Amorphophallus konjac (PROSEA)
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Introduction |
- Protologue: Wochenschr. Gärtn. Pflanzenk. 1: 262 (1858).
- Family: Araceae
- Chromosome number: 2n= 26 (24, 36)
Synonyms
Amorphophallus rivieri Durieu ex Carrière (1870), Hydrosme rivieri (Durieu ex Carrière) Engl. var. konjac (Koch) Engl. (1879), Amorphophallus mairei H. Lév. (1915).
Vernacular names
- Devil’s tongue, konjac (En)
- Philippines: pungapung (Tagalog), bulangan (Mangyan)
- Vietnam: củ nưa, khoai nưa.
Origin and geographic distribution
A. konjac originates from southern and south-eastern China, Vietnam and possibly Laos. It occurs wild and cultivated and easily escapes from cultivation. Its cultivation is most important in China and Japan (“konnyaku”), but it is also known in Indo-China and the Philippines, and occasionally elsewhere (e.g. Hawaii).
Uses
The flour resulting from milling dried A. konjac tubers is recommended as an adjunct in low-calorie diets and to control the blood cholesterol level. The tuber is used in traditional medicine in Vietnam as a poultice against furuncles.
The tubers of A. konjac can be made edible in times of food scarcity, usually after peeling, slicing and repeated washing and boiling in water to remove toxic and irritating substances. In Japan, they are used to prepare a traditional dish (“ito konnyaku”) which is gel-like in appearance and texture and is made by adding slaked lime to a colloidal solution of the flour in water, and then heating this. The glucomannan in the tubers has film-forming characteristics useful in preparing stabilizers and emulsifiers for food, drinks, cosmetics and in drilling fluids. Glucomannan is also a good sieving additive for capillary electrophoresis. The tubers can also be used to prepare acid and alcohol.
The tubers of A. paeoniifolius (Dennst.) Nicolson (synonym: A. campanulatus Decne.) are used in traditional medicine in India. They are, however, far more important as a tuber crop. They are considered aperient, carminative and expectorant, and are also applied externally as an irritant to treat acute rheumatism; the seeds are used for the same purpose. The tubers are administered internally to treat dysentery and haemorrhoids. In the Philippines, the tubers are considered caustic, and are also used as antirheumatic poultice. In India, the roots are used against ophthalmia, and applied to boils and as an emmenagogue. The fermented juice of the petioles is used to cure diarrhoea in India and Papua New Guinea. The tuber is given in Thailand as a food supplement for diabetic patients.
Some Amorphophallus species are used as poisonous plants. In Peninsular Malaysia, the juice of A. paeoniifolius and A. prainii Hook.f. mixed with latex from Antiaris toxicaria Lesch., is sometimes used as dart poison.
Properties
In tests with mice A. konjac powder exerted a positive effect on the rate of induced lung cancer, and it prolonged the survival time of the mice. There were no noticeable adverse reactions of the powder. The powder markedly lowered the cholesterol levels in the serum and liver of rats eating hypercholesterolemic diets. Some aromatic compounds (e.g. serotonin) with peroxynitrite scavenging activity have been isolated from the powder. The tubers of A. paeoniifolius exhibit antiprotease activity.
The carbohydrates of the tubers consist of starch, but mainly (more than 50%) of a glucomannan, a polysaccharide of mannose and glucose, which, in combination with water becomes very viscous and is therefore attractive for many industrial processes. Fresh Amorphophallus tubers are irritant due to the presence of calcium oxalate.
Botany
A perennial herb with subterranean tuber up to 30 cm in diameter and 20 cm long, weighing up to 10(-13) kg. Leaves usually solitary, tripartite, up to 2 m in diameter, each of the 3 segments highly dissected; leaflets elliptical, 3-10 cm × 2-6 cm; petiole up to 100 cm × 8 cm, smooth or with scattered punctiform warts at the base, dirty whitish-pinkish with large green spots and smaller white dots. Inflorescence a spadix 15–110 cm long, female in lower part, becoming male higher up via a transitional zone, with a well-developed asexual part at the top, spadix partly enveloped by the spathe 10-60 cm × 10-55 cm, spathe limb erect, undulate or folded longitudinally with spreading margins, outside dark purplish-brown with scattered blackish-green spots, inside dark brown, glossy. Fruit unknown.
The plants normally start flowering when 4 years old. Seed does not develop.
Because A. konjac has been cultivated in China for about 2000 years and also naturalizes easily after escaping from cultivation, it is not clear which characters belong to the true wild plant. It can best be subclassified into cultivar groups and cultivars.
Amorphophallus originates from and is mainly distributed in the Old World, especially in the tropics from Africa to the Pacific Islands, but also extends to temperate areas in China and Japan. The total number of species is approximately 170.
Ecology
A. konjac usually grows in secondary vegetation, in forest margins and thickets, village groves, usually under some shade, up to 2500 m altitude. The optimum average temperature ranges from 20-25°C. In China, it develops best when soil moisture is at 75% of the field capacity, preferably dropping to 60% when the tuber is maturing.
Management A. konjac can be propagated from tubers or tuber parts and by tissue culture. Planting holes of 60 cm × 60 cm × 45 cm are recommended, the bottom filled with a mixture of soil, manure and fertilizer. Planting is done at the beginning of the rainy season. In estate farming, A. konjac is interplanted with cereals such as maize and sorghum, or grown under shade trees. Mulching is recommended. In Japan, a permanent cropping system exists, in which young and old plants are grown mixed together like a semi-natural vegetation, in which only older tubers are harvested at the end of the growing season. In this system, a minimum input of chemical fertilizers, herbicides and pesticides is combined with large amounts of mulch, and remarkably few diseases and pests occur. In China and Japan, the tubers cultivated for food are harvested one year after planting, when they are small but sweet and juicy. For industrial purposes, they are harvested after 3 years.
Genetic resources
Germplasm collections of A. konjac are available in the Institute of Botany (Kunming, China) and the Gunma Agricultural Experiment Station (Konnyaku Branch, Gunma-ken, Japan).
Prospects
Studies in animals and humans indicate a potential use for the glucomannans in the tubers, particularly as anti-atherosclerosis agents. Complementary studies are needed, also to define quality criteria for the drug. It is unlikely that A. konjac will gain more prominence in agriculture as a food crop. However, in Thailand the market is expanding due to low-calorie value of the tubers and because the product decreases fat and cholesterol absorption.
Literature
118, 245, 369, 580, 683.
Other selected sources
121, 671.
Main genus page
Authors
R.H.M.J. Lemmens
