Amaranthus is a genus of plants commonly known as amaranths. Some species are known by variants of the common name "pigweed". Some members are annual and others are perennial. The plant can grow from tall with a succulent, hollow stem. Parts of the plant vary from green to reddish. Catkin-like cymes of densely packed flowers grow in the summer or fall.
There are 75 recognized species in the genus, some monoecious and some dioecious. The name amaranth was originally applied to the closely related genus Celosia. Amaranthus is cosmopolitan in tropical regions. Some species are cultivated as leaf vegetables, pseudocereals (with amaranth grain being collected), and ornamental plants.
Description
Amaranth is a herbaceous plant or shrub that is either annual or perennial across the genus. The plant has a primary root with deeper spreading secondary fibrous root structures. Amaranths contain concentric rings of vascular bundles and fix carbon efficiently with a C4 photosynthetic pathway. Amaranth can grow from tall with a cylindrical, succulent, fibrous stem that is hollow with grooves and bracteoles when mature varies in flower, leaf, and stem color with a range of striking pigments from the spectrum of maroon to crimson. The leaves are approximately and of oval or elliptical shape that are either opposite or alternate across species, although most are whole and simple with entire margins.
Catkin-like cymes of densely packed flowers grow in the summer or fall. The inflorescences are in the form a large panicle that varies from terminal to axial, color, and sex. The tassel of fluorescence is either erect or bent and varies in width and length between species. The flowers are radially symmetric and either bisexual or unisexual with very small, bristly perianth and pointy bracts. The flowers vary interspecifically from the presence of 3 or 5 tepals and stamens, whereas a 7-porate pollen grain structure remains consistent across the family.
Species in this genus are either monoecious (e.g. A. hybridus,) or dioecious (e.g. A. palmeri). The fruits are in the form of capsules referred to as a unilocular pixdio that opens at maturity. The top (operculum) of the capsule releases the urn that contains the seed. The seeds are circular, 1 to 1.5 millimeters in diameter and range in color with a shiny, smooth seed coat. The panicle is harvested 200 days after cultivation with approximately 1,000 to 3,000 seeds harvested per gram.
Taxonomy Amaranthus shows a wide variety of morphological diversity among and even within certain species. Amaranthus is part of the Amaranthaceae that is part of the larger grouping of the Carophyllales. Although the family (Amaranthaceae) is distinctive, the genus has few distinguishing characters among the 75 species present across six continents. This complicates taxonomy and Amaranthus has generally been considered among systematists as a "difficult" genus and to hybridize often.
In 1955, Sauer classified the genus into two subgenera, differentiating only between monoecious and dioecious species: Acnida (L.) Aellen ex K.R. Robertson and Amaranthus. Although this classification was widely accepted, further infrageneric classification was (and still is) needed to differentiate this widely diverse group. Mosyakin and Robertson 1996 later divided into three subgenera: Acnida, Amaranthus, and Albersia. The support for the addition of the subdivision Albersia because of its indehiscent fruits coupled with three elliptic to linear tepals to be exclusive characters to members of this subgenus. The classification of these groups are further supported with a combination of floral characters, reproductive strategies, geographic distribution, and molecular evidence.
The phylogenies of Amaranthus using maximum parsimony and Bayesian analysis of nuclear and chloroplast genes suggest five groups within the genus: .
Amaranthus includes three recognised subgenera and 75 species, although species numbers are questionable due to hybridisation and species concepts. Infrageneric classification focuses on inflorescence, flower characters and whether a species is monoecious/dioecious, as in the Sauer (1955) suggested classification. Bracteole morphology present on the stem is used for taxonomic classification of Amaranth. Wild species have longer bracteoles compared to cultivated species. A modified infrageneric classification of Amaranthus includes three subgenera: Acnida, Amaranthus, and Albersia, with the taxonomy further differentiated by sections within each of the subgenera. According to one source, 10 species are dioecious and native to North America, while the rest are monoecious and cosmopolitan.
There is near certainty that A. hypochondriacus is the common ancestor to the cultivated grain species, but their later domestication remains unclear. There has been opposing hypotheses of a single as opposed to multiple domestication events of the three grain species. There is evidence of phylogenetic and geographical support for clear groupings that indicate separate domestication events in South America and Central America. A. hybridus may derive from South America, whereas A. caudatus, A. hypochondriacus, and A. quentiensis are native to Central America and elsewhere in North America.
Species
Species include:
Amaranthus acanthochiton – greenstripe Amaranthus acutilobus – a synonym of Amaranthus viridis Amaranthus albus – white pigweed, tumble pigweed Amaranthus anderssonii Amaranthus arenicola – sandhill amaranth Amaranthus australis – southern amaranth Amaranthus bigelovii – Bigelow's amaranth Amaranthus blitoides – mat amaranth, prostrate amaranth, prostrate pigweed Amaranthus blitum – purple amaranth †Amaranthus brownii – Brown's amaranth Amaranthus californicus – California amaranth, California pigweed Amaranthus cannabinus – tidal-marsh amaranth Amaranthus caudatus – love-lies-bleeding, pendant amaranth, tassel flower, quilete Amaranthus chihuahuensis – Chihuahuan amaranth Amaranthus crassipes – spreading amaranth Amaranthus crispus – crispleaf amaranth Amaranthus cruentus – purple amaranth, red amaranth, Mexican grain amaranth Amaranthus deflexus – large-fruit amaranth Amaranthus dubius – spleen amaranth, khada sag Amaranthus fimbriatus – fringed amaranth, fringed pigweed Amaranthus floridanus – Florida amaranth Amaranthus furcatus Amaranthus graecizans Amaranthus grandiflorus Amaranthus greggii – Gregg's amaranth Amaranthus hybridus – smooth amaranth, smooth pigweed, red amaranth Amaranthus hypochondriacus – Prince-of-Wales feather, prince's feather Amaranthus interruptus – Australian amaranth Amaranthus minimus Amaranthus mitchellii Amaranthus muricatus – African amaranth Amaranthus obcordatus – Trans-Pecos amaranth Amaranthus palmeri – Palmer's amaranth, Palmer pigweed, careless weed Amaranthus polygonoides – tropical amaranth Amaranthus powellii – green amaranth, Powell amaranth, Powell pigweed Amaranthus pringlei – Pringle's amaranth Amaranthus pumilus – seaside amaranth Amaranthus quitensis - Mucronate Amaranth Amaranthus retroflexus – red-root amaranth, redroot pigweed, common amaranth Amaranthus saradhiana - purpal stem amaranth, green leaf amaranth Amaranthus scleranthoides – variously Amaranthus sclerantoides Amaranthus scleropoides – bone-bract amaranth Amaranthus spinosus – spiny amaranth, prickly amaranth, thorny amaranth Amaranthus standleyanus Amaranthus thunbergii – Thunberg's amaranth Amaranthus torreyi – Torrey's amaranth Amaranthus tricolor – Joseph's-coat Amaranthus tuberculatus – rough-fruit amaranth, tall waterhemp Amaranthus viridis – slender amaranth, green amaranth Amaranthus watsonii – Watson's amaranth Amaranthus wrightii – Wright's amaranth
Names and etymology
The word amaranth derives from Ancient Greek (), meaning 'unfading', a name used across cultures. Its development is associated with the Greek word for 'flower', (), giving rise to the idea of the 'unfading flower'. Amarant is an archaic variant of the name.
The botanical name Amaranthus likewise comes from the Ancient Greek . The noun is formed from the privative prefix (), meaning 'without', and the verb (), meaning 'to consume' or 'to exhaust'. Classical references to the plant appear in the works of Dioscorides (Book 3, section 9; Book 4, sections 55 and 57).Dioscorides, De Materia Medica, 3.9; 4.55, 4.57.
The name amaranth was first applied in Europe to the related genus Celosia. At the time, plants of the genus Amaranthus were not yet known in Europe. Amaranthus and Celosia share long-lasting dried flowers, which contributed to the transfer of the name.
The Latin form , which includes the letter h, arose from an erroneous association with the Greek word (), meaning 'flower'. This element appears in the names of many plants, such as Agapanthus, and influenced later botanical spelling.
In the languages of Indigenous peoples who cultivated amaranth in the Americas since ancient times, the plant is known by a variety of names. In Nahuatl, it is called . In Quechua, it is known as and . In Maya, it is called or . Additional names include ahparie in the Purépecha language, in Huichol, and guegui in the Tarahumara.
Distribution and habitat The genus most likely originated in Central America. The native range of the genus is cosmopolitan in tropical regions. It is found in elevations ranging from lowlands to mountain ranges such as the Himalayas.
Ecology Amaranth weed species have an extended period of germination, rapid growth, and high rates of seed production. Farmers have considered them problematic since the mid-1990s, partially due to the reduction in tillage and herbicide use, as well as the evolution of herbicide resistance in several species.Wetzel et al. (1999). Use of PCR-based molecular markers to identify weedy Amaranthus species. Weed Science 47: 518–523. In the United States and Canada, nine species of Amaranthus are considered invasive and noxious weeds: A. albus, A. blitoides, A. hybridus, A. palmeri, A. powellii, A. retroflexus, A. spinosus, A. tuberculatus, and A. viridis.[Assad, R., Reshi, Z. A., Jan, S., & Rashid, I. (2017). "Biology of amaranths". The Botanical Review, 83(4), 382–436.]
A new herbicide-resistant strain of A. palmeri has appeared; it is glyphosate-resistant and so cannot be killed by herbicides using the chemical. Also, this plant can survive in tough conditions. A. palmeri (Palmer amaranth) causes the greatest reduction in soybean yields and has the potential to reduce yields by 17–68% in field experiments. According to one source, A. palmeri is among the "top five most troublesome weeds" in the southeast of the U.S. and has already evolved resistances to dinitroaniline herbicides and acetolactate synthase inhibitors.Culpepper et al. (2006). Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) confirmed in Georgia. Weed Science 54: 620–626. This makes the proper identification of Amaranthus species at the seedling stage essential for agriculturalists. Proper weed control needs to be applied before the species successfully colonizes in the crop field and causes significant yield reductions.
An evolutionary lineage of around 90 species within the genus has acquired the carbon fixation pathway, which increases their photosynthetic efficiency. This probably occurred in the Miocene.
Amaranths are recorded as food plants for some Lepidoptera (butterfly and moth) species including the nutmeg moth and various case-bearer moths of the genus Coleophora: C. amaranthella, C. enchorda (feeds exclusively on Amaranthus), C. immortalis (feeds exclusively on Amaranthus), C. lineapulvella, and C. versurella (recorded on A. spinosus).
Conservation Amaranthus pumilus (seabeach amaranth) has been listed as an endangered species in the U.S. since 1993.
Cultivation