Amaranthus (PROSEA Cereals)

Plant Resources of South-East Asia
Introduction

Amaranthus L. (grain amaranth)

Protologue: Sp. pl.: 989 (1753); Gen. pl. ed. 5: 427 (1754).

Family: Amaranthaceae

Chromosome number: 2n= 32 (A. caudatus, A. hypochondriacus); 2n= 34 (A. cruentus); some confusion in species identities and chromosome counts warrants further studies

Major species and synonyms

Amaranthus caudatus L., Sp. pl.: 990 (1753), synonyms: A. mantegazzianus Passerini (1865), A. edulis Spegazzini (1917).

Amaranthus cruentus L., Syst. nat. ed. 10, 2: 1269 (1759), synonyms: A. paniculatus L. (1763), A. sanguineus L. (1763, pro parte).

Amaranthus hypochondriacus L., Sp. pl.: 991 (1753), synonyms: A. flavus L. (1759), A. frumentaceus Buch.-Hamilt. ex Roxb. (1832), A. leucocarpus S. Watson (1875).

Vernacular names

General:

Grain amaranth, amaranth (En)
Amarante (Fr)
Amaranto (Sp)
Indonesia: bayam (general), bayem (Javanese, Sundanese, Balinese)
Malaysia: bayam
Papua New Guinea: aupa
Philippines: halom, kulitis
Cambodia: phti:
Laos: hôm
Thailand: phakkhom-suan
Vietnam: rau dền.

A. caudatus :

Love-lies-bleeding, red-hot-cattail, foxtail (En, ornamental form)
Queue de renard (Fr)
Indonesia: bayam ekor kucing
Malaysia: bayam selaseh
Thailand: phakkhom-baidaeng (Bangkok)
Vietnam: rau dền duôi.

A. cruentus :

Cock's comb (En, ornamental form)
Brède (Fr)
Laos: hôm dè:ng.

A. hypochondriacus :

Prince of Wales' feather, prince's feather (En, ornamental form).

Origin and geographic distribution

Grain amaranths are crop species of New World origin (A. caudatus from Andean Peru and Ecuador, A. cruentus and A. hypochondriacus from Mexico and Central America). Paleobotany suggests some domesticates appeared as early as 4000 BC. The closest relatives are generally weeds of river banks, ditches and disturbed mesic habitats, distributed over large tropical and subtropical regions. Little is known about the ecogeography and evolutionary dynamics of these putative ancestors. There are two hypotheses on the origin of grain amaranth:

the 3 cultivated species were independently domesticated from weedy species in 3 regions: A. caudatus from A. quitensis Kunth in South America, A. cruentus from A. hybridus L. in Central America, and A. hypochondriacus from A. powellii S. Watson in Mexico;

the 3 cultivated species had a single origin: first A. cruentus arose from A. hybridus in Central America; subsequently, hybridization with local weed populations led to the selection of A. caudatus and A. hypochondriacus.

None of the hypotheses is conclusively supported by hybridization studies.

Grain amaranth cultivation was recorded in Incan and Aztec civilizations. It is now much reduced in importance, although recently it has been rediscovered as a crop. Three very significant dispersal events were:

post-Columbian migration of ornamental types to Europe;

18th Century introductions of leafy (vegetable) types (mainly A. cruentus) to Africa;

colonial transfers of grain types to India, Nepal and Sri Lanka and then recent dispersal to many South-East Asian and African countries, by migrants from the Indian subcontinent.

Thus, on a small scale these species are now cultivated in all continents and over a wide range of climatic conditions.

Uses

The primary use of grain amaranth in human food includes many forms of milled and popped/puffed seed products. Sweet snacks (alegria, laddoo) of popped amaranth and various flour mixes with other cereals are traditional and common in Mexico, Guatemala, India and Nepal. Useful formulations have been developed for feeding infants, and for many potential baking and breakfast cereal products. Amaranth seed can be milled to whole-grain flour for use in baking. Bread made from amaranth flour is usually described as nutty and pleasant tasting. Amaranth meal or flour is especially suitable as the sole or predominant cereal ingredient for unleavened (flat) bread. For making yeast-raised bread or other leavened foods, it must be blended with wheat meal or wheat flour because it lacks functional gluten. The whole grain can be sprouted for use as a nutritious vegetable.

In multiple-use cropping, seedlings are harvested once or several times as a vegetable, and the grain or total biomass is harvested at maturity for food or forage. Large-scale forage production has been reported, particularly in China and Argentina. In addition to their excellent nutritional qualities, amaranths are traditionally valued for their use in medicine, folk festivals, and as dye sources.

Production and international trade

No data are available on grain amaranth production and trade. Ethnobotanists and germplasm collectors have nonetheless found numerous farmers in countries such as India and Nepal growing grain amaranth very profitably. Some reports from the 1990s mention several thousands of ha under the crop in China, similar large production areas in the Argentine pampas, and about 3000 ha in the United States. Conservatively, acreage estimates for India and Nepal range up to 4000 ha. In Peru, there are over 1000 ha under grain amaranth in the high Andean region alone. The United States imports a large quantity of seed from Mexico to satisfy its growing speciality market. The many international meetings and general surveys suggest that production and trade have certainly increased since 1980.

Properties

Amaranth grain is renowned for the excellent quality of its protein because of the high lysine content (3.2-6.4%). The overall high protein score (13-18%) and 7-8% fat (oil) with potential antioxidant properties, make grain amaranth interesting for human nutrition. The starch mainly consists of amylopectin, with only 5-7% amylose. The rather small starch granules (1-3μm in diameter) have drawn wide attention for industrial uses. Similarly, significant amounts of squalene (4-11% of the total oil fraction) may find an important world market niche in products such as lubricants in computer industry and in cosmetics. 1000-seed weight is in the range of 0.4-1.1 g.

Description

Erect, monoecious annuals, much branched, up to 2.5 m tall, with extensively branched taproot.
Leaves alternate, with long petioles, simple and entire.
Flowers in axillary clusters (compound dichasia, also called glomerules), upper clusters often leafless and arranged like terminal, panicled spikes, unisexual; each flower solitary in the axil of a bract, with 2 bracteoles and (3-)5 tepals; male flower with as many free stamens as tepals; female flower with ovoid or oblongoid ovary, style branched into (2-)3(-4) stigmas; ratio of male to female flowers within glomerules varies with species and cultivar.
Fruit a laterally compressed utricle, usually with circumscissile dehiscence.
Seed lenticular, pale or ivory in grain cultivars to shiny black or brown in weedy and vegetable forms.

A. caudatus.

Inflorescence usually lax, long, thick and pendulous, almost unbranched in ornamental forms, usually branched in grain forms; individual glomerules quite large and relatively far apart, giving the spikes a peculiar knobby appearance; bracts not protruding beyond the utricle, with slender midrib.
Tepals strongly recurved, very broad towards the tip, the inner ones spatulate, the outer ones more obovate; style branches (stigmas) spreading.
Seed predominantly pale (ivory), but some plants with reddish or dark seed occur; outside South America reddish-seeded forms are most common.

A. cruentus.

Inflorescence lax, relatively small, soft; bracts very small, not protruding beyond the utricle, with slender midrib.
Tepals 5, straight, the inner ones oblong and pointed; style branches (stigmas) erect.
Utricle cap constricted into a narrow column below the base of the style branches.
Seed usually pale yellowish, but dark seed also occurs.

A. hypochondriacus.

Inflorescence large, thick and erect, with prickly appearance because of the large (as long as the style branches) and long-pointed bracts.
Tepals long, slightly recurved, acute; style branches (stigmas) thickened at base.
Utricle cap large, gradually sloping to the base of the style branches.
Seed predominantly pale, but mixed with varying amounts of dark forms. Ornamentals always with dark seed.

Growth and development

Seedlings emerge 3-5 days after sowing, and vegetative development is rapid. Landraces vary in the mode of transition from the vegetative to the reproductive stage, depending mainly on photoperiod and temperature. Flowering begins 60-110 days after seedling emergence. Monoecy, protogyny and variable male/female flower ratios may frequently produce high rates (30-35%) of cross pollination but the crop is usually handled as a self-pollinated crop. Most types of grain amaranth mature in 5-6 months. However, in some highland areas they may take up to 10 months to mature. A single plant may yield as many as 50 000 or more seeds. Most cultivars show large inter-plant variation in growth and yield characteristics.

Other botanical information

In addition to the difficulties of identifying Amaranthus grain species because of the highly variable expression of floral characters used in keys, numerous crop-weed hybrid populations exist, which are grouped as A. hybridus L., A. quitensis Kunth or as feral races of A. hypochondriacus. Even dark versus light seed colours appear polymorphic in all of these taxa. Morphological descriptors including taxonomic keys and various genetic markers have shown grain amaranth collections to be highly diverse. Several races or morphological groups have been successfully described on the basis of plant form, local adaptation and origin. However, this is likely to change when more information from genetically designed studies becomes available. Allozyme variation studies show a remarkable contrast with the morphological observations: crop and weedy species can be broadly identified as two, partially overlapping, taxonomic groups, but the three species are not distinct from each other. It has not been adequately recognized that hybridization occurs naturally in the weedy or cultivated populations. Genetic studies of certain key ecophysiological characters such as photoperiod response and sex ratio may clarify the relations between the different groups. Two important aspects are noteworthy:

domestication involved an increase in grain yield, a great diversity of inflorescence forms and colours, and a selection for pale seed; early flowering (without a short-day requirement) also evolved in many non-tropical areas; weedy relatives are generally more branched, their seeds are dark and shatter readily;

Mexico, Guatemala and the Andean region require extensive studies as primary centres of diversity, whereas India and Nepal too have developed rather rapidly as a secondary centre.

Typical vegetable amaranths (A. blitum L., A. dubius C. Martius ex Thell., A. tricolor L.) originated from South-East Asia but emigrants have taken them to other regions as well. Local uses of weedy A. hybridus L., A. spinosus L., A. retroflexus L., A. viridis L. and others as a forage or a vegetable have also produced wide uncontrolled dispersal.

Ecology

Information on the wide adaptability of grain amaranth is abundantly available in the literature but often based on pooling of both wild and cultivated species. Grain amaranths are C₄-cycle plants, giving higher yields at higher light intensities and temperatures, and being efficient in water use. In the weedy species and in A. caudatus the photoperiodic response is marked, but in populations of A. cruentus and A. hypochondriacus sampled from a wide range of latitudes and elevations it is either highly variable or rather weak. In Thailand, several A. caudatus and A. hypochondriacus accessions performed well under cooler, drier highland conditions whereas the Mexican A. cruentus accessions did better in a warmer and more humid environment.

Grain amaranth prefers well-drained neutral or basic soils (pH > 6), but some landraces are remarkably well adapted to acid and saline soils.

Propagation and planting

Grain amaranth is traditionally grown in small plots by direct sowing or transplanting, and in sole cropping as well as intercropping systems. In the Himalayan region, for example, black gram, soya bean, maize and millet fields are bordered with amaranth. Larger plantings and current experimental trials emphasize optimal plant density for achieving high yields, e.g. rows 50 cm apart and plants within rows thinned to 20 cm distance to give nearly 100 000 plants per hectare. Branching habit and flowering and fruiting pattern can be influenced by choosing planting time, plant density and cultivar. Although dense plantings (> 200 000 plants per ha) show self-thinning, a yield-depressing effect may occur. The seed-bed must be well prepared, and the seed placed 1-2 cm deep in firm moist soil. With mechanical row drilling, only 1.5-3.0 kg seed/ha is used. In small-scale cultivation the seed is broadcast and then raked in, or sown in pockets, and the young plants are thinned out. In that case the sowing rate must be higher.

Husbandry

Crop establishment is fast and grain amaranth competes well with weeds. The stand must be weeded manually or mechanically at least once during the first month. The weedy amaranths must be removed because their black seeds are impurities in the pale cereal seed. In semi-arid areas with rainfed cultivation, acceptable yields are obtained with only 200 mm rain during the growing season, provided initial soil moisture is high enough.

The mineral uptake is rather high. On poor soils, fertilizer applications of 100 kg N, 100 kg P and 200 kg K are needed for high yields. Too much nitrogen makes the crop susceptible to lodging and to fungal diseases and pests.

Besides research for large-scale production, attention for the traditional cropping practices of grain amaranth must not be neglected, since water and fertilizer inputs are usually minimal in these indigenous systems.

Diseases and pests

Damping-off caused by Pythium may occur when the seed-bed is too wet. Wet rot caused by Choanephora cucurbitarum and some other fungal diseases (Albugo, Alternaria, Cercospora, Phoma, Rhizoctonia) may cause problems. The crop is more susceptible to these diseases under humid conditions, high plant density and high doses of nitrogen. Plant parasitic nematodes are reported to occur but do not seem a serious problem.

The pests reported to cause economic damage sometimes are mainly leaf-eating caterpillars of the genera Heliothis, Hymenia and Spodoptera, stinkbugs (e.g. Lygus on the inflorescence), stem-boring larvae of weevils, grasshoppers, and aphids. The trend to large-scale commercial production will most probably result in numerous diseases and pests, requiring screening for resistance and breeding.

Harvesting

Grain amaranth is harvested once-over by cutting the inflorescences at full maturity before the seed shatters. The period from flowering to harvest varies from (3-)4-5(-6) months, depending on cultivar, ecological conditions and cultural practices. A few United States cultivars are now sufficiently uniform to be machine-harvested. The main difficulty in mechanical harvesting is that the central inflorescence matures and dries out while the numerous inflorescences on lower side-branches are still developing. High-density planting modifies plant structure to a point where a single inflorescence is formed, making mechanical harvest more efficient.

Yield

Seed yields vary widely from as low as 500-800 kg/ha to as high as 2500-4000 kg/ha. Most of these data are estimates based on small-plot harvests. Overall, the common impression of low yields and poor marketability is quite erroneous, as has been demonstrated in Peru, Mexico, Nepal, India, Kenya and other countries.

Handling after harvest

Plants are dried on the ground. Sometimes they are covered with cloth sheets. Careful and rapid handling assures clean seed. After threshing, the grain should be dried to 12-13% moisture content for good storage.

Genetic resources

Several genebanks (e.g. Rodale Research Centre and the USDA Station at Ames in the United States, the National Board for Plant Genetic Resources (NBPGR) in India, and institutes in Bolivia, Guatemala, and Mexico) have thousands of accessions, albeit with extensive duplication and inadequate records.

Breeding

Grain amaranth is an extremely interesting plant for genetic research: genetic studies have been initiated to identify marker loci for pigmentation patterns, inflorescence forms and seed characters. Selection in landraces has shown promise. Over a dozen cultivars have been released in Peru, Mexico, United States, Argentina, and India. Breeding is most advanced in the United States. Important selection criteria are suitability for mechanical harvesting, high yield potential, resistance to lodging (dwarf growth, strong stem), early maturity, non-shattering, good drying of the seed-head with synchronous dry-down. Most cultivars are either A. cruentus or A. hypochondriacus, some are derived from interspecific crossing. For successful breeding, more systematic cooperation at international level is necessary to meet local needs. Breeders will require a series of superior germplasm resources as well as a set of elite lines or gene pools, selected genetic stocks such as male-sterile lines and non-shattering types, and of wild-weedy relatives which have also proven to be useful. Research is needed on hybridization barriers among species as well as on the biosystematic identity of the individual species; only then can the information be indisputably related to species.

Prospects

The potential importance of grain amaranth lies in its combination of agronomic characteristics, nutritional qualities and food applications. Grain amaranth is becoming popular for production and biological research. Prospects are good for new industrial products based on the crop, and for improved cultivars and agronomic practices. Confusing information on taxonomic, genetic and ecological aspects should be clarified, to assure this crop continues to make progress in its newly expanding global setting.

Literature

Grubben, G.J.H. & van Sloten, D.H., 1981. Genetic resources of amaranths: a global plan of action. International Board for Plant Genetic Resources (IBPGR), Rome, Italy. 57 pp. (including a provisional key to some edible species of the family Amaranthaceae, by Laurie B. Feine-Dudley).
Joshi, B.D. & Rana, R.S., 1991. Grain amaranths. The future food crop. Shimla Science Monograph No 3, National Bureau of Plant Genetic Resources (NBPGR), Regional Station, Phagli, Shimla, India.
Kauffman, C.S., 1992. Realizing the potential of grain amaranth. Food Reviews International 8: 5-21.
Kulakow, P.A. & Jain, S.K., 1990. Grain amaranth - Crop species, evolution and genetics. Proceedings of the Fourth National Amaranth Symposium. pp. 105-114.
National Research Council, 1984. Amaranth, modern prospects for an ancient crop. National Academy Press, Washington, D.C., United States. 80 pp.
Parades-Lopez, O. (Editor), 1994. Amaranth: biology, chemistry and technology. CRC Press, Boca Raton, Florida, United States. 223 pp.
Stalknecht, G.F. & Schulz-Schaeffer, J.R., 1993. Amaranth rediscovered. In: Janick, J. & Simon, J.E. (Editors): New crops. Proceedings of the Second National Symposium. John Wiley & Sons, New York, United States. pp. 211-218.
Williams, J.T. & Brenner, D., 1995. Grain amaranths. In: Williams, J.T. (Editor): Cereals and pseudocereals. Chapman & Hall, London, United Kingdom. pp. 129-185.

Authors

S.K. Jain & H. Sutarno

See also the Amaranthus (PROSEA Vegetables) page