“Ancient botanical treatises and pharmacopoeias attribute various properties to the sundew, or Drosera, whose red droplets of mucilage do not dry out in the sun. Certain extracts of these plants serve as treatment for corns, verrucas, and burns. Infusions and other extracts are used against coughs, respiratory disorders, tuberculosis, arteriosclerosis, inflammations, intestinal illnesses, and syphilis. These preparations are diuretic, soothing and even aphrodisiac”..
Drosera extracts are still being used against infections and ailments of the respiratory tract. Plumbagin and related compounds occur in the Droseraceae and are thought to be responsible for its therapeutic properties. Although plumbagin occurs in many species of Drosera the compound is also extracted from species of Plumbago (). Frequent harvesting of natural populations of Drosera in Europe have resulted in the plants becoming increasingly scarce and alternate sources of plants are therefore being sought.
Vegetative propagation of Drosera and the production of plumbagin in vitro may serve as an alternative to the utilization of natural populations.
Distribution and General Morphology of Drosera
The genus Drosera was the first of the carnivorous plants to be described and also the first plant with a carnivorous habit to be understood. In the first edition of Dodoens’ Cruydeboeck in 1554, Drosera intermedia or Drosera anglica was illustrated under the section for mosses and named Rosa solis.
The Droseraceae (from the Greek word drosos – dew) are present throughout the world except in the arctic regions. The distribution of Drosera is ubiquitous, but the genus is best represented in Australia with about 54 species. In addition to a worldwide distribution, there is a large diversity in the morphology of the species. Sizes range from the small pygmy variety (1 cm in diameter) to very large forms (up to 1 m in length). The growing conditions (and cultural requirements) vary according to the natural habitat of the species. A number of species are from temperate and tropical climates, while others have developed tuberous-like growth habits to endure drought periods.
The leaves of the Droseraceae are alternate, rarely in whorls, often in basal rosettes, and may or may not have stipules. On the upper surface they are covered with sessile or stalked glands. The “trap”, which may be either thread-like or disk-shaped, can move, and the glandular hairs have at their extremities a drop of mucilage which shines in the sun (the dew of the Sonne or Sonnedewe – sundew). The flowers are regular, bisexual, and are often in coil-like racemes, but occasionally are solitary. They have five sepals more or less connate at the base, five petals (which may be almost any shade from white, yellow through pink, purple red to orange, depending on the species), up to 20 free stamens. The ovary is superior with two to five fused carpels with one locule, with three to numerous ovules on a basal placenta. The fruit is capsule-shaped and dehisces locucidally into two to five valves. The seeds are usually black and numerous, containing endosperm and small basal embryos.
The trapping action of the Drosera is an active mechanism; the leaves and tentacles bend inward at the point where the prey is situated, bringing the maximum possible number of tentacles into contact with it. The tentacles curve around the insect and direct it towards the digestive glands situated in the center of the leaf. The whole process can last from a few minutes to a few hours. One to two weeks are required for the tentacles to regain their initial shape. The insect is digested by means of proteolytic enzymes and ribonucleases which the plant secretes. Often bacterial intervention occurs.
One of the earlier reports of the medicinal usage of Drosera plants appears in Gerard’s New Herbal in 1633, which although recommended for use against “consumption of the lungs” also included a warning “that they have sooner perished that used the distilled water hereof, than those that abstained from it”. The aphrodisiacal qualities of Drosera were also recorded with respect to the action of female cattle and sheep after “only taste of it”. Turner (1568) and Culpepper (1813) also reported medicinal properties related to Drosera. Plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone) and related compounds occur in the Droseraceae, and are thought to be responsible for its therapeutic properties.
Extracts of Drosera were applied to remove corns and warts and as a cosmetic for the removal of freckles and sunburn, however, whether these cures were a result of the naphthoquinones was questioned. Drosera extracts and in particular those from D. rotundifolia are widely used in phytotherapy, where plumbagin has been used in bronchial treatments, in particular whooping cough. Grieve (1959) also lists it as an agent against phthisis, chronic bronchitis, and asthma.
A number of researchers have reported the use of Drosera extracts as antispasmodic agents. Denoel (1949) advocated the therapeutic application of plumbagin against tuberculosis because of its liposoluble properties. Heise and Steenken (1941) and Lloyd and Middle-brook (1944) showed that plumbagin had an inhibitory action on the growth of the tuberculosis bacterium. Didry et al. (1986) proved that pure plumbagin could inhibit both Gram-positive and Gram-negative cocci and bacilli; however, when plumbagin was supposedly absent from Drosera extracts, very little antibiotic activity was observed. It was shown that in extracts of Drosera peltata antimicrobial activity was related to the plumbagin content.
Kubo et al. (1980) reported that plumbagin exhibited relatively specific antimicrobial activity against yeasts and is a potent insect antifeedant against the larvae of African army worms. Kubo et al. (1983) also showed that plumbagin was an insect ecdysis inhibitor and a naturally occurring chitin synthetase inhibitor.
The growing importance of Drosera extracts in pharmaceutical preparations has led to the recent description of suitable analytical procedures for standardizing their naphthoquinone content.
There are several naphthoquinones which are known only from Drosera. This is also true for a chlorinated naphthoquinone isolated from D. anglica and D. intermedia which is apparently unique to higher plants. Droserone was isolated by Rennie (1887) from D. whittakeri and characterized by Asano and Hase (1943), while Moussli (1930) described a product called “droseroside”, which gave a red reducing sugar after hydrolysis.
The major naphthoquinone of the Droseraceae is synthesized by the novel homogentisate ring-cleavage pathway. Using a number of 14C precursors, it was shown that only tyrosine is incorporated in significant amounts when fed to plants. This work was confirmed using sterile-grown plants and cell suspensions so as to rule out the possibility of fungal contamination (the homogentisate ring-cleavage pathway does occur in the fungi). In Plumbago plumbagin is not produced from tyrosine. Juniper et al. (1989) speculated that the homogentisate ring-cleavage pathway is a modification and occurs in the Droseraceae as a result of low nitrogen availability in the environment. Ramentaceone, which is found in the Droseraceae, is also produced by the homogentisate ring-cleavage pathway.
Conventional Propagation of Drosera
Propagation is usually best achieved by seed or leaf cuttings. Root cuttings have been used in a few cases. Germination of seeds is variable depending on the species. The genus contains members which are capable of self-fertilization. The seeds are generally collected and stored at low temperatures (2-7 °C) and sown the following spring. The seed trays are watered with a fungicide and kept moist. The resultant plantlets are potted out. Plantlet regeneration from leaves is widely practised. Leaf sections only or sections including the petiole are used: “so frequent and vigourous is the last method used (budding from leaves) that it would seem to rival that by seed”. A treatment of the leaf sections with fungicide and hormones is often recommended. Asexual reproduction is also possible via root cuttings and secondary bulbs (tubers). The most successful means of reproduction of the pygmy varieties is by using “gemmaea”, small spherical/flattish structures at the base of the leaves. These structures form in response to reduction in photoperiod and temperature. These structures are removed from the plant and propagated as if seedlings.
Summary and Conclusions
Extracts of Drosera are still utilized against a number of infections, with plumbagin known to occur in the Droseraceae, being thought to be responsible for its therapeutic effects. Due to frequent harvesting of natural populations of Drosera in Europe, the plants are becoming increasingly scarce, and alternative sources of plants are being sought.
In vitro culture of Drosera plants has been attained relatively frequently using sterilized seeds as initial explants with subsequent manipulation for multiplication purposes. All plant portions can be utilized as explants, but sterilization of the vegetative explants poses a problem. Using a three-stage culture regime, Crouch and van Staden (1988) have reported that 1500 plantlets can be formed from a single mature leaf explant within a period of 8 months.
Although it was found that in vivo-grown Drosera plants had a higher plumbagin content than in vitro-grown plants, Plumbago had a greater plumbagin content than either in vitro- or in vivo-grown Drosera plants investigated.
The described techniques have been found effective and easy for the production of Drosera plants either for conservation or horticultural reasons. At present, the low concentration of plumbagin found in Drosera would appear to preclude the culture of the genus as a commercial source of naphthoquinone.