674

19: Essential-oil plants

Santalum album L.

Sp. pl.: 349 (1753).

SANTALACEAE

2n = 20, (18)

Sirium myrtifolium L. (1771), Santalum ovatum R. Br. (1810), Santalum myrtifolium (L.) Roxb. (1814).

East Indian sandalwood, white sandalwood, yellow sandalwood (En). Bois santal (Fr). Indonesia: cendana (general), ai nitu (Sumba), hau meni (Timor). Malaysia: chendana. Burma (Myanmar): san-ta-ku. Thailand: chantana.

The exact origin of Santalum album is not known. It is probably native to the Outer Banda arc of islands in south-eastern Indonesia, of which Timor and Sumba are the most important ones. Its current distribution extends from Bondowoso District in East Java eastwards to Timor, Sulawesi and the Moluccas and as far as northern Australia. The earliest records of sandalwood trade from Timor date back to the 3rd Century AD. Large stands also occur in southern India. It has been suggested that Santalum album is native to southern India, but it is more often assumed that it was introduced into India about 2000 years ago. It occurs naturalized in India from Uttar Pradesh to southern Karnataka and to south-western Andhra Pradesh. Small stands are found in Rajastan, Maharashtra and Uttar Pradesh. Santalum album was introduced into several other tropical countries e.g. the Mascarene Islands, where it also naturalized, China (Guangdong), Sri Lanka and Taiwan. In south-eastern Indonesia where it once was a common forest tree, natural stands have been decimated and exploitable natural stands now remain only in Timor. In Sumba, once called Sandalwood Island, it has become rare. Santalum album is commercially cultivated in India, Indonesia, China and Australia and it has been tried in southern Africa and several Pacific islands.

Since time immemorial Hindus, Bhudists, Chinese and Muslims have used sandalwood as incense for its sweet fragrance in their ceremonies. It has been suggested that the original source of the incense in India may have been Pterocarpus santalinus L.f. (red sandalwood), and that this was gradually replaced by Santalum album after its introduction from Indonesia. Formerly, in China the most expensive coffins were made of sandalwood, while in India it was the preferred wood for funeral pyres. Even today it is customary to add at least a single piece of sandalwood. When supplies of Santalum album became scarce, sandalwood from Australian Santalum species was used for such purposes. Larger pieces of heartwood are now mostly used for wood sculptures and carvings (e.g. statues, 'keris' handles). Incense is made from sandalwood sawdust mixed with gum arabic and saltpetre and often with other aroma materials and made into sticks (agarbattis or joss-sticks). Sawdust from sapwood and spent heartwood, remaining after distillation, are used for the same purpose. Fine sawdust is also put in sachets used to scent stored clothes. In Indonesia finely ground sandalwood mixed with water is rubbed on the body for its cooling effect.
Sandalwood oil, steam distilled from the heartwood of Santalum album, is an indispensable aroma material in perfumery, where its outstanding fixative properties and excellent tenacity, blending ability and highly attractive fragrance have made it a basic component of countless perfumes, cosmetics and toiletries. An essential oil is also obtained by acid hydrolysis of distilled sandalwood chips and sawdust. This oil differs in scent and appearance from true sandalwood oil.
Sandalwood oil has slight antiseptic properties but is now rarely used medicinally, except in aromatherapy. A few drops taken internally give relief in chronic bronchitis. Sapwood ground to a powder and mixed with water was taken in Indonesia against gonorrhoea. In China the oil is used to treat vomiting, stomach-ache and gonorrhoea. A compound extracted from the bark exhibits hormone activity in insects, disrupting their development. It also has a chemosterilant effect, but is not used commercially.
Santalum album is sometimes grown as an ornamental and as a low-branching wind-break. In India it is considered as a practical agroforestry species. The leaves provide green manure. The fruits are edible. The seeds yield a red drying oil mainly used as lamp oil, while fresh leaves yield a pale yellow wax.

No reliable production figures are available for sandalwood, as much of the trade remains unrecorded. About 30% of the world production of sandalwood and sandalwood oil is exported from the ports of Kupang and Dili in Timor, whereas most of the remainder comes from southern India. Towards the end of the 1960s about 30 t of sandalwood oil (then worth US$ 3.5 million) plus an equivalent amount of sandalwood were exported annually from Timor. Smaller amounts were sold as incense sticks, carvings and spent wood. The total value of the exports of sandalwood and its products from Indonesia was about US$ 9.5 million annually. Because of overexploitation, exports of sandalwood oil and heartwood gradually declined between 1989 and 1994 to a yearly average of 12 t oil (only 7 t in 1994) and 680 t heartwood. Domestic consumption in India exceeds its exports, but is poorly documented. In the early 1990s India exported about 48 t sandalwood oil annually. In 1986, total world production of sandalwood oil stood at about 200 t.

The heartwood of the trunk, main branches and roots of Santalum album contain an essential oil. The essential oil concentration is highest in the roots and may reach 10%, followed by the trunk (4—8%) and branches (2—4%). The volatile oil is a moderately viscous, pale yellow to yellow liquid with an extremely soft, sweet-woody and almost animal-balsamic odour. It has little or no particular top note, while its very tenacious fragrance remains constant for many days. The oil blends so well and with so many fragrance materials that it has become a common blender-fixative used in countless perfumes from oriental-floral to aldehydic and from woody-floral to fougères. The oil is also used as a base for co-distillation of the most delicate flowers, e.g. Rosa spp., Mimusops elengi L., Anthocephalus chinensis (Lamk) A. Rich. ex Walp. and Pandanus odoratissimus L.f. Sandalwood oil is commonly used in the production of attars in India. It is of little importance as a flavour material as it has a bitter, resinous taste. The oil contains 90% or more sesquiterpene alcohols, of which the characteristic components are santalols, including (Z)-'ALFA'-santalol (45—47%) and (Z)-'BETA'-santalol (20—30%). The oil also contains the related sesquiterpene alcohols epi-'BETA'-santalol, (E)-'BETA'-santalol and spiro-santalol. Other components of sandalwood include several cis-nuciferols and cis-lanceol. The foreruns of distillation contain degradation products of the sesquiterpene compounds representing the spicy, smoky by-notes of sandalwood oil, and N-furfuryl pyrrole with its powerful, peculiar odour. Sandalwood oil has been approved for food use by the Food and Drug Administration (FDA) of the United States under paragraph 172.510 and has been 'generally recognised as safe' (GRAS No 3005). The oil is registered by the Council of Europe under No 420n. A monograph on the physiological properties has been published by the Research Institute for Fragrance Materials (RIFM). The oil is used as a flavour component in major food products, including alcoholic and non-alcoholic beverages. Average maximum food levels in food products and drinks is generally below 0.001%. Sandalwood oil has a sedative effect on inhalation in mice. See also: Composition of essential-oil samples and the Table on standard physical properties.
Sapwood and heartwood of Santalum album are sharply demarcated. Sapwood is white to whitish-yellow and unscented, heartwood is scented, light yellowish-brown when freshly cut, turning dark brown on exposure and ageing to dark reddish-brown. It is dull to somewhat lustrous, with an oily feel, moderately hard to hard, heavy (specific gravity 0.90—1.14), mostly straight and extremely close grained (occasionally slightly wavy grains in radial plane) and very fine and even textured. Growth rings, when present, are clearly visible under low magnifications, delimited by a band of darker, denser fibrous tissues towards the outer margin; individual rings frequently fluctuate in diameter at different points of the circumference. Vessels are small to extremely small and rays are not visible to the naked eye. The heartwood is resistant to termites.
The seed contains up to 60% of a dark red, viscous, drying oil. The oil thickens when exposed to sunlight or when heated. The pale yellow wax obtained from the leaves has a melting point of 30°C and contains 75% unsaponifiable compounds, mainly n-octacosanol, n-tricontanol, palmitone and 10-hydroxy-palmitone. There are 5000—8000 seeds per kg.

Most Santalum species yield fragrant wood, and some species in the Pacific, e.g. Santalum yasi Seem. may match the quality of Santalum album. Sandalwood oil is sometimes adulterated e.g. with oils from Australian Santalum spp. (which lowers the laevorotation), with araucaria oil (Neocallitropsis pancheri (Carr.) Laub.), Atlas cedarwood oil (Cedrus libani A. Rich. subsp. atlantica (Endl.) Battand. & Trabut), amyris oil (Amyris balsamifera L.), Indian bastard sandal oil (Erythroxylum monogynum Roxb.), various East African wood oils (e.g. Brachylaena huillensis O. Hoffm.) and with bleached copaiba balsam (Copaifera spp.). It is also occasionally mixed with various odourless solvents such as benzyl benzoate or dialkyl phtalates.
The main components of sandalwood oil can be synthesized and several synthetic substitutes have been developed, such as sandela, santalidol and several camphanyl cyclohexanols and trimethylcyclopentenyl alkanols.

Small, evergreen, semi-parasitic, spineless, glabrous tree up to 20 m tall and with a girth of over 1.5 m, occasionally shrubby and sometimes scandent, up to 4 m tall; trunk terete, bark coarse, greyish-brown to reddish-brown; lower branches sometimes drooping; branchlets slightly angular-striate. Leaves opposite or decussate; petiole thin, 2-ribbed, 5—15 mm long, yellowish; blade ovate, lanceolate-elliptical or oblong, 2.5—8 cm x 1.5—4 cm, base obtuse, cuneate or acute, margins undulate, flat or slightly recurved, apex acuminate, slightly discolorous, pale green above, glaucous beneath, with 6—10 pairs of secondary veins, reticulate venation evident. Inflorescence a terminal or axillary panicle or raceme, 2—5 cm long; peduncle 4—20 mm long; bracts caducous; pedicel usually very short but up to 3 mm long; flowers bisexual, 4(—5)-merous, perianth-tube campanulate, about 2 mm long, 4-lobed, lobes triangular-ovate, initially yellowish, turning brownish-red, with a hair tuft behind the stamens; nectaries 4, on the middle of the perianth tube, alternating with the disk lobes; stamens 4, as long as hair tufts; disk prominently 4-lobed, lobes erect-recurved, fleshy, 1.2—1.5 mm long, alternating with stamens, orange-brown at first, turning blackish-red; pistil with superior to half-inferior ovary, short style and small, slightly 3-lobed stigma. Fruit an ellipsoidal, 1-seeded drupe, about 1 cm long, with small apical collar; exocarp blue to blackish-red, mesocarp succulent or firm, endocarp smooth. Seed without testa. Seedling with epigeal germination.

Santalum album is a semi-parasite, depending on a host for inorganic nutrients and water, but capable of photosynthesis and autotrophic for assimilates. Soon after germination, often within 3 months when nutrient reserves in the seed become depleted, the roots attach themselves to those of nearby grasses, herbs, shrubs or trees by means of haustoria. In the haustorium an intrusive wedge of cells is formed which invades the root core of the host, forming a link between the xylem systems. Over 300 species have been recorded as hosts. Suitable host plants include Casuarina equisetifolia L., Senna siamea (Lamk) Irwin & Barneby, Acacia spp., Breynia cernua (Poiret) Muell. Arg. and Imperata cylindrica (L.) Raeuschel. In Timor, Calotropis gigantea (L.) Dryander is often grown as the primary host followed by Acacia glauca (L.) Moench and Senna siamea. Although sandalwood is the main beneficiary of the link, some transfer may occur in the opposite direction. Bitter compounds found in the leaves of Strychnos nux-vomica L. have been detected in the leaves of adjacent sandalwood trees, while the characteristic taste of sandalwood has been noted in Eugenia spp. growing nearby.
Although Santalum album is evergreen, in Indonesia individual trees shed their leaves twice per year, at the beginning of the rainy season and in the dry season. Leaf-fall of individual trees is not simultaneous and bare trees and trees in full leaf can be seen next to each other.
Under normal conditions young trees grow slowly, only gradually developing a core of heartwood. In India, trees growing under favourable conditions start to make heartwood when about 10 years old and 7.5 cm in diameter, but there is considerable individual variation. Stem diameter increases at a rate of 0.5—1 cm per year, depending on the environment. Heartwood formation accelerates when trees are about 20 years old and is at its maximum in trees of 30—60 years old and 40—75 cm diameter at breast height. The proportion of heartwood to sapwood plus bark is 0.65—0.85 in healthy trees; in trees affected by spike disease it may be only 25%. The environment and resulting growth rate have a substantial effect on the quality of heartwood and oil. Trees on mountainous, rocky, and dry soils develop the hardest wood and the highest content of oil. The darker the wood, the higher is the oil content. The highly prized dark marks called birds' eyes which have a very high oil content are the result of damage to the wood.
Santalum album coppices well when young; damaged roots produce large numbers of root suckers. Flowering starts when trees are 3—4 years old, but seed is mostly collected from 20-year-old trees. In Indonesia Santalum album flowers year-round with a peak from December to January, and a peak in fruit maturation from March to June. It produces large amounts of seed and abundant numbers of seedlings are often found on disturbed locations near old trees. Seed viability is affected by maturity. Physiologically mature seed from red fruits gives about 80% germination. When fruits turn black the seed becomes dormant and germination becomes very uneven. Germination is promoted by light.

Santalum L. comprises 16—25 species occurring mainly in Australia and the Pacific. Santalum album is the only species occurring naturally in Asia, Santalum macgregorii F. Muell. and Santalum papuanum Summerh. are found in New Guinea. Santalum spicatum (R. Br.) A. DC. (synonym Eucarya spicata (R. Br.) Sprague & Summerh.), yielding West Australian sandalwood oil, occurs widely in western and southern Australia. It is the most important source of Australian sandalwood and was once extensively cut and exported, but at present only small quantities are harvested (see also the chapter on minor essential-oil plants). Small-leaved and large-leaved forms are occasionally distinguished in Santalum album, but such forms belong to the normal natural variation and do not merit formal classification.

Santalum album is found in regions with an annual rainfall of 600—2000 mm. However, 850—1350 mm is considered optimal and 2500 mm is only tolerated on freely draining locations. Its natural habitat has a pronounced dry season and a short rainy season of 2—3 months. Trees are intolerant of waterlogging, especially when young, but are less affected when mature. Permanently wet sites are unsuitable. During periods of drought, considerable amounts of water are extracted from the host plants and wilting symptoms often show first on the latter. Santalum album grows naturally up to 1500 m altitude, with the best quality heartwood being produced at 600—900 m. It requires a sunny climate and is most common in open forest and at the edges of deciduous forest. Long periods of excessive heat or intense sunshine, however, severely reduce growth and are often fatal to seedlings, whereas in older trees bark may split and form deep cracks. Under extreme conditions the bark may peel off, exposing the wood. Most soils on Timor and Sumba are heavy clays derived from marine deposits, but Santalum album now occurs mostly on shallow stony soils. The best wood is from trees growing in open forest on rather poor and stony soils. It also grows well on laterite. On fertile loamy soils growth is fast and trees become large, but the oil content of the heartwood is low and the quality of the oil poor. Santalum album is intolerant of saline and highly calcareous soils but tolerates sodic soils.

Propagation of Santalum album is usually by seed. The blue to purple-black ripe fruit is juicy and sweet and much liked by birds, which eat the outer fleshy part and drop the hard seed. In plantations, Santalum album is often sown directly in the field, because seedlings must parasitize a host when still very young. Sowing is done after the first showers of the monsoon rains. As only 30—40% of the seeds germinate in the field, about five seeds are planted per hole near a number of primary host plants. Secondary host plants should be well established on the site before sowing or planting. In the field, seed is spaced at about 2.5 m x 4 m. Shading seedlings to protect them from sun, extreme temperatures and wind is beneficial. About 50% shade is usually optimal. Growing seedlings in a nursery and transplanting is possible, provided seed is sown together with seed of a primary host plant. Seedlings are transplanted when about 1 year old and 25—30 cm tall. They may grow to 60—70 cm in the second year. Natural regeneration often occurs by root suckers, which are also suitable for vegetative propagation. Methods using hypocotyl segments, shoot and leaf cuttings have also been developed.

Young plantations of Santalum album must be protected from rodents and grazing animals. Fire is also a major cause of mortality in young plantations. Where Santalum album is a major forest tree, seedlings are ring-weeded until well established. Host plants overtopping seedlings should be pruned or lopped.

The gravest peril to Santalum album in India is spike disease, resembling witches' broom disease and caused by a mycoplasma-like organism (MLO). It is transmitted by insects. Trees of all ages are susceptible and once infected, they often succumb within 3 years. The MLO spreads through the phloem, thereby blocking the vessels. The first symptoms are shortening of internodes, severe narrowing of leaves and reduction in leaf size, leaves turning yellow and red before falling and death of root tips and haustoria. When symptoms develop, infection is usually severe and advanced, as symptoms may be masked for some time. Several insects have been implicated in the transfer of spike disease, e.g. Coelidia indica, Moonia albimaculata, Cocostirphus tuberculatus, Nezara viridula and most recently Redarator bimaculatus, but none of these reports has been confirmed. A high density of vectors is required to infect sandalwood trees. Such density is only sustained where additional suitable host plants are available. Clearing rank vegetation around sandalwood reduces the incidence of spike disease and is recommended in India. No chemical control methods giving permanent recovery have yet been developed. Injection of benomyl and tetracycline has given temporary recovery for about 6 months. When spike disease is found in a plantation it is important not only to control vectors, but also to completely uproot and remove affected trees, as several other species may develop witches' broom type symptoms when grown near diseased sandalwood trees. Lantana camara L. as host for Santalum album seems to increase the risk of contracting spike disease. Natural stands of Santalum album are generally free from fungal diseases. In Indonesia, konmeo disease, a leaf-gall disease of unknown origin, occasionally provokes some damage. In India mottled sponge rot caused by Ganoderma applanata, spongy or butt rot by Ganoderma lucidum, sooty mould by Asterina congesta and leaf curl by Santalum virus 2 result in some damage. Nursery diseases and pests include pathogenic fungi (Fusarium spp., Phytophthora spp.) and nematodes. Sandalwood trees may be damaged by insects, e.g. the red coffee borer Zeuzera coffeae, Chionapsis sp. and Valanga, but none is a widespread, serious pest.

The cutting of sandalwood is strictly regulated. In Indonesia sapwood should be at least 2.5 cm thick and heartwood should have a diameter at breast height of at least 12 cm. Trees are preferably cut during the dry season when they do not have a new flush of leaves. The number of trees cut and the weight of sandalwood delivered at the government depot have to be registered. Trees are sometimes dug out around the base and winched down, to collect as much of the roots as possible. After felling the sapwood is removed leaving a 2—3 cm thick covering of sapwood to protect the heartwood during transport and against loss of oil in storage. Before transport to the collection point the trunk and main branches are cut into billets for ease of transportation. Sawdust is carefully collected and used.

The yield of sandalwood oil varies; it is highest in the roots and lowest in chips of a mixture of heartwood and sapwood. Average yield of oil from good billets and roots is 4.5—6.25%. Trees yield 75—150 kg heartwood.

For sandalwood oil extraction, billets are first reduced to chips and then milled to powder. In small traditional stills the wood used to be soaked in water for 2 days before being water distilled for a long time. In modern steam stills, charges of 750—1000 kg are distilled for 48—72 hours at a steam pressure of 1.4—2.8 kg/cm². In general, higher pressures increase oil yield, but above 3 kg/cm² the oil may acquire an undesirable harsh note. After distillation the oil is separated from the distillation water and purified by steam distillation followed by vacuum distillation to remove all traces of water. Solvent extraction of sandalwood yields sandalwood concrete but the process is not economic.

No substantial germplasm collections of Santalum album are known of.

Little breeding work has been done, apart from the identification of superior seed trees of Santalum album in India and Indonesia.

Although synthetic substitutes are available, few even approach the fragrance and tenacity of sandalwood oil, so the demand for the natural oil for high quality perfumery is expected to remain very strong. To protect dwindling natural stands and maintain production, expansion of plantations of Santalum album must be a first priority. Botanical and agronomic research to back up its cultivation is urgently needed.

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Razali Yusuf

Yusuf, R., 1999. Santalum album L.. 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