Members of the genus Amaranthus (family Amaranthaceae) are widely distributed throughout the world's tropical, subtropical, and temperate regions. The genus contains about 60 species.Willis, J.C. 1973. A Dictionary of the Flowering Plants and Ferns.
Cambridge University Press.
Growth habits vary from prostrate to erect and branched to unbranched; leaf and stem colors range from red to green, with a multitude of intermediates; and seed colors range from black to white.
DISTRIBUTION AND ECOLOGY
Historically, people have cultivated amaranths in environments ranging from the true tropics to semiarid lands and from sea level to some of the highest farms in the world. Ecotypes have evolved that tolerate alkaline sandy soils with pH as high as 8.5, as well as the acidic clays of hillside slash-and-burn fields of the tropics.
Although traditionally cultivated within 30° latitude of the equator, amaranth can be grown in higher latitudes using strains that will initiate flowering in spite of the longer daylength
(photoperiod) than that of the tropics. Most grain amaranth cultivation has been concentrated in highland valleys, such as those in the Sierra Madre, Andes, and Himalayas. Generally, traditional farmers in those areas have exchanged seed only with neighbors, so that adjacent semiarid, highland, and subtropical lowland gene pools have remained largely distinct.
Amaranths, as already noted, are among the group of plants that carry on photosynthesis by the specialized C4 pathway. They are one of the few C4 crop species that are not grasses.
The C4 pathway is a modification of the normal photosynthetic process that makes efficient use of the carbon dioxide available in the air by concentrating it in the chloroplasts of specialized cells surrounding the leaf vascular bundles. The photorespiratory loss of carbon dioxide, the basic unit for carbohydrate production, is suppressed in C4 plants.
Consequently, plants that use the C4 pathway can convert a higher ratio of atmospheric carbon to plant sugars per unit of water lost than those possessing the classical C3 (Calvin cycle) pathway.
Even when their stomata are partially closed, plants having the C4 pathway are able to maintain relatively high rates of carbon dioxide fixation. Since stomata close when the plant is under environmental stress (such as drought or salinity), C4 plants, such as amaranth, perform better than C3 plants under adverse conditions. Also, the reduced stomata! opening reduces the water lost by transpiration.
Through osmotic adjustment, the plants can tolerate some lack of water without wilting or dying. This is also an adaptation for surviving periods of drought.
The potential ability to photosynthesize at high rates under high temperature is another physiological advantage of C4 photosynthesis. Research on Amaranthus caudatus cv "edulis" has shown peak photosynthetic activity to occur at 40°C.
Many of the amaranths are sensitive to length of day. For example, strains of Amaranthus hypochondriacus from the south of Mexico will not set flower in the summer in Pennsylvania.
They do, however, mature in the greenhouse during the short-day conditions of winter. The reverse happens with Amararathus cruentus from Nigeria. It remains vegetative for a long period in its equatorial home. However, it goes to seed very early when introduced into the long-day conditions in Pennsylvania and can be used to breed for early-maturing traits.
Amaranthus caudatus, on the other hand, is known to be a short-day species. It usually flowers and sets seed only when daylength is less than 8 hours. However, some
Amaranthus caudatus, such as the ornamental "love-lies-bleeding," will set seed at longer day-length conditions.
Elevation is not a severe limitation. Amaranths grow satisfactorily from sea level to above 3,200 m, but only Amaranthus caudatus is known to thrive at altitudes above 2,500 m.
Grain amaranth grows best when the daily high temperature is at least 21°C. Various accessions have showed optimal germination temperatures varying between 16°C and 35°C. The speed of emergence is increased at the upper end of this range.
Although Amaranthus hypochondriacus and Amaranthus cruentus tolerate high temperatures, they are not frost hardy. Growth ceases altogether at about 8°C and the plants are injured by temperatures below 4°C. Amaranthaus caudatus, however, is native to areas high in the Peruvian Andes and is more resistant to chilling than the other species.
Field observations indicate that amaranth grows well on soils containing widely varying levels of soil nutrients. Initial studies in Pennsylvania show that young grain amaranth plants grow taller with fertilizer, but the grain yield has thus far shown little improvement. Vegetable amaranth, on the other hand, requires high soil fertility, particularly potassium and nitrogen.
Grain amaranth requires well-drained sites and appears to prefer neutral or basic soils (pH values above 6). However, this has not been studied carefully, and, with the wealth of amaranth germplasm that exists, it is likely that types that tolerate acid conditions can be found, especially as vegetable amaranth is often cultivated in tropical lowlands where acid soils are common.
Although the genus is not known for high salt tolerance, an apparent ability to withstand mild salinity and alkalinity is apparent in some species of amaranth. Moreover, Amaranthus tricolor has demonstrated tolerance for soil with high aluminum levels.
For seeds to germinate and establish roots, amaranths require wellmoistened soil, but once seedlings are established, grain amaranths do well with limited water; in fact, they grow best under dry, warm conditions. Vegetable amaranths, on the other hand, require moisture throughout the growing season. Grain amaranths have been grown in dry-land agriculture in areas receiving as little as 200 mm of annual precipitation, and, at the other extreme, vegetable amaranths are routinely grown in areas receiving 3,000 mm of annual rainfall. Indeed, in West Africa, vegetable amaranth production continues even during the torrential rainy season.
Of all the 60 or so amaranth species, only a handful are now used as crops. A few of the others are serious weeds. The main weedy types are A. viridis, A. spinosis, A. retroflexus, and A. hybridus. These are dark-seeded plants with widespread distribution. A. retroflexus ("pigweed") is one of the world's worst weeds.
These weeds are distinctly different from the grain amaranths highlighted in this report. They tend to be indeterminate and to produce seeds at many different parts of the plant and scatter them during a long part of the season. On the other hand, the cultivated grain types tend to mature over a rather short period and have a dominant seedhead with fewer side branches.
Like other Amaranth species, the hardy weeds prefer hot, bright sunlight. Their seed is readily spread by birds and water. They frequently occur as pests in pastures, crops, or along roadsides, usually in unshaded areas in competition with other weeds and grasses. In urban areas they are commonly seen in abandoned lots or poking up through cracks in the pavement.
The seeds of weedy amaranths have remarkably long viability; some have germinated after 40 years. (Much less is known about the viability of the seed of cultivated amaranths, except that seeds stored at room temperature in desiccators may keep well for several years. In high humidity the seeds quickly lose viability, and they also lose their popping quality with age or desiccation.) Of the weedy species, only some types of A. hybridus are worth serious attention as vegetable crops or as breeding parents for grain amaranth improvement.
The species and varieties cultivated for grain do not carry the same potential for weediness. They lack the strong taproot that is seen in A. retroflexus, for example, and are generally much less aggressive in their habits.