Issue No 10, July 1998.

NOTICE: Hard copies of the Australian New Crops Newsletter are available from the publisher, Dr Rob Fletcher. Details of availability are included in the Advice on Publications Available.

21.1 Australian native citrus: wild limes from the rainforest to the desert...

Recently, there has been increasing interest in food crops indi- genous to Australia. These '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 bushfood industry was estimated to be worth $10-12 million in 1995/6 and growth is expected to boom by the time of the Sydney Olympic Games in 2000. Export potential is high and exports are expected to contribute to growth (Graham and Hart, 1997). One of the strengths of the bushfood 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 bushfood industry in Australia has developed over the last 15 years from initially comprising only 'wild harvested' product.

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

To date, at least 50 bushfoods and bushfood products have been researched, developed and processed for market. A further 150 species have been 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 lime' group. Several species of native citrus have been wild harvested in recent years.

Graham and Hart (1997) in their report of the prospects for the Australian native bushfood industry included wild lime (Microcitrus species and Eremocitrus glauca) in the fourteen crops with the most commercial potential.

The Australian native citrus have a strong potential within both the bushfood and citrus industries for both commercial growers of raw produce and for the retail (home grower) market.

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:

WT Swingle (1915) described the rainforest limes as

Only limited records exist of traditional Aboriginal use for these native citrus species. However early Australian colonists used them to make excellent drinks and marmalades (Low, 1991).

Cultivation of native citrus was largely ignored in Australia until CSIRO began investigating the use of some 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 spp. (Sykes, 1997).

Exotic citrus species have been cultivated for thousands of years. Most citrus species can no longer be found in the wild, as cultivation has lead to hybridisation. Hybrids can result from either natural or deliberate interbreeding of related species.There is confusion in attempting to define citrus species, as each variety may be a complex hybrid of three or even four species. Resulting hybrids may then be given a new botanical name and species status (e.g. Microcitrus 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 Citrus and Microcitrus).

There is a lack of standardisation of common names within the Australian bushfood and citrus industries, with the six different native citrus species and their hybrids listed under the general category of 'native citrus' or 'wild limes'. Botanical names are the only positive method for identification of the true native citrus species.

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 Citrus and Microcitrus.

There are two genera of true citrus fruit trees in 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 their very small juvenile leaves and the minute size of their flowers.

There are five species of Microcitrus found in Australia, all of which are endemic to rainforest habitats on the east coast. Their distribution originally extended from Cape York Peninsula to the Clarence River on the north coast of New South Wales although much of this area has since been cleared for development.

Each species now has a limited distribution, with two of the species endemic to Queensland currently listed as rare in the wild. Eremocitrus glauca is monotypic and is endemic to the semi-arid regions of Queensland, New South Wales and South Australia.

The natural distribution of the finger lime is from the Richmond River, in northern NSW to Mt Tambourine in Queensland (Floyd, 1989), with reference to its occurrence in the southern portion of the Moreton Bay district (Ross,1983). It is found growing in sub-tropical rainforest as an under-storey 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 borne 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 in 1998, yielded an average of 186g of fruit per tree. Grafted selections have been observed to yield more fruit in their second 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 true pigmented variety of finger lime, called the red-pulp finger lime (Microcitrus australasica var. sanguinea) which has an attractive pale pink to dark crimson pulp on maturity.

The red-pulp finger lime 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 present author.

The seed of the finger lime is monoembry-onic (Smith, 1996) and seedling trees may not bear true-to-type. Progeny grown by CSIRO researchers from seed from a single parent tree have borne fruit with the entire range of skin colours (Lewis, B, pers comm, 1997). Size ranges from a small fruit of 6.3 x 1.2cm, weighing 7g at maturity, to a large fruit of 12.8 x 2.4cm weighing 42g, suitable for commercial processing.

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:

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, desserts, jellies or pastries. They contain up to 82mg of Vitamin C per 100g of raw edible portion (Miller et al, 1993). The finger lime has not yet been traded commercially (Cherikoff, pers comm, 1998) and a farm gate price is still to be determined.

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 round pulp-vesicles or variation in colour.

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 currently sells for $7/kg at the farm gate (Cherikoff, pers comm,1998).

This is the last of the Australian native citrus species to be discovered and is named after Mrs RW Garraway. 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 under-storey shrub and has been recorded at a height of 15m. Due to its limited distribution, this species is now classified as rare 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 the tree bears fruit from April to November (Cooper, 1994). The fruits are also 'finger-shaped', with a green skin and greenish-white 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.

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 Microcitrus, 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 and is protected under the 1992 Act.

Named in honour of Professor JH Maiden, this species was originally described as a variety or subspecies of M. inodora. It also has a limited distribution in far north Queensland. Similar to the M. inodora, the species is distinguished by two spines in each leaf axis, but the fruit are smaller, with a distinctly sunken apex (Alexander, 1983).

Also known as the limebush or native 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 wide range of soil types (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).

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).

The following two open-pollinated Citrus seedlings, with obvious Microcitrus characteristics, have recently been released by CSIRO to the bushfood industry for commercial experimentation and development.

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 (Microcitrus australasica var. sanguinea) (Lewis, pers comm, 1997). The cross has produced a tree of 2-3 metres in height, which produces small, blood-red fruit of approximately 30-50mm x 20-30mm (about the size of an oval cumquat).

The pulp of the fruit is red in colour, the intensity of which varies from season to season and 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, marma- lades, sauces and syrups (Beal, 1998).

This variety is an open pollinated seedling selected from a Faustrimedin, 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 Microcitrus 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).

Both Eremocitrus and Microcitrus species can be propagated from seed, although germination from seed of Microcitrus australasica is erratic (Floyd, 1989). Due to a wide genetic diversity, monoembryonic seed and compatability with exotic citrus varieties, Microcitrus australasica seedlings may not bear true to type.

Cuttings are slow to strike (up to 6 months on a heat bed) with a 50% strike rate. Both seed and cuttings may take many months to develop root systems (Nicholson, 1985). Seedling trees of Microcitrus australasica bear fruit from 5 to17 years (Sykes, pers comm, 1997) and their growth is slow. Seedlings of Eremocitrus 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.

Both Microcitrus species and Eremocitrus glauca are compatible with Citrus and when grafted onto rootstocks they grow quite vigorously, forming smooth unions (although some graft incompatibilities with Eremocitrus glauca have been reported (Bitters et al, 1964)). Root- stocks may be selected to suit soil type and climatic conditions.

The process of budding Microcitrus requires greater skill than for other 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 reliable yield of high quality fruit.

Dr Steve Sykes, from the CSIRO Division of Horticulture at Merbein, Victoria, has been crossing native citrus with other citrus hybrids and researching the breeding of both new rootstocks and citrus scions.

Microcitrus species have been used for their dwarfing characteristics and the red-pulped fruits of the Microcitrus australasica var. sanguinea for breeding new pigmented varieties of Citrus. Eremocitrus 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.

Dr Sykes has recently released two selections of Eremocitrus glauca, as well as the limes named above, to the bushfood industry for commercial experimentation and development.

These varieties have 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, breeding and selecting a range of superior true native citrus species for commercialisation.

Microcitrus and Eremocitrus glauca selections have been grafted onto different rootstocks for long-term evaluation in trial plantings. These are being assessed for 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 for a variety of red-pulp finger lime (Microcitrus australasica var. sanguinea) 'Rainforest Pink Pearl'.

Seed has also been collected by the author from wild populations of Microcitrus australasica from throughout their natural distribution for in situ conservation and for further breeding programs.

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 arboreta. This 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 in situ conservation of these species if it is ever to be used for breeding in the future.

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. The Australian 'bush nut' was introduced into Hawaii in the 1880's, cultivars were developed for Hawaiian conditions and then returned to Australia for commercial use. But 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 in situ.

Bushfood 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.

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

Bailey, FM (1904) Contributions to the flora of Queensland. Queensland Agricultural Journal 15: 491-495.

Beal, A (1998) Commercialisation of Native Citrus. The Australian Rainforest Bush-foods 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. California Citrograph 49: 443-8.

Miller, JB 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.

Low, T (1991) Bush Tucker, Australia's Wild Food Harvest. Angus and Robertson, Sydney.

Maiden, JH (1889) The Useful Native Plants of Australia. Compendium. Melbourne. Reprinted 1975.

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 Microcit-rus); a citrus breeder's viewpoint. Australian Bush-foods Magazine 3.

Contact: Dr Rob Fletcher, School of Land and Food, The University of Queensland Gatton College, 4345; Telephone: 07 5460 1311 or 07 5460 1301; Facsimile: 07 5460 1112; International facsimile: 61 7 5460 1112; Email: r.fletcher@mailbox.uq.edu.au

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originally created by: GK; latest update 6 June 1999 by: RF