Bowiea volubilis Harv. ex Hook.f. (Sea Onion)

2015

When Bowiea volubilis Harv. ex Hook.f., a member of the family Hyacinthaceae, (also known as the sea/climbing onion) was first described, it was stated in Curtis’s Botanical Magazine that “though possessing little beauty, this is certainly one of the most curious plants ever introduced into Europe” (Dyer 1941). More recently, B. volubilis has attracted attention as a source of cardiac glycosides. The plant has long been known among tribes of southern Africa as a strong medicinal herb. The strong toxic properties of this plant have been the cause of many deaths due to overdoses administered by herbalists. Jaretsky suggested that the toxic properties of the bulb were due to cardiac glycosides similar to digoxin and digitoxin. Katz identified the cardiac glycosides and named them bovoside A, B, C, D, and E.

Morphology and Distribution

There is uncertainty as to the actual number (one to three) of Bowiea species. The three described species of Bowiea include the common form B. volubilis Harv. ex Hook.f., B. kilimandscharica Mildbraed, and B. gariepensis v Jaarsveld. There is debate as to whether the latter two are synonym and sub-species respectively. Dahlgren et al. indicated that Bowiea is pronouncedly peripheral in the Hyacinthaceae and may merit the rank of a separate family.

Bowiea is a geophyte with a depressed, globose bulb up to 15 cm in diameter, which consists of thickened fleshy scales, the outer layers becoming green when exposed to sunlight. There are one to a few leaves which are rudimentary, approximately 2-4 cm long, filiform, and which wither and fall early during growth. The “stem” (inflorescence) is annual in nature and leafless, scrambling, and copiously and intricately branched. The branches are reflexed and spreading, succulent, green in color, and function as the assimilatory organ. The pedicels are curved, with those towards the base being mostly sterile. There are numerous green, diurnal flowers with six spreading segments that are fused at the base; these segments are persistent and are reflexed after fruit set. There are six stamens and the anthers are introrse. The ovary is conical and trigonous with numerous axile ovules; the style is terete with a clavate three-lobed stigma. Seeds are few, oblong, and flattened with shiny black testas, contained within a loculicidal ovoid capsule.

Bowiea’s distribution is fairly wide in southern Africa, with localities from Namibia through the Cape Province, Natal Transvaal and north into Zimbabwe, Zambia, Tanzania, and central Africa.

Medicinal Importance

The bulbs of Bomea are highly sought after by the indigenous populations for use as a poison and a medicine, and for their magical properties. Bomea volubilis has long been used for treating a variety of ailments, and African herbalists have probably used it for centuries. Unfortunately, accidental (and intended) overdoses, and perhaps local differences in plant toxicity, have resulted in fatalities. The symptoms of overdose are vomiting, salivation, palpitations, and cramps followed by death within minutes or hours, depending on the amount ingested. Walker (1914) recorded cases of Bomea poisoning with the postmortem results showing congestion and signs of irritation of the gastrointestinal tracts, stoppage of the heart in diastole, and congestion of the liver and other organs. Van der Riet was supposedly the first to extract a poisonous alkaloid from the bulb, and reported that it gave (with hydrochloric acid) a color reaction characteristic of veratrine. Walker (1914) isolated resinous bodies from the bulb. Lewis (Meltzer 1928) extracted a highly toxic impure alkaloid from the bulb. De Almeida (1930), after extracting and subjecting the extract to pharmacological experiments, suggested that the active principle was colchicine. A great deal of work has been conducted on the bulb, its extracts, and its effects on a number of laboratory animals to determine the minimum lethal dose (Meltzer 1928).

Jaretsky first suggested that cardiac glycosides were probably responsible for the action of the bulb, with the two glycosides being probably of the scillaren type. Research workers have confirmed the digitalis action of the Bowiea extract and reported that the glycoside of Bowiea is slightly more cumulative than digoxin, but not nearly as much as digitoxin. Katz did extensive work on the chemistry of Bowiea (), from which he isolated a number of highly active cardiac glycosides of the scillaren type. A number of researchers subsequently studied the constituents of Bowiea (). Jaretsky and Scheermesser (1938) registered a German patent for a glycoside preparation for human cardiac use.

It has been reported that the cardiac glycoside action of the bulb is estimated to be 30 times more intense than that of Digitalis, and that of the flower, 60 times. It is evident that the range and toxicity of substances in Bowiea make it a valuable source of cardiac glycosides and a fascinating experimental plant.

Collection and Conventional Propagation of Bowiea

Traditional means of Bowiea propagation include seed production and germination (with associated variability), the production and separating of offsets, and scaling (a slow propagation alternative).

Bowiea bulbs are collected from the Natal/KwaZulu area in South Africa and are sold in urban herbal medicine shops for high prices (approximately ten times more than most medicinal plants; Cunningham 1986). This is coupled with their scarcity in the Natal/KwaZulu area. The popularity of medicinal plants amongst rural and urban Zulu populations ensures that exploitation and future conservation concerns are likely to increase in direct proportion to population growth in these areas.. Law enforcement as the only management strategy has failed to control overutilization of indigenous medicinal plants, and illegal gathering takes place in state forests and conservation areas. An alternative management strategy is to provide a legal source of supply through cultivation to satisfy the demand of both the urban herb trader and the rural herbalist. In vitro culture provides an alternative means for mass production of these endangered plants.

In vitro culture of Bowiea volubilis can play an important role in the boosting of plant stocks from which larger numbers of bulbs could be produced using traditional propagation techniques. However, more importantly, these data indicate the potential for biotechnological manipulation of this plantlet not only from a conservation outlook, but also as a source for new and more effective compounds in the battle against heart disease.

Selections from the book: Medicinal and Aromatic Plants VII (1994).