of seeds is limited to the immediate post-fire period. Seed germination in the inter-fire period may be very low, notwithstanding abundant annual seed deposit into the soil. For some, this low germination rate may be attributed to dormancy which requires some fire-related signal to be broken.

Keeley et al. (1985) suggest that seeds are not dormant but may be chemically or otherwise inhibited by post-fire environmental conditions once the vegetation has been re-established.

The ground-breaking news that smoke or smoke-derived extracts could have an amazing effect on breaking dormancy and increasing seed germination of many species was first discovered by J.H. de Lange, a Ph.D student, and C. Boucher in South Africa. They demonstrated it using the rare and difficult Audouinia capitata (Bruniaceae), a threatened fynbos species from South Africa as well as 12 other fynbos species and species from other countries.

The results of their experimentation were indisputably convincing and promoted further wide research that confirmed the results experimentally in the Australian context.

Shauna Roche et al.(1994) first reported amazing success using smoke to germinate seed of Australian plants. In 1995, Kingsley Dixon et al. reported considerable success using the method to revegetate mining sites in Western Australia with difficult and stubborn endemic species natural to the area of operation. Roche, Dixon et al (1997) reported success with four Western Australian species. Even in species without obvious need of fire such as the humble lettuce (Drewes et. al. 1995) and celery (Thomas and van Staden 1995), seed germination was improved using smoke.

In respect of Grevillea, Dixon (1995) reported success with G.wilsonii. Morris (2000) demonstrated it with seven east Australian species, G.buxifolia ssp.buxifolia, G.diffusa ssp.filipendula, G.juniperina, G.linearifolia, G.mucronulata, G.sericea ssp.sericea, and G.speciosa. Smoke however appears to be only one of a number of fire-related germination cues in Grevillea. Roche & al. (1997) have confirmed that the age of seed in 181 Australian species greatly affects viability, with decline over one year ranging between 10-80%. When fresh seed was used almost 70% of species responded positively to smoke whether applied prior to or after sowing. Only 10% of species achieved optimum germination with seed ageing alone but when smoke was applied as a treatment after soil storage, 60% of species responded positively. Experiments also showed that for some sensitive species high concentrations of smoke may even be inhibitory.

In 1995, two schools of thought emerged on the effects of heat. Auld & Tozer showed that germination largely occurs in the first year after fire and that heat was not the signal that induced germination. Edwards & Whelan on the other hand detected a slight germination increase in G.macleayana after subjecting the seed to short heat exposure and to scarification. They postulated that Grevillea seeds were dormant due to a hard seed coat and that this was only broken by scarification or cracking after heat exposure.

The germination response of seven east Australian Grevillea species was tested by E.C. Morris after exposure to smoke, heat and scarification and to all of these in varying combinations. At the same time, Edwards & Whelan's hypothesis was tested using different and more species. All three germination treatments when administered singly increased germination; smoke increased germination in all seven species, heat in four and scarification in a different four. Smoke and heat treatments combined led to the highest germination response in four species whereas heat and scarification combined decreased germination rates in three species. Germination stimuli have been shown to be additive in effect.

It was found that treated Grevillea seed consistently germinated after 12 days with most germination occurring between days 30-40, after which the germination rated declined until day 60. Germination of the control group (no treatments) was low, ranging from 0 to a maximum of 20%. Seed treated with smoke tended to germinate quicker than seed sown after other treatments. The effect of smoke exposure on germination of all seven species was highly significant.

Seed to be treated with heat exposure were placed on a petri dish in a fan-forced oven heated to 80^oC for ten minutes. Heat treatment increased germination relative to controls by about 10-13% in G.buxifolia and G.diffusa and up to four-times more in G.juniperina, G.sericea, G.speciosa and G.mucronulata showed no increase in germination response after heat. Recent studies by Tieu (in press) have shown that longer exposure to higher temperatures (3 hrs at 100^oC) led to release of dormancy.

Scarification (slicing through the seed coat to the embryo along one side using a scalpel) as a treatment in itself greatly increased germination in G.mucronulata and G.juniperina and only slightly in two other species.

When combined with a heat treatment, only seed of G.buxifolia increased in germination above the unheated sample. Scarification of smoked seed significantly increased germination in all three species that received the test compared to seed that had bean smoked only. Scarification of smoked and heated seed did not significantly increase germination in any species and actually depressed germination in two species, significantly in G.speciosa.

This finding overturns the hypothesis of Edwards and Whelan and de-facto almost all unpublished opinion on the seed-coat/dormancy issue among amateur propagators. Depending on weather and soil conditions, water continually penetrates the seed coat to the embryo and moves back out. Grevillea seed coats do not repel water like Acacia seeds and therefore improved germination resulting from removal and/or scarification of the coat is doing more than just allowing moisture penetration to the embryo. So why do they not germinate with the seed coat on? It has been shown experimentally and widely reported that when the seed coat is removed, Grevillea seed germinates readily (Langkamp 1987, Olde & Marriott 1994, Morris 2000). How-ever, since the seed coat is not preventing water uptake, what are the signals that initiate germination and what is the function of the seed coat?

One model suggests that the seed coat contains inhibitors. Another proposes that the seed coat acts simply as a mechanical barrier that prevents elongation of the radicle or prevents the exit of inhibitors contained in the embryo, thus facilitating irregular germination and ultimately enhancing species survival. A third model suggests that the seed coat restricts gas exchange, such as oxygen uptake.

Morris, Tieu & Dixon (2000) have investigated the role of the seed coat in seed dormancy using two Grevillea species. Most of the models proposed for Grevillea do not affect dormancy. However, the seed coat itself or the seed coat in combination with the embryo does contain germination inhibitors. Slight embryo dormancy was demonstrated for Grevillea wilsonii but this was easily overcome with smoke cues. Incorporation of smoke water in the germination medium also increased germination of G.wilsonii.

The conclusion that smoke can stimulate germination in Grevillea seed is indisputable, having been confirmed in a number of published experiments using a range of eastern and western species in both Grevillea and other families and genera. There is also plenty of unpublished evidence among amateur propagators. The identity of the active ingredient in smoke and the way the smoke cue acts within the seed is uncertain. Perhaps it doesn't matter greatly. Smoke produced from burning any woody vegetative material is sufficient. It is very complex containing thousands of chemicals in almost infinite combinations.

Evidence for the role of heat in stimulating germination is mixed and results are inconclusive. Some species are affected not at all (e.g. G.sericea, G.mucronulata, G.buxifolia); some are affected some of the time (Auld & Tozer achieved 60% germination of G.linearifolia after heat treatment; Morris achieved 13%). Perhaps the method and temperatures used need to be altered. However inconclusive, heat treatment can produce an increased germination response in some species.

The mechanism by which scarification/removal of the seed coat increased germination in unsmoked seed needs further explanation since the coat is not involved in excluding moisture.

The results of Morris' work suggest that there are multiple germination cues and that the stimulatory effects of the cues could be independent and additive or interactive. There may be a small, non-dormant fraction of seeds in the soil bank that germinate readily and quickly. Scarification or breakage of the seed coat over time could induce a further small fraction to break dormancy. Clearly the fire-related signals of heat and smoke, especially in combination, break dormancy in a large proportion of seeds, thus contributing to the post-fire flush of germination.

Bibliography

Auld T.D. & Tozer M. (1995) Patterns in emergence of Acacia and Grevillea seedlings after fire. Proc. Linn. Soc of NSW 115:5-15.

de Lange J.H. & Boucher C. (1990) Autecological studies on Audouinia capitata (Bruniaceae). Plant-derived smoke as a seed germination cue. S. Afr. J. Bot 56 (6): 700-703.

Dixon K.W, Roche S, Pate JS. (1995) The promotive effect of smoke derived from burnt native vegetation on seed germination of Western Australian plants. Oecologia 101: 185-192.

Drewes F.E. Smith M.T. & van Staden J. (1995). The effect of a plant-derived smoke extract on the germination of light-sensitive lettuce seed. Plant Growth Regulation 16:205-209.

Edwards W, & Whelan R. (1995) The size, distribution and germination requirements of the soil-stored seed-bank of Grevillea barklyana (Proteaceae). Aust. Journ.Ecology 20: 548-555.

Keeley J.E et al (1985) Role of allelopathy, heat and charred wood in the germination of chapparral herbs and suffrutescents, Journal of Ecology 73: 445-458.

Kullman (198-) Seed Germination Records of Western Australian Plants, Kings Park Research Notes No. 7.

Lamont B & Miiberg P. (1997) Removal of the testa during germination or establishment increases germinant mortality, decay and water loss. Seed Science Research 7:245-252.

Langkamp P (1987) Germination of Australian native plant seed. Inkata Press. Sydney.

Levyns M.R. (1935) Germination in some South African seeds. Journ. S. Afr. Bot. 1:161- 170.

Morris E.C. (2000) Germination response of seven east Australian Grevillea species (Proteaceae) to smoke, heat exposure and scarification. Aust. J. Bot 48:179-189.

Morris E.C, Tieu A & Dixon KW (2000) Seed Coat Dormancy in Two Species of Grevillea (Proteaceae) Ann. Bot. 86:771-775

Olde P,M. & Marriott N.R. (1994-95) The Grevillea Book Vols 1-3, Kangaroo Press, Kenthurst.

Roche S., Dixon K., & Pate J. (1994) Smoke - a new process for germinating Australian plants. Australian Horticulture 91(9):46-48.

Roche S., Koch J.M, Dixon K.W. (1997) Smoke enhanced seed germination for mine rehabilitation in the southwest of Western Australia. Restoration Ecology 5:191-203

Roche S., Dixon K.W. & Pate J.S. (1997) Seed ageing and smoke: partner cues in the amelioration of seed dormancy in selected Australian native species. Aust Journ.Bot. 45:783-815

Thomas T. & van Staden J. (1995) Dormancy break of celery (Apium graveolens L.) seeds by plant derived smoke extract. Plant Growth Regulation 17:195-196.

Tieu Anle, Dixon K.W., Sivasithamparan K., Plummer J., & Sieler I. (1999) Germination of Four Species of Native Western Australian Plants using Plant-derived Smoke. Aust. J. Bot. 47:207-219

From the newsletter of ASGAP's Grevillea Study Group, July 2001.

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Australian Plants online - December 2001
Association of Societies for Growing Australian Plants