Distribution and Morphology
The name Callitris is derived from the Greek word kallistos, and means most beautiful (). It was first named by Ventenat in 1808 (), and is a relatively small genus that belongs to the division of Gymnospermae, order Coniferales, family Cupressaceae (). Appreciable nomenclature complexities occur and therefore the reports on the number of Callitris species varies. In the Index Kewensis the names of 39 species are listed (Hooker and Jackson 1895).
Although present in North Africa with two species, Callitris quadrivalvis and Callitris articulata (), most species are found in Australia, New Caledonia, Tasmania, and New Zealand (). Callitris, vernacularly named cypress pine, is found in all states of Australia and covers approximately 4300000 ha of forest (). The most common and most important species is C. columellaris, also known as the white cypress pine (). Therefore, the greater part of the literature on Callitris deals with this species. Confusingly enough, previously used names for C. columellaris are: C. glauca, C. intratropica, C. arenosa and C. hugelii (). In addition, recently another new name, C. glaucophylla, has been introduced for this species by Thompson and Johnson (), while these authors like to consider C. intratropica, C. columellaris and the new one as three distinct species. Callitris species are very well adapted for dry, arid regions. Only one member, Callitris macleayana, is a tree which grows in the relatively humid climates of eastern Australia, often at the margin of rainforests ().
Callitris species are evergreen bushes or trees with hard bark and short erect branches which divide into branchlets at the closely pressed, sheath-like bases, while the buds are hidden by the leaves (). The leaves of juvenile plants are narrow, pointed, and in whorls of four, whereas the adult leaves are in alternating whorls of three. The morphology and development of transfusion tracheids in the leaves have been described by Gadek and Quinn (). The male and female strobili occur on the same plant. The male strobili stand solitarily or with several together at the ends of the shoots, sporophylls occur in whorls of three to four. Female strobili are solitary or in clusters composed of six to eight scales and bear numerous ovules. A detailed description of cone and ovule development has been given by Takaso and Tomlinson (). The cones are globular, ovoid, or pyramidal in form, six to eight scales are arranged in one whorl, alternate ones shorter and narrower than the others. The scales are thick, woody, and broad at the base, pointed at the apex, and not enclosed by bracts. The wood is dark or light-colored with distinct heartwood and sapwood.
The genome size has been determined to be 2n = 22 chromosomes for Callitris rhomboidea, Callitris verrucosa, and Callitris columellaris ().
Callitris species are amongst the most useful Australian conifers. The tree Callitris columellaris is recommended for afforestation in Central Australia, as it is known to withstand drought better than many other trees ().
The wood of Callitris is hard and has a strong aromatic smell due to the large amounts of phenolic compounds, which makes it an extremely durable timber, renowned for its resistance to termite attack (). The volatile components derived from the Callitris columellaris heartwood are held responsible for this feature. Even small quantities of l-citronellic acid, one of the volatile constituents, are said to be toxic and repellent, and sufficient to deter several species of termites ().
Other important properties of the timber are: small shrinkage, high density, and low corrosiveness. The timber is valuable and is being increasingly used for structural frames, flooring, furniture, turnery, and veneers ().
The idea of feeding sheep with Callitris columellaris sawdust in times of drought, as proposed by Hunter (), seems doubtful, since the presence of cytotoxic lignans and other toxic compounds cannot be excluded. Many sesquiterpene lactones have cytotoxic properties (). In addition, grazing plants containing sesquiterpene lactones with an α-methylene-γ-lactone, such as columellarin (), can disrupt a variety of metabolic pathways and may result in the degranulation of tissue mast cells, with the liberation of histamine and other physiologically active compounds (). These effects may have a significant role in the toxicity of poisonous plants containing sesquiterpene lactones, when ingested by livestock ().
The trunk of several Callitris species yields a yellow resin or gum, which is marketed as sandarac resin or gum () and is used as a pharmaceutic aid in ointments and plasters as well as a tablet coating that dissolves in the intestine (). Fitzgerald et al. () searched for anti-tumor compounds in the leaves of a series of conifer genera. Libocedrus, Chamaecyparis, Juniperus and some Callitris extracts exhibited tumor-necrotizing activities. The active compounds were shown to be either desoxypodophyllotoxin or podophyllotoxin (). Especially leaf extracts from Callitris drummondii () showed a significant cytostatic effect in sarcoma 37 tumor-bearing mice in 80-100% of the mice after a single subcutaneous dose of 1 mg/g. Also ethanolic extracts of stems and leaves of Callitris columellaris have been shown to possess anti-tumor capacities (Aynehchi 1971).
Extracts of Callitris rhomboidea have been prepared to perform an anti-viral screening, but no anti-viral activity could be found (). C. glaucophylla derived materials are applied as traditional bush medicines by the aboriginal communities of the Northern Territory of Australia. Aboriginal knowledge of phytotherapy has evolved over more than 40000 years and has been studied and scientifically presented by Barr et al. (). Aromatic infusions of leaves and twigs or profusely smoking leaves are applied as a therapeutic aid. The main therapeutic activities are: antiseptic and decongestant. Preparations are used to cure: gastrointestinal disorders, symptoms of colds and flu, sore throat, cough, sores, boils, cuts, allergy rash, and itchy skin disorders, as well as general malaise.
Callitris drummondii needles are appropriate material from which to establish tissue culture. The cell suspension predominantly produces podophyllotoxin-β-D-glucoside as well as several other lignans in minor quantities. As the growth of the cultures is slow, the podophyllotoxin yields after one growth cycle are relatively low, namely 6-7 mg/1.
With respect to podophyllotoxin production using tissue culture material, a series of studies was performed in the last decade (). Despite the podophyllotoxin production in all Podophyllum cell cultures, no economically attractive yields could be obtained so far. The search for new cytostatics is still in progress because of the severe side effects of the generally used anti-tumor agents and because of the insensitivity developed by many neoplastic malignancies for the applied therapeutics.
Since Callitris drummondii is the first species that has been used for the initiation of tissue culture, a systematic investigation into the lignan content of in vivo and in vitro material of other Callitris species may yield additional interesting cytotoxic lignans.
Selections from the book: “Medicinal and Aromatic Plants V”, 1993.