658

19: Essential-oil plants

Corymbia citriodora (Hook.) K.D. Hill & L.A.S. Johnson

Telopea 6: 388 (1995).

MYRTACEAE

2n = 22

Eucalyptus citriodora Hook. (1848), Eucalyptus melissiodora Lindley (1848), Eucalyptus maculata Hook. var. citriodora (Hook.) Bailey (1900).

Lemon-scented gum, spotted gum, lemon-scented iron gum (En). Thailand: yukhalip. Vietnam: b[aj]ch d[af]n d[or], b[aj]ch d[af]n chanh.

Corymbia citriodora is endemic to Queensland (Australia). It occurs mainly in the region from north-west of Maryborough to north of Rockhampton and west for up to 400 km. There are also extensive stands on the tablelands inland between Mackay and Cairns, and an occurrence west of the Great Dividing Range north of Hughenden.
Corymbia citriodora has been extensively planted as an ornamental tree in many regions of the world, and has been planted for commercial purposes in South America, especially Brazil (6 million trees), southern China, India, Sri Lanka, Congo (Zaire), Kenya and most countries in southern Africa and in Fiji. In South-East Asia it is mainly planted in Peninsular Malaysia. In Thailand it was introduced in 1949, but commercial plantations no longer exist.

The pleasant, lemon-scented essential oil from the leaves of Corymbia citriodora is widely used in less expensive perfumes, soaps and disinfectants. It has antibacterial and insecticidal activity. The citronellal-rich oil is a preferred natural source for the production of hydroxycitronellal, citronellylnitrile and menthol. Hydroxycitronellal is one of the most widely used of all perfumery materials. Other minor constituents recovered during fractionation of the essential oil, such as citronellol, are also used by the fragrance industry.
The timber of Corymbia citriodora is used for general and heavy construction such as frame and bridge construction, flooring, cladding, tool handles and case manufacturing. The wood of young trees has been successfully used for certain grades of pulp and paper, but wood from old trees is generally not suitable because of its high extractive content and high density. Young trees or coppice stems produce straight poles or posts which can be pressure impregnated with preservatives for many industrial purposes. In Brazil large plantations have been established for charcoal production.
Corymbia citriodora is widely used in park and avenue plantings as an attractive, large ornamental noted for its bark colour, straight trunk, branching habit and glossy leaves. However, its crown is too sparse for shelter-belts. It is a source of nectar and pollen in apiculture and gives a light amber honey.

Estimates of annual world production of Corymbia citriodora indicate that in 1991 only three countries produced substantial quantities of this oil: China with 900—1100 t, Brazil with 400—600 t and India with 50 t. About 400 t are exported annually from China; Brazil exports about half of its annual production. The oil is no longer produced to any extent in Australia. Corymbia citriodora oil was priced at US$ 6/kg by dealers in New York in May 1997.

Commercial essential oils of Corymbia citriodora are colourless to pale yellow with a strong, very fresh rosy-citronella scent and a sweet balsamic-floral dry-out. They consist principally of citronellal and other monoterpenes including citronellol, neral, isopulegols and smaller amounts of 1,8-cineole, linalool, geraniol, 'ALFA'-terpineol, 'GAMMA'-cadinene and 'BETA'-caryophyllene. Oils are assessed on their odour characteristics and the portion of citronellal in the total aldehyde content. Producers of hydroxycitronellal prefer the higher citronellal levels.
At least four chemical forms are recognized in the natural stands of Corymbia citriodora in Australia; their citronellal contents range from 1% to 91%. In addition to the form yielding citronellal-rich (65—91%) 'type' oils, there are: a 'var. A' form yielding oils containing citronellol (about 50%) and citronellal (1—14%), an 'intermediate form' with citronellal (20—50%) and guaiol, and a 'hydrocarbon form' with citronellal (<10%) and hydrocarbons. The presence of such diverse chemotypes in nature may, in part, account for the substantial variation in citronellal content between oil batches from different regions where Corymbia citriodora is cultivated. See also: Composition of essential-oil samples and the Table on standard physical properties. Myrtillin, found in the leaf extract, is reported to induce temporary hypoglycemia. Other compounds reported from the leaves include several fatty acids (including shikimic acid), flavonoids and sterols.
A monograph on the physiological properties of the essential oil has been published by the Research Institute for Fragrance Materials (RIFM). An exudate from the bark contains citriodoral, a compound with antibiotic properties. The bark also contains about 10% tannin.
The wood of C. citriodora has a white or cream sapwood up to 60 mm wide and is very susceptible to Lyctus borer attack; the heartwood is pale grey-brown to dark brown. The sapwood is permeable but the heartwood extremely resistant to preservative treatment. The grain is straight or interlocked and occasionally wavy, the texture open and coarse. The timber is hard, strong and tough. Air-dry density of wood from natural stands is about 1000 kg/m³ (12% moisture) with basic density of 800 kg/m³. It saws easily, planes well, but is rather difficult to nail and prone to checking and collapsing during drying. The timber is susceptible to marine borer and termite attack. Chemical pulp of reasonable quality can be obtained from wood of young (9-years-old) trees.
There are about 109 000 viable seeds/kg seed and chaff mix.

Both citronellal and the derived hydroxycitronellal are produced from two other sources, citronella oil from the citronella grasses (Cymbopogon nardus (L.) Rendle and Corymbia winterianus Jowitt) of Sri Lanka and Indonesia, and turpentine. Price and the slight odour differences caused by accompanying minor constituents determine customer preference. Natural citronellal has a marketing advantage over synthetic, turpentine-derived citronellal, especially in flavour applications.

Medium to large, often straight stemmed tree, 25—40(—50) m tall, of handsome appearance, with pale grey, cream or pink powdery bark, smooth throughout, decorticating in flakes, and somewhat sparse foliage. Twigs slender, slightly flattened, light green, tinged with brown. Leaves petiolate, strongly lemon-scented when crushed; seedling leaves opposite for a few pairs then alternate, peltate, ovate, 6.5—17 cm x 2.3—7.5 cm, pale green, slightly discolorous, setose; juvenile leaves alternate, ovate to broad-lanceolate, up to 14—21 cm x 4.5—8 cm, in some plants setose, pale green and peltate for many pairs, in others becoming glabrous and glossy green soon after the seedling stage; stems, petioles and leaves setose with bristle glands up to 0.5 cm long in both the seedling and juvenile stages; intermediate leaves petiolate (up to 2.2 cm), disjunct, broad-lanceolate, 13—30 cm x 2—5 cm, apex acute to obtuse; adult leaves on flattened petioles 13—22 mm long, disjunct, lanceolate to narrow-lanceolate, 7—22 cm x 0.5—2.2 cm, concolorous, very glossy, green, venation very densely reticulate with numerous large island oil glands, lateral veins just visible, at 35°—50°, intramarginal vein distinct, up to 1 mm from margin, stomata on both surfaces. Inflorescence an umbelliform condensed and reduced dichasium (usually called a conflorescence), combined into clustered terminal or sometimes axillary, corymbose panicles; peduncle terete, 3—8 mm long; umbels 3-flowered; pedicel 1—6 mm long; buds pedicellate, clavate, up to 10 mm x 6 mm, scar absent; operculum hemispherical, 3—4 mm long, 4—5 mm wide, apiculate; flowers creamy-white; hypanthium hemispherical, 5—6 mm x 4—5 mm; stamens numerous, prominent, 6 mm long, spreading 12 mm across, all fertile with subulate filaments and oblong, dorsifixed anthers dehiscing by parallel slits; pistil inferior with 3-celled ovary. Fruit a capsule, truncate-ovoid to urceolate, 8—15 mm x 7—11 mm, brown, often warty, 3-locular; disk about 2 mm wide. Seed dorsiventrally compressed, 2—3 mm x 1.5—2.5 mm, glossy red-brown, with a median dorsal keel. Seedling with epigeal germination.

Lignotubers develop early in the life of Corymbia citriodora seedlings. As in other eucalypts no dormant buds develop and trees will grow whenever conditions are favourable. Corymbia citriodora grows rapidly during its early years. In Congo trees reached a mean height of 13 m in 4 years. In southern China it can average 2.5—3.7 m growth in height per year and reach a diameter at breast height of 25 cm in 5 years. In fuelwood trials in southern Nepal, Corymbia citriodora reached 5 m in 3 years from planting. A mean annual height increment of 2 m can be attained in southern Africa over the first 5 or even 10 years. Trunk form varies from straight to sinuous; frequent forking and sparse foliage are less satisfactory crown characteristics. Flowering usually starts within 2 years after planting and seeds are produced abundantly by 5 years of age. Pollination is mainly by insects but also by birds and small mammals. Flowering in natural stands in Australia has been observed in all months of the year with a peak during April—June. The main seed collection months are September through January with seedfall timed to coincide with the rainy season. In wild stands it is often difficult to obtain good seed crops; in contrast, plantations often have abundant seed crops. In Thailand the optimum time for seed collection is April.

Corymbia citriodora belongs to the spotted gums, part of the bloodwood group of eucalypts. Under the informal classification of Pryor and Johnson (1971), Corymbia citriodora (as Eucalyptus) was placed in the series Maculatae of the subgenus Corymbia of Eucalyptus, along with its close relatives Eucalyptus maculata Hook. and Eucalyptus henryi S.T. Blake. Hill and Johnson (1995) have placed the bloodwoods in a new genus Corymbia K.D. Hill & L.A.S. Johnson. Under this classification, which is adopted here, Eucalyptus citriodora becomes Corymbia citriodora of the series Maculatae and section Politaria. Four closely related species are recognized in this section: Corymbia citriodora, Corymbia maculata (Hook.) K.D. Hill & L.A.S. Johnson, Corymbia henryi (S.T. Blake) K.D. Hill & L.A.S. Johnson and a new species Corymbia variegata (F. v. Mueller) K.D. Hill & L.A.S. Johnson. The latter one was reinstated from synonymy, to cater for the populations in northern New South Wales and south-eastern Queensland that are morphologically intermediate between Corymbia citriodora and Corymbia maculata and whose leaves are not lemon-scented. In regions where the distributions of Corymbia citriodora and Corymbia variegata overlap intergrading populations occur with a gradient in the amount of citronellal in the leaves.
Hybrids between Corymbia citriodora and Corymbia torelliana (F. v. Mueller) K.D. Hill & L.A.S. Johnson have appeared spontaneously in Australia, India, Nigeria and Papua New Guinea, and have been created artificially in India. Hybrids with other Corymbia spp. are also found occasionally.

The climate in the natural range of Corymbia citriodora is warm humid to warm subhumid. Temperature varies from the higher rainfall, coastal areas to the drier inland locations. In the wetter sites the mean maximum temperature of the hottest month is about 30—32°C and the mean minimum of the coldest month about 9—12°C; for inland locations the corresponding values are 34—36°C and 5—10°C. Light frosts may occur at higher elevations. The mean annual rainfall is about 650—1600 mm with a well developed summer maximum, most distinct in the north. Late winter and spring are dry. Climatic requirements for Corymbia citriodora derived from both its natural distribution area and the sites where it grows well as an exotic are estimated to be: mean annual temperature 17—28°C, mean maximum temperature hottest month 28—39°C, mean minimum temperature coldest month 8—22°C, absolute minimum temperature over —3°C, mean annual rainfall 650—2500 mm and a dry season (i.e. less than 40 mm/month) of 0—7 months. Although it originates from summer rainfall zones it can also grow successfully in locations with a winter rainfall or uniform rainfall distribution pattern.
In its original habitat Corymbia citriodora occurs mostly on poor gravelly soils, podzols and residual podzols of lateritic origin, usually well drained and undulating. Other stands occur on deep red loams, hard gravelly clays and on sandstone-derived soils. In Sao Paulo (Brazil) it grows well on lateritic soils, in Congo (Zaire) excellent growth is obtained on rich volcanic and young alluvial soils, while in India it is grown on various soils ranging from fertile loams to poor acid sands. It is not very tolerant of waterlogging.

Seedlots of Corymbia citriodora vary in germination rate but usually average 30—50%. Rapid and complete germination is achieved under moist, warm conditions (25—30°C is optimal in the laboratory) in the presence of light. In seed tests a leachate from the seed has been found to inhibit germination on filter paper. Direct seeding into carefully prepared ground has been successful in southern Africa, but is unreliable and not generally recommended, as favourable weather and freedom from weeds are critical to success. Containerized planting stock is preferred, because bare-rooted seedlings tend to show poor lateral root development and losses due to transplanting shock. The seeds are relatively large for a eucalypt and can be sown directly without pretreatment into containers filled with a sterilized freely draining mixture of loam and sand and covered with a light sprinkling of fine sand. Corymbia citriodora, like other eucalypts, is highly susceptible to damping-off and other fungal pathogens in the nursery. Disease problems can be limited by good hygiene, reducing watering and shade, and allowing good ventilation. Seedlings are planted out in the field when they reach a height of about 25 cm, 10—12 weeks after sowing. This should coincide with the onset of the rainy season in tropical countries. Corymbia citriodora can be multiplied vegetatively by micropropagation, but so far this technique has only been applied experimentally. Propagation by cuttings is very difficult.
Intensive site preparation by ploughing is beneficial; on compacted soils deep ripping may also be used. Spacing varies, depending on the purpose of the plantation. When grown solely for oil production with frequent coppicing, 3 m x 1.5 m (2222 plants/ha) is appropriate. For fuelwood, poles or charcoal production with harvesting cycles of 7 years, a spacing of 3 m x 2 m (1667 plants/ha) is common. Several planting arrangements have shown favourable economic returns in India: block plantings at 1 m x 1 m spacing in a farm forestry programme; planting along bunds at 1—2 m spacing in an agroforestry programme; and interplanting essential oil-yielding grasses (Cymbopogon spp.) with Corymbia citriodora at 2—3 m x 2 m spacing in a social forestry programme, to obtain essential oil as well as fuelwood and pole wood.

The ability of young Corymbia citriodora to compete with weeds is so poor that inadequate weed control may lead to the complete failure of a planting. Mechanical and manual cultivation are the most common means of control. Corymbia citriodora has a strong self-pruning ability and is only hand pruned when leaves are harvested for oil production. Light demand is high and frequent and regular thinning is a prerequisite for healthy, vigorous plantations. In China, where it is grown principally for poles and fuelwood on 20 year rotations, thinning is prescribed at ages 3—5, 7 and 10—12; the first thinning reduces the initial stocking rate from 4000 to 2000 stems/ha; the two additional thinnings reduce the final stocking rate to 900 stems/ha. A handy indicator of growth stagnation and the need to undertake the first thinning has been developed in Brazil, based on measurements of the outer rows of a plantation. The first thinning should be carried out when the mean diameter of the trees in the second row exceeds that of the trees in the third row by 10% or more.
Plantations are not normally irrigated but good results have been achieved in trials in Pakistan, where irrigated Corymbia citriodora intercropped with such crops as wheat, maize, berseem (Trifolium alexandrinum L.) and sesame (Sesamum orientale L.) grew to 16.4 m in height with a diameter at breast height of 17.7 cm at 7 years.
Although organic or chemical fertilizers are rarely used, fertilizer application on sites of very low soil fertility may be necessary to increase productivity. For example, in the savanna (cerrado) region of Brazil, application of 100—150 g of an NPK mixture (10-28-6) per plant regardless of soil type or time of planting used to be standard practice for Corymbia citriodora and other eucalypts. Today a blend of several macro- and micro-nutrients is applied, depending on the nutritional status of coppice leaves from a particular site. A microcomputer programme has been developed to assist in determining the optimum treatments. When planted on sites with severe water deficit (more than 4 months of drought), Corymbia citriodora is very sensitive to boron deficiency and application of 1.0—1.5 g B per plant reduces dieback and increases volume growth up to 30%. The effect of fertilizers on oil production has given variable results, sometimes increasing production but often giving no response.

Within Australia Corymbia citriodora has remained relatively free of diseases and pests. In Brazil, it has been damaged sometimes by a range of diseases including: damping-off and leaf spot caused by Cylindrocladium spp., a rust (Puccinia psidii), and a stem canker (Cryphonectria cubensis). Gummosis and cankers from infection by Endothia havanensis have also been noted. In China, gummosis induced by Cytospora sp. and Macrophoma sp. has caused severe damage. In India, it is susceptible to a range of diseases including: Cylindrocladium seedling blight, a rust (Melampsora sp.), pink disease (Corticium salmonicolor), and Ganoderma root rot. The root rot fungus Pseudophaeolus baudonii attacked plantings of Corymbia citriodora in Sege (Ghana), causing 50% mortality over 3 years. Most problems arise on sites with high rainfall and humidity. Prevention is the best cure, so appropriate nursery techniques should be applied and planting sites should be selected carefully.
Corymbia citriodora is very susceptible to termites. In India, Microcerotermes minor can cause 20—30% mortality and Odontotermes horni over 10%. Use of dangerous pesticides such as dieldrin, aldrin and chlordane for the protection of seedlings against termites has been phased out in most countries. Carbosulfan, a non-persistent carbamate insecticide, is being used as an effective replacement in several African countries. A range of defoliating insects and a stem borer (Apate indistincta) have been noted as causing occasional damage to plantations.

Optimum rotation length for plantations of Corymbia citriodora depends on site, management and end products required. In southern China and Brazil, the rotation length for plantations for roundwood, fuel and citronellal oil production is 20—21 years, after which time the clear-felled areas are replanted. However, there are numerous plantings elsewhere that are much older and are regularly harvested for oil and wood. Elsewhere in Brazil, the initial seedling crop is commonly harvested for wood in the 7th year after planting, yielding 110 m³/ha. The harvest is followed by two coppice harvests at 7-year intervals yielding 120 m³/ha and 88 m³/ha of wood respectively. The best time of the year for harvesting the seedling crop is in the winter or dry season. The development of new growth from buds on the stump begins shortly after harvest and within 10 months several shoots have become dominant. At this point, rigorous thinning to 2—3 stems per stump is carried out.
When Corymbia citriodora is grown principally for oil production the aim is to maximize the yield of oil per ha and to minimize harvesting costs or, as most harvesting is done by hand, to minimize the cost of labour. In Brazil the smaller branches are cut from the stem 18 months after planting. This is repeated every 6 months or so, until the branches are too high to be reached (after about 3 years). The stem is then cut at about knee height and 2—3 stems are allowed to grow over a period of about 12 months, when the harvesting cycle is repeated. A cutting height of 0.8 m gave greater oil production than a 0.4 m cutting height after an initial harvest at 2 years in Taiwan, but other reports indicate that coppicing gives higher yields than pollarding. Most Corymbia citriodora oil produced in India comes from smallholders who harvest at irregular times depending on convenience and oil prices. In north-western India, major harvests are undertaken in April and November with a supplementary cut in July if regrowth has been vigorous. Although 2—3 harvests per year may give maximum yields, harvesting once a year is often most profitable. Overcutting may also have an adverse effect on tree longevity. In China, tall trees in wood-producing plantations are climbed, and all but the very top foliage is removed for oil production in small roadside stills. Many factors influence oil yield, including seasonal differences. Harvesting in February, April, July and October gave maximum yields at Kodaikanal in India, while February and November are the best months to harvest in Cuba. Local experimentation should be undertaken to determine the optimum time to harvest on a particular site.

Reports of concentration of essential oil in fresh leaves of Corymbia citriodora vary from 0.5—5.0%, but mainly fall within the range of 1—3%. Oil content and citronellal content in that oil are greatest in young leaves (4 months) and both decline with leaf age. The reduction in oil content and citronellal content from 4—9 months is significant but is offset by an increase in leaf numbers over this period. Coppiced trees give 2—5(—10) kg of leaves per tree annually. Theoretically, therefore, an annual harvest of 1 ha of coppiced plantation (2000 stems/ha) would yield about 150 kg of oil. Mature trees yield 300—500 kg of leaves when felled for timber.
Mean annual increments in wood production of 15 m³/ha have been recorded in China and Brazil with a maximum of 25 m³/ha achieved on some sites in China. In the Guinea zones of Nigeria mean annual increments of 9.2—14.7 m³/ha have been reported.

Extraction of oil from Corymbia citriodora leaves is done by water or steam distillation. Foliage from the harvesting operation should be distilled either the same day or the following day. Evidence suggests that both oil yield and aldehyde content of the oil decline rapidly after harvesting. The crude oil of Corymbia citriodora is not subjected to any particular treatment before marketing, beyond clarification and filtration to ensure freedom from water and suspended matter. Storage in a cool location away from light is recommended.

In the wild Corymbia citriodora is locally abundant over a wide area and is not considered to be at risk. In addition, it is very widely planted and local landraces in many countries are another reserve of genetic material. Seed trees have seldom been tested for their oil characteristics in Australia, and seed exported from Australia for plantation establishment could be representative of one or more of the four chemical forms of Corymbia citriodora. When propagating Corymbia citriodora for oil production, care should be taken that the seed has been selected from provenances with oil characteristics that meet the requirements of the intended market. The Australian Tree Seed Centre of CSIRO Forestry and Forest Products in Canberra maintains a collection of seed of provenances of Corymbia citriodora including some individual tree collections that have been tested for oil characteristics.

Despite the early identification of Corymbia citriodora as a useful multipurpose tree and its widespread dissemination, few comprehensive provenance trials exist. The sporadic occurrence of seed crops in the natural stands contributes to the difficulty of making a comprehensive seed collection for such trials. Existing provenance trials mostly show genetic variability between and within populations in growth traits and point to the need to select provenances locally before embarking on a planting programme. Brazil is most advanced in the improvement of the species. It started selection work in the 1970s using only imported seed of approved quality. In the 1980s it identified certified seed production areas; these are phenotypically superior stands in which some management practices have been conducted. In the state of Minas Gerais, a tree improvement programme was initiated in 1978, with elite trees selected for growth characteristics in a seed production area. The aim was to establish seedling seed orchards and clonal seed orchards by grafting. A study of mating system parameters indicated reasonably high levels (14.7%) of self fertilization and low levels of heterozygosity in progeny from these stands.

Corymbia citriodora is already widely planted for roundwood, fuel, essential oil and amenity throughout the tropics and subtropics. Although not of outstanding growth rate, it gives acceptable growth on a wide range of sites. Corymbia citriodora is also a useful multipurpose tree grown for wood and oil, which can be profitable for smallholder farmers. In general, it is considered that the potential of Corymbia citriodora may not have been fully exploited. No breeding or selection work is being done on Corymbia citriodora to improve the production of essential oil. Improvement of wood production is in its infancy.

Boland, D.J., Brophy, J.J. & House, A.P.N., 1991. Eucalyptus leaf oils: use, chemistry, distillation and marketing. Inkata Press, Melbourne, Australia. 252 pp.
Coppen, J.J.W., 1995. Flavours and fragrances of plant origin. Non-Wood Forest Products No 1. Food and Agriculture Organization of the United Nations, Rome, Italy. 101 pp.
Coppen, J.J.W. & Hone, G.A., 1992. Eucalyptus oils: A review of production and markets. Natural Resources Institute Bulletin 56. Chatham, United Kingdom. 45 pp.
Dung, N.X., Hang, N.T., Versluis, K., Tinh, H.M., Be, N.V. & Loi, L.V., 1995. Results of the study on Eucalyptus citriodora from Vietnam. Proceedings of the NCST of Vietnam 7(1): 51—57.
Hill, K.D. & Johnson, L.A.S., 1995. Systematic studies in the eucalypts 7. A revision of the bloodwoods, genus Corymbia (Myrtaceae). Telopea 6: 185—504.
Poynton, R.J., 1979. The eucalypts. Tree planting in Southern Africa, Vol. 2. Department of Forestry, Pretoria, South Africa. 882 pp.
Pryor, L.D. & Johnson, L.A.S., 1971. A classification of the eucalypts, Australian National University Press, Canberra, Australia.
Sugimoto, S. & Kato, T., 1983. Composition of Eucalyptus oils. Kanzei Chuo Busekisho Ho [Reports of the Central Customs Laboratory] 23: 31—34.
Weiss, E.A., 1997. Essential oil crops. CAB International, Wallingford, United Kingdom. pp. 272—278.

J.C. Doran

Doran, J.C., 1999. Corymbia citriodora (Hook.) K.D. Hill & L.A.S. Johnson. In: L.P.A. Oyen and Nguyen Xuan Dung (Editors): Plant Resources of South-East Asia No 19: Essential-oil plants. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea