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Issue 16

Australian native citrus ~ Erika Birmingham

`wild limes' from the rainforest to the desert...

Erika Birmingham is the owner/operator of Byron Bay Native Produce, a small Australian native foods business on the north coast of NSW. She has been researching and developing Australian native citrus species as a commercial crop for the last six years and is a Consultant Qualified Person with the Plant Breeders Rights Office in Canberra, for native citrus species.

Recently, there has been an emergence of interest in food crops indigenous to Australia. These `bushfood' or `Bush Tucker' crops are introducing many new flavours into our Australian cuisine, which until now has been largely drawn from European and Asian cultures.

The Australian native foods industry is currently worth an estimated $14 million annually and growth has boomed in 2000 with the Sydney Olympic Games. Export potential is high and exports are expected to contribute to growth (Graham and Hart, 1997). One of the strengths of the bushfoods industry in the export market is its `clean, green' image.

As the market for new bushfood flavours has grown, so has the interest in the commercial cultivation of some of the thousands of edible plants endemic to Australia.

The native foods industry in Australia has developed over the last 15 years from an early infancy of `wild harvesting'.

Wild bushfood harvesting has several problems. The resulting product is often of poor quality and expensive, supply is inconsistent and pressures have been exerted on native plant communities in the wild. This form of harvesting is gradually being replaced by sustainable commercial cropping and a new wave of bushfood growers is emerging.

lime

Round lime - Citrus australis

At present, at least 50 bushfoods and bushfood products have been researched, developed and processed for market, with a further 150 species identified as having commercial potential. Bushfood products are now appearing in the general retail market and some have become household items (Graham and Hart, 1997). Current demand for many bushfood products outstrips supply.

One interesting group of new crops to emerge from the rainforests and semi-arid areas of eastern Australia is the edible native citrus or `wild limes'. Several species of native citrus have been wild harvested in recent years. "Native citrus are keenly sought after for use in the

Native Foods Industry, where their excellent culinary and beverage attributes...are proving to be powerful marketing tools" (Beal, 1997).

Graham and Hart (1997) in their report of the prospects for the Australian native bushfood industry included wild limes (formerly Microcitrus species and Eremocitrus glauca, now reclassified back into Citrus) in the fourteen crops with the most commercial potential. Australian native citrus have a strong potential within both the bushfood and citrus industries for commercial growers of raw produce and for the retail (home grower) market.

History

Exotic citrus species were first introduced into Australia in 1788, by members of the First Fleet. Six species of native citrus endemic to Australia were described by colonial botanists. Their potential for cultivation was recognised as early as 1899, when the colonial  botanist, FM Bailey, advised that native citrus was "...well worthy of cultivation for its fruit, which is juicy and of equal flavour with the West Indian Lime."

WT Swingle (1915) described the rainforest limes as "...very ornamental and should become better known for decorative purposes." Only limited records exist of traditional Aboriginal use for these native citrus species. Tim Low, in his book "Bush Tucker" says: "Anthropological records are so incomplete (in the subtropics) that we cannot even confirm that Aborigines ate such common fruits as native tamarind..." However, records do exist that such species as the finger lime were used by the early settlers for making cordials and marmalade.

Cultivation of native citrus was largely ignored in Australia until CSIRO began investigating the use of some of these native citrus species in breeding programs, in the 1960's. However, the native citrus species were used primarily for developing new hardy varieties of rootstocks for Australian conditions and as a source of genetic characteristics to contribute to improvement of mainstream Citrus (Sykes, 1997).

Classification

Exotic citrus species have been bred for thousands of years. Most true citrus species can no longer be located in the wild, as early cultivation has lead to hybridisation. Hence there is confusion in attempting to define citrus species, as each variety may be a complex hybrid of three or even four species. Hybrids can result from either natural or deliberate interbreeding of related species. Resulting hybrids may then be given a new botanical name and species status (e.g. Citrus virgata - the Sydney hybrid), under the International Code of Nomenclature for Cultivated Plants. This practice is no longer in common use and new hybrids are referred to as hybrids between the two relevant parent species.

Thus confusion has arisen in Australia, with the current commercialisation of both true native citrus species and `native citrus varieties' (cultivated hybrids between exotic and native Citrus). There is a lack of standardisation of common names within the Australian native foods and citrus industries, with the six different native citrus species and their hybrids often listed under the general category of `native citrus' or `wild limes'. Until the industries reach an agreement on the correct identification of Australian native citrus, botanic names are the only positive method for identification.

Cultivated hybrids are generally referred to by their cultivated variety name (`cultivar' name) or origin, e.g. the `Australian Blood'PBR lime is a hybrid between a native citrus species and an exotic citrus variety.

Australian native true citrus species

Until recently, there were two genera of true citrus fruit trees in

There is a lack of standardisation of common names within the Australian native foods and citrus industries, with the six different native citrus species and their hybrids often listed under the general category of `native citrus'or `wild limes'.

Australia, Microcitrus and Eremocitrus, both of which are members of the sub-family Aurantioideae of the family Rutaceae. The rainforest limes were separated by Swingle (1915) into the new genus Microcitrus due to the very small juvenile leaves and the minute size of the flowers. These genera have now been reclassified by Professor David Mabberley, of the Royal Botanic Gardens Sydney, back into Citrus.

There are six native species of Citrus (five endemic) in Australia. Their distribution originally extended from the Northern Territory, to the Cape York Peninsula and down the east coast of Australia, to the Clarence River, on the north coast of New South Wales. Much of this area has since been cleared for development. For this reason, each species now has a limited distribution, with two of the species endemic to Queensland currently listed as rare in the wild. Citrus glauca is endemic to the semi-arid regions of Queensland, New South Wales and South Australia and Citrus gracilis is endemic to the Eucalypt woodlands of the Northern Territory. Citrus x virgata `Sydney Hybrid" is a cross between Citrus australis and

C. australasica. finger

Citrus australasica - finger lime

The natural distribution of the finger lime is now from the Richmond River, in northern New South Wales to Mt Tambourine in Queensland (Floyd, 1989), with reference to its occurence in the southern portion of the Moreton Bay district (Ross,1983). It is found growing in sub-tropical rainforest as an understorey tree with an average height of 6m, on a range of soil types. The flowers occur singly in the leaf axils in spring and summer and fruit are born on the previous season's growth in autumn, on seedling trees. In coastal sub-tropical regions, flowering and fruiting may occur several times a year, on grafted trees. Seedlings trees have a long juvenile period of 5-17 years (Sykes, pers. comm., 1997) and few trees bear fruit annually. A wild population of 25 mature finger limes surveyed by the author, yielded an average of 186g of fruit per tree, in 1998. Grafted selections are variable, but generally bear more fruit in their third year than their mature counterparts in the wild.

The shape of the fruit is cylindric-fusiform or `finger-shaped', which is unique in the Orange sub-family. The species has a wide genetic diversity within its natural distribution and fruit varies considerably in size, shape, colour, quantity of seed and degree of acidity. The mature skin colours of the finger lime range between crimson, blood red, purple, black, yellow and green. The pulp is green on maturity and there is also a pigmented variety of finger lime, which has an attractive pale pink to dark crimson pulp on maturity. The red-pulp finger lime, although previously recorded as having a distribution from Terania Creek, in New South Wales to Mt Tambourine, in Queensland, has been observed growing throughout the natural distribution of the finger lime by the author. Previously unrecorded phenotypes, a yellow-pulped variety and a green/pink-pulped variety, have also been identified by the author.

The seed of the finger lime is monoembryonic (Smith, 1996) and seedling trees do not bear true to type. Size ranges from a small fruit of 6.3 x 1.2cm, weighing 7g at maturity, to a large fruit of 11.2cm x 2.9cm. weighing 60g, suitable for commercial processing. Fruit size varies with environmental conditions. The fruit contain from five to seven locules and have the unique characteristic of separate pulp-vesicles (Alexander, 1983). These pulp-vesicles, which have the appearance of caviar, are compressed within the skin and will burst out on cutting open the fruit.

In their book `Wild Food in Australia', Alan and Joan Cribb give a delightful description of eating the fruit: "For anyone who likes sour fruit these pulp cells are delicious; they burst pleasantly at slight pressure from the teeth and provide a most welcome refreshment" (Cribb, 1980). The finger limes can be used as a fresh fruit for garnish and for processing into a wide range of value-added products, such as salad dressings, beverages, sauces, marmalades or desserts. They contain up to 82mg of Vitamin C per 100g of raw edible portion (Miller et al.,1993). Farm gate price ranges from $8 -12.00/kg.

Citrus australis - round lime

Also called the Dooja or Gympie lime, this is the most vigorous of the Australian native citrus, growing to a height of 9-18m. It is endemic to south-eastern Queensland from Beenleigh to Gympie, in lowland sub-tropical rainforest. This species flowers in spring and bears rounded fruit which are 2.5-8cm in diameter, with a rough greenish-yellow skin on maturity and pale green pulp. The fruit contains an acidic juice, similar to the finger lime, but does not have the rounded pulp-vesicles or colour variation. The round lime is suitable for processing into a range of value-added products such as cordials, sauces, marmalades and lime flavouring. The skin is very thick (up to 7mm) and has potential for culinary use, such as grating into spice pastes, or for candied peel. The species may also have potential for essential oil extraction. The round lime fruit currently sell for $8-9.00/kg at the farm gate (Cherikoff, pers. comm., 2000).

Citrus garrawayae - Mt White lime

This species is endemic to the foothills and upland rainforest of the Cook District, Mt White on Cape York Peninsula in Australia and Goodenough Island in Papua New Guinea. It grows in deciduous vine thickets as an understorey shrub and has been recorded at a height of 15m. Due to its limited distribution, this species is now classified as rare in the wild and is protected under the Queensland Nature Conservation Act 1992. It is similar to the finger lime, but has broader leaves (Alexander, 1983). The flowers are single, axillary and seedling trees bear fruit from April to November (Cooper, 1994). Grafted trees bear flowers in Spring and fruit in Autumn. The fruits are also `finger-shaped', with a green skin and greenish-white to pale pink pulp on maturity. The fruit have fewer cells than the finger lime and a thicker skin (up to 2mm) containing large oil glands and weigh an average of 25g. The fruit may be used for processing into a range of value-added products, as for the round lime. The Mt White lime fruit have not yet been traded commercially (Ringer, S, pers.comm.2000).

Citrus inodora - Russell River lime

Also called the large-leaf Australian wild lime, this species is endemic to the lowland rainforest between Cairns and Innisfail, in north-eastern Queensland. It is a small tree, with an average height of 2-4m and differs from other species of native citrus by the presence of twin spines in each leaf axil and absence of perfume in the flowers (hence the name inodora). The fruit are green on maturity, oval (somewhat lemon- shaped) and up to 6.5 x 3.2cm in size (Alexander, 1983). This species is also classified as rare in the wild and is protected under the Queensland Nature Conservation Act 1992.

Citrus glauca - Desert limeglauca

Also known as the limebush or desert cumquat, this species was the `wild lime' featured in the Australian cookbook of the same name. The botanical name of this species is derived from eremos, the Greek word for `desert' and glauca, meaning `bluish', referring to the blue-grey colour of the leaves. The natural distribution of this species is the semi-arid regions of eastern Australia, from Rockhampton to Longreach in Queensland, south to Dubbo in central New South Wales and west to Quorn, in the Flinders Ranges of South Australia (Alexander, 1983). The desert lime is the only pronounced xerophyte in the orange subfamily, is extremely drought tolerant and able to withstand extremes of hot (45ºC) and cold (-24ºC) temperatures (Swingle and Reece, 1967). In its natural habitat of inland woodlands and brigalow scrubs, it is found growing on a range of soil types, especially heavy clay soils. (Low, 1988). The tree varies in size and form, from a dense multi-stemmed thicket of 2-3m in height, to a taller, more upright tree of 12m. The fruit is round to oblate in shape and approximately 2cm in diameter, weighing from 1-3g. The skin is a light yellow-green on maturity and contains large oil glands. The flower to fruiting time is the shortest of any citrus species, being from 10-12 weeks (Sykes, 1997). The species flowers mainly in spring and fruits ripen in summer. The acidic fruit is often seedless and can be used whole in cooking, or for processing into a range of value-added products, such as marmalades, sauces and cordials. The desert lime currently sells for $7.00/kg at the farm gate (Cherikoff, pers. comm., 1998).

Citrus gracilis - Humpy Doo lime

Also known as the Kakadu lime, this species has recently been formally described and named by Professor David J. Mabberley of the Royal Botanic Gardens Sydney. Named gracilis because of its graceful foliage, the species has a similar habit to the Citrus glauca. It is a straggling tree of up to 4m in height and bears pinkish-white flowers. It has characteristic large rounded fruits, of up to 8cm in diameter, which contain many locules. This fruit is recorded as having been eaten by Aboriginal people (Mabberley, 1998). The habitat of this species is Eucalypt woodland, with a grassy understorey on sandy or gravelly soils. As yet, little is known about the commercial potential of this species.

Hybrids

Citrus x virgata - `Sydney hybrid' (Citrus australis x Citrus australasica)

An example of a hybrid native citrus grown from seeds of the round lime and the finger lime by the US Department of Agriculture and given species status. The fruits of this species are similar to a lemon in shape, 35-50 x 20-28mm in size, with the tip abruptly rounded. The skin is 2mm thick and yellowish-green when ripe, containing numerous oil glands and the pulp-vesicles are tear-shaped (Swingle and Reece, 1967).

Cultivated hybrids

The following two open-pollinated citrus seedlings have recently been released by CSIRO to the bushfood industry for commercial experimentation and development.

`Australian Blood' PBR lime

This is a selected hybrid produced by open-pollination. It is thought to be a cross between an Ellendale mandarin (a mandarin and orange hybrid) and a seedling form of the Australian red-pulp finger lime (Citrus australasica) (Lewis, B, pers. comm., 1997). The cross has produced a tree of 2-3 metres in height, which bears small, blood-red fruit of approximately 30-50mm x 20-30mm (about the size of an oval cumquat). The pulp of the fruit is pink in colour, the intensity of which varies from season to season, however, the fruit do not have the separate rounded pulp-vesicles as found in the finger lime (Sykes, pers. comm., 1997). The `Australian Blood' PBR lime flowers in spring and fruit ripens in winter. The fruit is suited to manufacturing into a range of value-added products, including beverages, preserves, marmalades, sauces and syrups (Beal, 1998).

`Australian Sunrise' PBR lime (Citrus australasica x (Fortunella sp. x Citrus reticulata `Calamondin')

This variety is an open pollinated seedling selected from a Faustrimedin (Sykes, 1997), which is a hybrid of the finger lime with the calamondin; itself a hybrid between the cumquat and some variety of orange of the mandarin group (Citrus reticulata). The Faustrimedin is thus a trigeneric hybrid of Fortunella, Citrus and Citrus australasica and was originally bred in California in 1911 (Swingle and Reece, 1967). This hybrid citrus variety flowers in spring to early summer and fruit ripens in winter. The fruit are pear-shaped, approximately 30-45mm x 20-40mm, with a yellow-coloured skin on maturity. They are suited to manufacturing into a range of value-added products, as for the `Australian Blood' PBR lime (Beal, 1998).

Australian Native Citrus Propagation

All Australian native citrus species can be propagated from seed, although germination from seed of Citrus australasica is erratic (Floyd, 1989). Due to a wide genetic diversity, monoembryonic seed and compatability with exotic citrus varieties, Citrus australasica seedlings do not bear true to type. Cuttings are slow to strike (up to 6 months on a heat bed) with a 50% strike rate and both seed and cuttings may take many months to develop root systems (Nicholson, 1985). Seedling trees of Citrus australasica bear fruit from 5 to17 years (Sykes, pers. comm.,1997) and "Slow growth rates are frustrating..." (Nicholson, 1985). Seedlings of Citrus glauca grown under irrigation in California, began to flower and set fruit at 8-10 years old and reached a height of 5m after 7 years (Swingle and Reece, 1967).

Citrus species are now almost universally propagated by budding onto citrus rootstocks. Trees are selected with superior qualities and reproduced clonally by grafting buds (`budding') from the original parent trees onto selected citrus rootstocks. This enables trees of distinct, uniform and stable qualities to be cultivated. Because the scion is selected from mature trees, budding short-cuts the long juvenile period and enables trees to bear fruit in their second or third year in the ground.

Native citrus species appear to be compatible with Citrus and when grafted onto rootstocks they grow quite vigorously, forming smooth unions (although some graft incompatibilities with Citrus glauca have been reported (Bitters

Citrus species are now almost universally propagated by budding onto citrus rootstocks. Trees are selected with superior qualities and reproduced clonally by grafting buds (`budding') from the original parent trees onto selected citrus rootstocks. This enables trees of distinct, uniform and stable qualities to be cultivated. et al., 1964)). Rootstocks may be selected to suit soil type and climatic conditions. The process of budding native citrus species requires greater skill than for exotic citrus species, due to the minute size of the buds and high losses may be experienced. However, the result is a tree of increased vigour and disease resistance, which can flower and set fruit at an early age and provide a more reliable yield of high quality fruit.

Australian Native Citrus Breeding Programs

Dr Steve Sykes, from the CSIRO Division of Horticulture at Merbein, has been crossing native citrus with exotic citrus hybrids and researching the breeding of both new rootstocks and citrus scions. Citrus australasica and Citrus australis were used for their dwarfing characteristics and the red-pulped fruits of Citrus australasica were used for breeding new pigmented varieties of Citrus. Citrus glauca was used for cold  hardiness and resistance to salt and boron toxicity. He has since recognised a potential for some of these new hybrid varieties of Citrus to be cultivated for their ornamental qualities (Sykes, 1997).

limes

An assortment of limes- photo Erika Birmingham

Dr Sykes has released a selection of Citrus glauca `Australian Outback'PBR, as well as the `Australian Blood' PBR lime and `Australian Sunrise'PBR lime to Australian Native Produce Industries Pty Ltd, of South Australia, for commercial development. These varieties have currently been granted Interim Protection by the Plant Breeders Rights Office in Canberra. CSIRO is now focusing primary research on breeding new citrus varieties and plans to conduct further breeding programs with the `Australian Blood'PBR lime, aimed at increasing the fruit size of the hybrid citrus varieties (Sykes, 1997).

Other plant breeders have also emerged, who are breeding and selecting a range of superior true native citrus species for commercialisation. The author has collected seed and propagating material from throughout the natural distribution of native citrus species for ex situ conservation and breeding programs. Fifty varieties of native citrus have been selected from a wide gene pool of seedling trees. These have been grafted onto different rootstocks for long-term evaluation in trial plantings and to determine scion/rootstock compatibility. Superior varieties are being assessed for the following criteria: vigour, tree habit, fruit yield, ease of propagation, pest and disease resistance and a range of fruit qualities, such as skin and pulp colour, flavour, size, seedlessness and skin thickness. Interim Protection has been granted by the Plant Breeders Rights Office in Canberra on a variety of finger lime (Citrus australasica) `Rainforest Pearl'PBR, currently available from AT Eyles and Sons P/L in NSW and Victoria and Birdwood Nursery in Queensland.

Conclusion

In Australia, we are fortunate to have a valuable genetic resource in wild populations of true native citrus species. Our native citrus have been exported to many countries over the last century, where they remain in arboretums, but represent limited genetic diversity. Biodiversity of native citrus species is currently diminishing in the wild, due to continued clearing for development and other environmental pressures, with two species already placed on the rare and endangered list. This diminishing genetic resource should be protected by both in situ and ex situ conservation of these species for protection of biodiversity and for use in future breeding programs.

Limited funding has restricted the research and development of our unique Australian crops and the research base is small and fragmented. Government departments are currently funding a narrow range of native food crops, prioritising the development of existing products and markets.

More emphasis must be placed on adequate resources for R&D of Australian native food crops, the value of which can be measured in cultural, economic and environmental terms. Let us not repeat the mistakes of the Macadamia Industry, where the Australian `bush nut' was introduced into Hawaii in the 1880's, cultivars developed for Hawaiian conditions from a limited genetic diversity and then returned to Australia, only to find they did not perform well here (Bell, 1995).

In Australia, we need to  acknowledge the largely untapped genetic resource of native citrus from the rainforest to the desert. More emphasis should be placed on the conservation of our native citrus species ex situ. Citrus growers in Australia now have the opportunity to commercialise our Australian native citrus species for their unique qualities and in doing so, to help preserve our natural heritage.

Erika Birmingham, 2000

Byron Bay Native Produce

Telephone: 02 6687 1087

Facsimile: 02 6687 1087

International facsimile: 61 2 6687 1087

Email: erikab@nor.com.au

 References

Alexander, DMcE (1983) Some Citrus Species and Varieties in Australia. CSIRO Publishing, Melbourne.

Bailey, FM (1899-1902) The Queensland flora. Vols 1-6 HJ Diddams & Co, Brisbane.

Beal, A (1997) Australian Blood Lime PBR. Australian Native Produce Industries Pty Ltd.

Beal, A (1998) Commercialisation of Native Citrus. The Australian Rainforest Bushfoods Industry Association Technical Journal 6.

Bell, HFD (1995) Plant Breeding in Vegetatively Propagated Tree Crops. Proceedings from the Sixth Conference of ACOTANC, Lismore, NSW. Available from:

http://www.uq.edu.au/~gagkrego/acotanc/papers/bell.htm

Bitters, WP, Brusca, JA and Cole, DA (1964) The Search for new citrus rootstocks. Calif. Citrograph, 49, 443-8.

Brand Miller, J, James, KW and

Maggiore, PMA (1993) Tables of Composition of Australian Aboriginal Foods. Aboriginal Studies Press.

Cooper, W (1994) Fruits of the Rain Forest. RD Press, Australia.

Cribb, AB and JW (1980) Wild Food in Australia. Fontana/Collins Publication.

Floyd, AG (1989) Rainforest Trees of Mainland South-eastern Australia. Forestry Commission of NSW.

Graham, C and Hart, D (1997) Prospects for the Australian Native Food Industry. Rural Industries Research and Development Corporation, ACT.

Low, T (1988) Wild Food Plants of Australia. Angus and Robertson, Australia.

Mabberley, DJ (1998) Australian Citreae with notes on other Aurantioideae (Rutaceae). Telopea 7(4):333-344.

Nicholson, N and H (1985-9) Australian Rainforest Plants. Hugh and Nan Nicholson, The Channon, NSW. Volumes 1-4.

Ross, EM (1983) Rutaceae. In Stanley, TD and Ross, EM (1983) Flora of South-eastern Queensland 1: 440-470. Government Printer, Brisbane.

Smith, K (1996) Comparative Embryology of Microcitrus australasica and Microcitrus australis. University of New England, Armidale.

Swingle, WT (1915) Microcitrus, a new genus of Australian citrus fruits. Journal of the Washington Academy of Science 5: 569-578.

Swingle, WT and Reece, PT (1967) The botany of Citrus and its wild relatives. The Citrus Industry Vol. 1. University of

California, Division of Agricultural Science, USA pp190-430.

Sykes, SR (1997) Australian native limes (Eremocitrus and Microcitrus); a citrus breeder's viewpoint. Australian Bushfoods Magazine 3. 

glauca

Citrus glauca - Wild lime. Photo Brian Rogers

Erika Birmingham

Byron Bay Native Produce

Australian finger lime specialists since 1995.

PO Box 232, Bangalow, NSW 2479

Ph/Fax (02) 6687 1087

Email: erikab@nor.com.au

 

 

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