N.A.C. Brown, P.A. Botha T. Johnson & D. S. Prosch
The Seed Biology Research Programme at Kirstenbosch
Horticultural Research, Kirstenbosch Research Centre, National Botanical Institute
The Cape Floristic Region possesses the richest temperate flora in the world. The dominant vegetation of the Cape Floristic Region is the fynbos which is typified by the presence of members of the Restionaceae (Cape reeds and grasses), the Proteaceae (Sugarbushes, pincushions and conebushes), Ericaceae (Cape heaths) and a number of other families, including the Asteraceae (Everlastings) and the Bruniaceae. Many of these wildflowers are of outstanding horticultural potential. Seeds of many species are dormant and research has shown that very specific environmental cues are required for germination. It has also been shown that fire, a natural feature of the fynbos environment, provides the major cues for germination of wildflower seed. Recently, considerable progress has been achieved in understanding seed dormancy mechanisms in some of the most important fynbos families and this has led to a greater efficiency in the propagation of these wildflower species from seed.
The Cape Floristic Region (CFR) covers an area of 90 000 km2 (35 000 sq miles) in the south western Cape, at the southern tip of Africa. This is less than four percent of the area of South Africa, yet it contains 8600 plant species. It is by far the richest temperate flora in the world and over two-thirds of the plant species and seven of the plant families are endemics. Fynbos is the unique type of vegetation which is dominant in the CFR. It is a community of small shrubs, evergreen and herbaceous plants and bulbs and is exceptionally rich in species. It is perhaps best known as the home of the South African Proteas (Sugarbushes, pincushions and conebushes), Ericas (Cape heaths) and Helichrysums (Everlastings), and is also typified by the Restionaceae (Cape reeds or Cape grasses) (Brown et al., 1995).
Many of the wildflowers from these families are cultivated as ornamentals in parks and gardens around the world or are of importance as floricultural crops. Propagation of fynbos plants from seed is often difficult, as seeds of many species are dormant when shed and require very specific environmental "messages" or cues before they will germinate (Brown, 1993a). The fynbos occurs in areas with a Mediterranean climate (winter rainfall) and the environment is characterised by a number of stress factors such as summer drought, low soil fertility and periodic fires. The fires have a frequency of 5 to 40 years and are a natural phenomenon in fynbos. Seeds of many wildflowers are adapted to germinate in response to one or more of the cues provided by fire. Heat from flames may fracture the impermeable seed coat of hard-seeded species (e.g., Fabaceae) resulting in the coats becoming permeable to water. Dry heat may also break dormancy by providing a heat-pulse which stimulates the embryo directly and results in germination. (e.g., Restionaceae). Dry heat has also been reported to break seed dormancy of some South African Leucospermums (Pincushions, family Proteaceae) by complete desiccation of their oxygen-impermeable seed coats. When rain falls the dry coats, which are permeable to water, split suddenly and the embryo then obtains sufficient oxygen for germination. Fires also provide chemical messages or cues, such as the gases of ethylene and ammonia, which stimulate germination in some species of Erica. In addition to the more obvious cues provided by heat, it has recently been discovered that smoke from fynbos fires provides a major (yet unidentified) chemical cue which is responsible for stimulating the germination of seed of many fynbos species. To date, 216 species from 30 families have been tested and 117 of these have given a positive response. (See Table 1). Amongst those responding are the horticulturally important species of the Cape Reeds, Everlastings and Brunias, (Brown, 1993b: Van Staden, et al., 1995). Fire may also have an indirect effect on germination by causing changes in the soil temperature regimes in the immediate post-fire environment (Brown, 1993a). Fire thus provides the major cues for germination in the wild and these cues have to be simulated when attempting to germinate wildflower seed in the laboratory and nursery.
Two hundred and sixteen species from 30 families have been tested for a germination response to smoke and 117 of these from 18 families have given a positive response
A: Families in which a positive response was obtained B: Families in which no response was obtained Aizoaceae Amaryllidaceae Asteraceae Anacardiaceae Brassicaceae Asphodelaceae Bruniaceae Cuppressaceae Campanulaceae Cyperaceae Caryophyllaceae Haemodoraceae Crassulaceae Hyacinthaceae Ericaceae Iridaceae Fabaceae Montiniaceae Geraniaceae Rhamnaceae Mesembryanthemaceae Rubiaceae Penaeaceae Sterculiaceae Poaceae Proteaceae Restionaceae Rosaceae Scrophulariaceae Thymelaeaceae
The seed biology of the family Proteaceae, with approximately 330 members, has been extensively researched and the findings reviewed by Van Staden & Brown, (1977); Brits et al, (1995) and Brits, (1996). Members of the Cape Proteaceae have two distinct achene types. The one type is rounded (often ellipsoid), relatively hard and nut-like and stored in the soil. Germination is characterised by the splitting of the seed coat, due to cotyledon expansion, which is then followed by protrusion of the radicle. In the second type, the achene is winged, plumed or hairy (often flattened) and relatively soft. The latter type is produced mostly by serotinous species, i.e., species in which seeds are stored in the living plant canopy. In serotinous species, germination is first indicated by penetration of the seed coat by the radicle. Serotinous genera comprises Protea, Aulax and most of the Leucadendron spp. and make up approximately 37% of the Cape Proteaceae; with the remainder (excluding Brabeium) being nut-like. Nut-like and serotinous achenes show different germination patterns or syndromes.
I Proteaceae with nut-like achenes. These achenes do not germinate, or germinate poorly, in mature fynbos vegetation but seedlings recruit en masse during the first winter after fire. The breaking of dormancy in species with nut-like achenes is strongly dependent on moderately low seasonal air temperature. This is not a stratification requirement, but the low temperature requirement is a mechanism to promote germination during the favourable cool, moist western Cape winter period. Diurnal high temperature is also required for maximal germination. A range of fluctuating temperatures is equally effective in stimulating germination, e.g. 4 - 10 degrees celsius (night) and 20 - 28 degrees celsius (day). Seed-coat-imposed dormancy by means of oxygen exclusion is a characteristic of most Proteaceae with nut-like fruits (10/14 species of three genera tested), but not of serotinous species (13/15 species of three genera tested). Germination of achenes of Leucospermum species can be improved by a single 24h treatment with relatively low concentrations (0.01 to 0.1%) of hydrogen peroxide. A relatively slight increase in the level of oxygen available to embryos is usually sufficient to initiate germination under suitable environmental conditions. (Brits, 1996).
- The intact seed coat is readily permeable to water but poorly permeable to oxygen. To improve oxygenation, soak achenes in 1% H202 before making commercial sowings in seed beds. In the laboratory, incubate achenes in oxygen.
- Seed germination is strongly dependent on seasonal low temperature. Therefore, sow seeds in seed beds during autumn or early winter. In the laboratory. incubate seeds at an optimum low temperature of 8 or 9 degrees celsius.
- High temperature is also required for germination and should be alternated with low temperature on a daily basis. Commercial seed beds should thus be constructed in full sun. In the laboratory the optimum high temperature should be maintained for 8h per day followed by a period of low night temperature (16h).
- Seeds should be give a light dusting with a fungicide dressing to prevent pre- and post -emergence seedling infection
Serotiny is an adaptive response to cyclical fire in fynbos. In nature, seeds are shed following a fire and germinate en masse only after fire. Seeds have a low temperature requirement for germination 1-11 degrees celsius which allows the avoidance of drought by synchronizing germination with the first (wet) winter season following dispersal. In contrast to nut-fruited species, the seed coat in serotinous species apparently plays a lesser role in preventing oxygen diffusion to the embryo.
- Use freshly, harvested seed, as seeds lose viability with age.
- Germinate seeds at temperatures below 20 degrees celsius, preferably between 1-11 degrees celsius
- If seeds are of uncertain age and viability and/or incubation temperatures are above 20 degrees celsius, these factors may be counteracted by pre-soaking seeds in a solution containing GA3 or GA4 and GA7. (Brown & Drewes, 1991).
- Sow seeds in a well-aerated, well-drained, sandy soil and avoid waterlogging.
- Seeds should be give a light dusting with a fungicide dressing to prevent pre and post- emergence seedling infection. (Brown, Van Staden & Brits, 1996)
Cape Ericaceae (Ericoideae)
Seed germination studies in this family were reviewed by Brown et al., (1993). Ninety-five percent of the 857 species are confined to the southern tip of Africa and many are of importance in horticulture and floristry. In the wild fire is very important in the ecology of the Ericoideae and the vast majority of species regenerate only from seed after a veld fire. Seeds are very small and, in all but one species, are shed when ripe. Serotiny is rare in this family and is found only in Erica sessiflora.
Factors of Importance in Germination.
- Dry heat and the gases ethylene and ammonia stimulate germination of some species.
- Germination is stimulated by soaking seeds in GA3 and GA7 and GA4.
- Alternating day/night temperatures, as occur in winter in burnt fynbos, are important as a cue for germination.
- Germination is stimulated by plant-derived smoke and aqueous smoke extracts (Brown and Van Staden, 1997). In the first major germination study in this family, Brown et al., (1993), screened seed of 40 species to obtain an indication of how important the smoke cue was for germination. The improved germination following smoke treatment shown by 26 of the 40 species tested, suggested that under natural conditions smoke from fynbos fires provided an important cue for triggering seed germination in this family. Amongst the species responding to smoke treatment were a number of species of particular horticultural importance. The smoke treatment ensures a much greater efficiency when propagating from seed and this is of importance in plant-breeding programmes. Its use has resulted in many more plants of these species becoming available to the horticulture industry.
- Use fresh mature seed;
- Soak seeds in aqueous smoke extract or 'Kirstenbosch Smoke-Plus' seed primer for 24 h before sowing. or "smoke" seeds sown in seed trays. The trays should have a sand/loam/bark mixture and be well-drained.
- Alternatively, seeds may be pre-soaked in GA3 or GA4 and GA7 solution prior to sowing.
- Incubate seeds under alternating night/day temperatures, e.g. 10 degrees celsius (16h, night); 15-25 degrees celsius (8h,day). (Brown, Van Staden & Brits, 1996)
The Restionaceae is a family of evergreen, rush-like plants which is almost restricted to the southern hemisphere. There are about 320 species in Africa, (300 in Cape) and 100 in Australia. The African Restionaceae are relatively diverse in their seed dispersal mechanisms, which could be implicated in the survival of seeds during or after fires. The modes are:
- wind dispersal of unilocular, indehiscent ovaries, with a persistent perianth, which acts as a wing for the fruit (e.g. Thamnochortus, Staberhoa, Calopsis);
- myrmecochory of fruits containing elaisomes. The ovary is unilocular and indehiscent, and the ovary wall is heavily lignified (e.g. the 'nut-fruited' restiads: Anthocortus, Mastersiella, Hypodiscus, Willdenowia, Ceratocaryum, Cannomois). These seeds are also serotinous and are retained on the plant until the next season's seed crop is mature; and
- the so-called 'basic' condition, showing dehiscent ovaries, with 1-3 locules. Here the seed is released from the ovary after maturation, but it is not known how it is dispersed after release (Restio, Ischyrolepis, Askidiosperma, Chondropetalum, Dovea, Nevillea. Rhodocoma)(Linder, 1991, Brown et al., 1994).
Factors of Importance in Germination.
The poor germination achieved with seed of many species has been attributed to the limited seed set of some species and the difficulty in determining when seeds are mature and ready for harvest
- Heat treatment of seeds at 120 degrees celsius for 3 min gave a significant improvement in the germination of seeds of some species.
- In common with many other fynbos species, restios require alternating high and low diurnal temperatures as a cue for germination.
- Germination is stimulated by plant-derived smoke and aqueous smoke extracts (Brown and Van Staden, 1997). Brown et al., (1994) conducted a major study, in which seed of 32 species was screened to obtain an indication of how important the smoke cue is for germination in this family. The results of this study represented the first occasion that comparative germination data for South African species family had ever been obtained. Twenty-five of the 32 species tested showed a statistically significant improvement in germination following smoke treatment. Untreated seeds of 18 of the species responding, showed a high degree of dormancy with only 0.1% to 2.0% germination. These results suggested that under natural conditions smoke from fynbos fires provided an important cue for triggering seed germination in many species in this family. The four species that did not germinate were all myrmecochorus, nut-fruited species, which possibly require a different or additional heat cue for germination.
- Use fresh, mature seed.
- Seeds may be pre-soaked in aqueous smoke extract or 'Kirstenbosch Smoke-Plus' seed primer for 24 hours before sowing; or seeds may be smoked once sown in trays. Fill trays with a sand/loam/bark mixture which is well-drained.
- Incubate seeds under alternating night/day temperatures of 8 degrees celsius(16h)28 degrees celsius (8h) for optimum germination.
- Nut-fruited species remain difficult to germinate. On suggested dormancy breaking treatment is to heat fruits to 120 degrees celsius for 3 min. prior to pre-soaking in aqueous smoke extract. Germination cues for nut-fruited restios require further study. (Brown, Van Staden & Brits, 1996). Some experimental results suggest that more attention needs to be paid to the role of light in restio seed germination. (Johnson, Brown and Van Staden, unpublished data)
There are a number of everlasting species in fynbos. These produce colourful inflorescences or flower heads which are harvested and dried and are of importance to the dried wild flower industry. Amongst the most attractive of the everlastings which have horticultural potential are: (i) Syncarpha vestita[syn. Helichrysum vestitum] ( Cape Everlasting, Sewejaartjie), a robust woolly herbaceous shrub with large white flower heads, (<1m in height); (ii) S. speciosissima (syn. Helipterum speciosissimum) [Cape Everlasting], a small sprawling shrub with white or cream flowers, (<0.25m in height); (iii) Syncarpha eximia [Syn. Helipterum eximium] (Strawberry Everlasting), a robust small shrub with bright red flowers, (<0.3m in height); (iv) Edmondia sesamoides [syn. Helichrysum sesamoides] (Strawflowers), a sparsely branched small shrub, with white, rose or yellow flowers, (<0.5 m in height); (v) Helichrysum patulum, a woolly shrub, with yellow flowers, (<0.5m in height); (vi) H. foetidum [Yellow Everlasting] a tall biennial shrub with bright yellow or cream flowers, (>1 m in height); and (vii) Phaenocoma prolifera [Red Everlasting], a densely-branched granular-leaved shrub with flowers of different shades of red, (<0.5m in height).
Factors of Importance for Germination.
Achenes (one-seeded fruits), hereafter called seeds, are produced from the fertile flowers at the centre of the flower heads, the sterile flowers on the perifery provide the colourful papery bracts. The seed biology of some of the most important everlastings has recently been studied in a nursery trial (Brown, 1993b, Brown and Botha, 1998). In nature these wildflowers act as fire ephemerals. Seeds are dormant and germinate in response to the smoke of a veld fire, and a large numbers of seedlings then appear. The plants then gradually die out, until a flush of germination occurs after the next fire.
- Use only mature, plump, fully-formed seeds, in most species these are darker in colour than the immature ones;
- Pre-soak seeds in aqueous smoke extract or "Kirstenbosch Smoke-Plus" seed primer* for 24h; or smoke seed trays after sowing:
- Seeds should be give a light dusting with a fungicide dressing to prevent post -emergence seedling infection,
- Sow seeds in a sandy, well-drained soil medium,
- Incubate in full sun under autumn temperature conditions, e.g., alternating 10 degrees celsius (16h, night) x 20 degrees celsius (8h day).
- Seeds of some species are sensitive to light. Light may either promote or inhibit germination. There is also an interaction between the effect of light and the germination response to smoke.Bruniaceae, Campanulaceae, Geraniaceae, Penaeaceae, Poaceae and Thymelaeaceae
Germination in a range of species selected from a number of other fynbos families has also been studied. Those species showing a germination response to smoke included: Audouinia capitata and Berzelia lanuginosa (Bruniaceae), Lobelia and Roella triflora (Campanulaceae), Pelargonium crithmifolium (Geraniaceae), Endonema retziodes (Penaeaceae), Themeda triandra (Poaceae) and Passerina vulgaris and P. ericoides (Thymelaeaceae).Germination Protocol
(1) Use only mature, fully-formed seeds; (2) Pre-soak seeds in aqueous smoke extract or "Kirstenbosch Smoke-Plus" seed primer for 24h; or smoke seed trays after sowing: (3) Seeds should be give a light dusting with a fungicide dressing to prevent pre- and post- emergence seedling infection, (4) Sow seeds in a sandy, well-drained soil medium, (5) Incubate in full sun under autumn temperature conditions, e.g., alternating 10 degrees celsius (16h, night) x 20 degrees celsius (8h day).
Research has led to more knowledge of germination cues, particularly those related to fire viz. Heat, smoke and alternating temperatures. Research results have led to:
- Greater efficiency in germinating fynbos seeds.
- A considerable increase in the range of fynbos species available for cultivation by the horticultural industry.
- Guaranteed germination for many rare threatened and endangered species, which has contributed to their conservation.
- Improved germination of hybrid seed in plant breeding and plant improvement programmes involving native species.
- The smoke research results have been applied in Australia, USA, Europe and other parts of the world and have made a major contribution to native plant propagation and utilisation projects and to vegetation rehabilitation.
Brown, N.A.C. (1993a). Promotion of germination of fynbos seeds by plant-derived smoke. New Phytol. 123: 575-583.
Brown, N.A.C., (1993b) Seed germination in the fynbos fire ephemeral, Syncarpha vestita is promoted by smoke, aqueous extracts of smoke and charred wood derived from burning the ericoid-leaved shrub, Passerina vulgaris. Int. Jl. of Wildland Fire 3: 203-206
Brown, N.A.C. and Botha, P.A. 1998. Plant-derived smoke promotes the germination of Cape everlastings. Abs. 9th Biennial Int. Protea Ass. Conf. And Int. Protea Working Group Workshop, Cape Town, 12-15 Aug
Brown, N.A.C. and Van Staden, J. 1997 Smoke as a germination cue: a review. Pl. Growth Regul. 22: 115-124.
Brown, N.A.C., Botha, P.A. and Prosch, D. 1995. Where there's smoke. The Garden, Jl. Roy. Hort. Soc. 120 (7): 402-405.
Brown, N.A.C. & Drewes, F. E. (1991) Germination of achenes of of Proteaceae following pretreatment with promalin and its components. Proc. 6th Biennial Conf. Int. Protea Ass,. Perth, W. Australia: 323-333.
Brown, N.A.C., Jamieson, H. & Botha, P.A. (1994) Stimulation of germination in South African species of Restionaceae by plant-derived smoke. Pl. Growth Regul. 15: 93-100.
Brown, N.A.C., Kotze, G. & Botha, P.A. (1993) The promotion of seed germination of Cape Erica species by plant-derived smoke. Seed Sci. & Technol. 21: 179-185.
Brown, N.A.C., Van Staden, J. & Brits, G.J. (1996) Propagation of Cape Proteaceae, Ericaceae and Restionaceae from seed. Comb. Proc. Int. Plant Propagator's Soc. 46: 23-27
Brits, G.J. (1996) Ecophysiology of Leucospermum R.Br. seed germination. Ph.D. dissertation, Department of Botany, University of Cape Town.
Brits, G.J., Cutting, J.G.M., Brown, N.A.C., & Van Staden, J. (1995) Environmental and hormonal regulation of seed dormancy and germination in Cape Fynbos Leucospermum R.Br.(Proteaceae) species. (A working model). Pl. Growth Regul. 17: 181-193
Linder, H.P. (1991) A review of the southern African Restionaceae. Contr. Bolus Herb. 13: 209-264.
Van Staden, J., Drewes, F.E. & Brown, N.A.C. (1995) Some chromatographic characteristics of germination stimulants in plant-derived smoke extracts. Pl. Growth Regul. 17:241-249.
Van Staden, J., & Brown, N.A.C. (1977) Studies on the germination of South African Proteaceae - a review. Seed Sci. & Technol. 5: 633-643.
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