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Biology of Amaranths.

Introduction

Amaranthus is a cosmopolitan genus of annual or short-lived perennial plants (Dorling, 2008), including domesticated and endangered species, restricted endemics and widespread weeds (Sauer, 1950), which are commonly referred as 'Amaranths' or 'Pigweeds' (Bensch et al., 2003). The genus name, Amaranthus comes from the Greek word "amarantos", ([phrase omitted]) which means "unfading", "immortal", "everlasting" or "non-wilting", in view of the fact that its flowers last for a long time.

It has been reported that genus Amaranthus consists of 87 species, of which 14 are distributed in Australia, 17 in Europe, and 56 in America (Jacobsen et al, 2000). However, due to fewer studies on Amaranthus systematics, the number of species is still tentative. There is some disagreement regarding the number of species in this genus with different reports like 60 species (Uphof, 1968; Singh et al., 1983; National Research Council, 1984; Budin et al., 1996; Wiersema & Leon, 1999), 70 species (Pratt et al., 1999; Costea & DeMason, 2001 ; Mosyakin & Robertson, 2003), over 75 species (Sauer, 1993), 86 species (USDA, ARS, 1999) and 100 species (Hanf, 1984) [Table 1- Appendix],

Presently, Amaranthus is distributed in many parts of the world, including Central and South America, Africa, China, India, and the United States (Budin et al, 1996). Approximately 25 species of Amaranthus have been reported in Asian region (Das, 2016). In India the species of genus Amaranthus are mainly found in Himalayas--from Kashmir to Bhutan--and in South Indian hills.

Morphology

Genus Amaranthus is characterized by the following traits: Annual or (rarely) short-lived perennial life history, herbaceous habit with prostrate to erect stem. Leaves are alternate, ovate to linear and have an indented or notched apex and smooth margins. Flowers are imperfect, in compound dichasia packed into inflorescences. The plants of Amaranthus are monoecious (A. albus, A. blitum, A. caudatus, A. hybridus, A. powellii, A. retroflexus and A. spinosus) or dioecious (A. tuberculatus, A. palmeri and A. rudis). The inflorescence is terminal and/or axillary with three to five tepals and stamens. Monoecious species are generally self-pollinated, wind pollinated. Fruit is utricle or pyxidium. Seeds are lenticular and tiny (0.9 to 1.7 mm diameter and 1000-seed weights from 0.6 to 1 g) that are typically dispersed by wind, water, or birds, having extended period of germination with prolific seed production and a base chromosome number of 16 or 17 (Stevens, 1932; Sauer, 1967; Kigel, 1994; Pratt et al., 1999; Franssen et al., 2001b; Costea & Tardif, 2003; Mosyakin & Robertson, 2003; Costea et al, 2004; Steckel, 2004). In addition, this genus has [C.sub.4] photosynthesis, unlike its closest related genera (Weaver, 1984; Mitich, 1997; Bensch et al., 2003; Sage et al, 2007). Morphological terminology in Amaranthus, as used in different floristic and taxonomic treatments, is rather confusing, especially regarding the ternis applied to flowers and inflorescence. Within each Amaranthus species there are several races defined by their common branching pattern, height, inflorescence size and form, days to maturity, seed size and colour, and other morphological characteristics (Espitia-Rangel, 1994; Brenner et al, 2000).

Taxonomy

Genus Amaranthus is often difficult to characterize taxonomically, due to few distinguishing characters among its species, small and difficult-to-see diagnostic parts, broad geographical distribution, large number of hybrid forms, complicating the taxonomy and thus has generally been considered by systematists as a "difficult" genus (Costea & DeMason, 2001). The genus Amaranthus was first reported by Linnaeus (1753). Various species of the genus were at one time recognized as separate genera, particularly the dioecious species and the monoecious species with dehiscent or indehiscent fruits (Linnaeus, 1753; Kunth, 1838). These genera were later placed within Amaranthus by Sauer (1955), and Robertson (1981), and are presently recognized as subgenera. In the most recent taxonomic work, the genus has been divided into three subgenera: Acnida, Amaranthus, and Albersia (Mosyakin & Robertson, 1996; Costea et al., 2001). Subgenus Acnida is generally delimited to include all the dioecious species of Amaranthus, whereas subgenus Amaranthus and subgenus Albersia split the monoecious species using combination of inflorescence position, number of tepals, and fruit dehiscence (Mosyakin & Robertson, 1996). Several authors have suspected that this infrageneric taxonomy may not correspond well to evolutionary history (Eliasson, 1988; Mosyakin & Robertson, 2003) but, despite its wide geographical distribution and close association with human activities, currently there is no well-sampled, well-supported phylogenetic study of this genus. Miiller and Borsch (2005) and Sage et al., (2007) placed Amaranthus in the order Caryophyllales, family Amaranthaceae, subfamily Amaranthoideae, tribe Amarantheae, sub-tribe Amaranthinae, genus Amaranthus and according to Sauer (1967) into the section Amaranthus.

Origin

The centre of origin of Amaranthus is believed to be Central America, with evidence of its cultivation dating back as far as 6700 BC (Myers & Putnam, 1988; Putnam et al., 1989; Sauer, 1993; Kigel, 1994; Mposi, 1999). The Aztec civilization of central Mexico represents the first recorded instance of Amaranthus use, and the crop figured prominently in Aztec culture during the 1400 to 1500 AD. Amaranthus, which the Aztecs called 'huautli' was their staple food and was incorporated into their religious ceremonies. Its seeds were ground by Aztec women and mixed with honey, other sweets, and sometimes with human blood (National Research Council, 1984), then molded into various forms (including animals, natural features, and Gods) for consumption at religious ceremonies and other occasions (Brenner et al, 2000). Despite the known Aztec custom of human sacrifice, the association of human blood with the figures is unclear (National Research Council 1984).

After the arrival of the Spanish conquistadors in Mexico in the early 1500 AD, Spanish attempted to suppress Aztec culture and religion; so upon their conquest Amaranthus, as a crop almost disappeared in America (Sauer, 1950). However, according to Spanish missionaries, the use of Amaranthus as food and in traditional cultural practices continued at a reduced level until some 50 years after the Spanish conquest, but subsequently declined (Early, 1992). Even today Amaranthus is grown in limited quantity in that area, most of which is popped and mixed with honey to make a confection called, "alegria" which means "happiness".

Sauer (1967) reports the introduction of Amaranthus into Spain in sixteenth century, from where it had spread throughout the Europe. Around 1700 AD, it was known as a minor grain plant in central Europe and Russia and by the early nineteenth century it reached Africa and Asia. Till mid-1990s, South Asia was the world's only region where Amaranthus production was increasing (Brenner et al., 2000). It was in 1970s that research on this plant began in the US, only after new evidence revealed grain amaranth protein to be of high quality (Senft, 1980; Lehmann, 1996). Today, it has spread around the world in different regions of Europe, Asia and Africa (Myers & Putnam, 1988; Putnam et al., 1989; Mposi, 1999). Thus Amaranthus is a historic as well as a contemporary plant.

Habitat Preference

Amaranthus has high genetic variability, with diversity in plant form (erect to prostrate), plant height, number of inflorescences (one to several), seed colour, protein content, seed yield, resistance to pests and diseases, and adaptation to soil type, pH, climate, rainfall and day-length (Kulakow, 1990). Although Amaranthus can grow on a wide range of soil types and soil moisture levels, it has been reported to grow well in loamy or sandy-loam or silty-loamy soils with good water holding capacity (Whitehead & Singh, 1993; Ghorbani et al., 1999; Palada & Chang, 2003) with pH range between 4.5 and 8.0 (National Research Council, 1984; Stallknecht & Schulz-Schaeffer, 1993; Palada & Chang. 2003).

Amaranthus is extremely adaptable to adverse growing conditions and tolerates drought and low fertility (O'Brien & Price, 1983). Field studies have shown that it grows well on soils varying widely in levels of soil nutrients (National Research Council, 1984; Myers, 1998) and responds well to good soil fertility and organic matter (Schippers, 2000). Agricultural fields are a great habitat for annual plants like Amaranthus, which grow naturally in open or disturbed areas and receive full sunlight (Pratt et al, 1999). Dieleman et al, (2000) pointed out that the distribution of Amaranthus species in agricultural fields is associated with high levels of nitrates and low levels of phosphate and potassium.

Phenology

Phenology is the study of periodic biological events that take place at different levels, for example in organs, tissue or cells (Aim et al., 1991). The analysis of phenological stages makes it possible to accurately estimate crop-weed competition (Ghersa & Holt, 1995). Thus phenological surveys are of great importance in weed science (Brainard et al., 2005) and can help us in the development of a realistic and practical model for weed control (Elmore, 1996; Swanton et al., 1999).

Phenology of various Amaranthus species has been reported by several workers. Forcella et al. (1997) reported that emergence of Amaranthus species begins in early April and continues until the end of May. Emergence of A. tuberculatus commences from late May and continues to early August (Hartzler et al., 1999), while as flowering and seed set continue until the first frost. Germination of A. albus and A. blitoides occurs from the middle of May to the beginning of June. The first seeds of A. blitum germinate at the end of June or beginning of July. Flowering of A. albus and A. blitoides begins at the end of June or beginning of July, and of A. blitum at the end of July or early August, continuing until senescence is induced by the first fall frost (Stevens, 1924). Shedding of seeds (A. albus and A. blitoides) and of fruits enclosing the seeds (A. blitum) extends throughout the rest of the growing season, due to the indeterminate growth pattern of inflorescences and the continuous formation of new flowers (Costea & Tardif, 2003). A. retroflexus emerges at the end of May and its senescence stage is from November to February; however its phenological stages are slightly dephased for different latitudes, with shorter developmental stages at higher latitudes (Huang et al., 2000; Iamonico, 2010). A. spinosus emerges in June, flowers in July, fruit develops in August, followed by seed dispersal in September and finally shows senescence in November (Chakravorty & Ghosh, 2012).

Physiology

Amaranthus is a dicotyledonous, herbaceous mesophyte that utilizes specialized [C.sub.4] carbon-fixation pathway for photosynthesis in which the first photosynthesis product is a four carbon-compound (National Research Council, 1984; Stallknecht and SchulzSchaeffer, 1993; Myers, 1998; Wang et al., 1999). Amaranthus belongs to the group of NAD-malic enzyme-type of [C.sub.4] metabolism and it exhibits the typical Kranz anatomy ([C.sub.4] anatomy) of leaves, cotyledons and bracts (Wang et al., 1993; Costea & Tardif, 2003). Their photosynthetic pathway is characterized by the use of the mitochondrial NAD-ME to decarboxylate malate in the Kranz bundle-sheaths (Long et al., 1994; Long & Berry, 1996). The combination of various anatomical characteristics of Amaranthus, such as [C.sub.4] metabolism, well developed root system, stomatal conductance, and maintenance of leaf area (Spreeth et al, 2004), results in increased efficiency of using C[O.sub.2] under a wide range of temperatures (from 25[degrees]C to 40[degrees]C), with higher light intensity and moisture stress environments (Williams & Brenner, 1995) which enables this plant to adapt under diverse geographic and environmental conditions (Kigel, 1994). Plants that use the [C.sub.4] carbon fixation pathway tend to require less water than the more common [C.sub.3] carbon-fixation pathway plants (National Research Council, 1984). This is the reason that Amaranthus performs well under adverse temperature and moisture conditions as compared to many [C.sub.3] plants such as wheat and soybeans (Stallknecht & Schulz-Schaeffer, 1993; Schippers. 2000; Spreeth et al., 2004), indicating that Amaranthus can be grown in areas that are not suitable for other crops (Breene, 1991; Lehmann, 1996; Brenner et al., 2000; Rana et al., 2007). Previous studies showed that [C.sub.4] weed species like A. palmen, A. retroflexus and A. nul is respond positively to elevation in temperature (Guo & Al-Khatib, 2003), thus temperature elevation due to climate change could promote the invasion potential of [C.sub.4] weed species like A. retroflexus by enhancing their growth and seed production (Hyvonen, 2011).

Amaranthus has been referred to as drought tolerant (Grubben & van Sloten, 1981; Liu & Stutzel, 2002) and this drought tolerance is due to its [C.sub.4] photosynthetic pathway, a deep and extensive root system, ability to go dormant under extreme drought conditions (O'Brien & Price, 1983) and ability to shut down transpiration through wilting and then recovering easily when moisture is available (Myers, 1996). Plant height, dry matter production, and leaf area expansion of Amaranthus species respond positively to increasing day/night mean temperature (Flint & Patterson, 1983). Flower initiation depends on photoperiod and most of the Amaranthus species flower when day-length is shorter than 12 h (Fuller, 1949; Huang et al, 2000; Palada & Chang, 2003). Plants grown under short-day conditions (8 h) require 14 to 16 days to initiate flowering, whereas plants grown under long-day conditions (16 h) need approximately 45 days (Costea et al., 2004). O'Brien and Price (1983) reported that short days and water stress may promote flowering in this plant.

Simbolon and Sutarno (1986) studied responses of Amaranthus species to reduced light intensity and found them to be moderately tolerant to shade. Previous research shows that [C.sub.4] plants have higher light-saturated photosynthetic rates and are better adapted to high levels of irradiance compared to [C.sub.3] plants that saturate at relatively lower light intensities (Stoller & Myers, 1989; Regnier & Harrison, 1993). Shading affects the survival and growth of plants by altering their physiological and morphological response to light environment. Weeds like Amaranthus show physiological and morphological adaptations to reduced irradiance and the "adaptive plasticity" to adjust to a light-limited environment (Stoller & Myers, 1989). Stoller and Myers (1989) reported that A. albus respond to shade by decreasing light saturated photosynthesis, dark respiration rates, and leaf thickness, while increasing chlorophyll content per unit leaf volume and specific leaf area. A. powellii, A. retroflexus and A. tuberculatus respond to shade by reducing biomass and leaf appearance rates and by increasing specific leaf area and stem elongation (McLachlan et al., 1993a, b: Steckel et al, 2003; Brainard et al., 2005). Characterizing these physiological and morphological responses of this genus to shading are likely to provide valuable information for improving our mechanistic models of crop-weed competition and weed population dynamics (Brainard et al., 2005).

Chromosome Analysis

The genus Amaranthus provides fascinating material for geneticists to work with. Basic genetic studies of this genus were initiated in late 1950s with chromosomal counts (Grant, 1959b; Pal, 1964) and identification of polyploids (Khoshoo and Pal 1972).

Chromosome number varies among Amaranthus species (Table 2). In A. albus, A. palmeri and A. retrojlexus chromosome counts of 2n = 34 (Grant 1959b; Sharma and Banik, 1965; Weaver & McWilliams, 1980) and 2n = 32 (Heiser & Whitaker, 1948; Mulligan, 1984; Rayburn et al., 2005) have been reported. In A. spinosus, somatic counts of 2n = 34 (Baquar & Olusi 1988; Al-Turki et al., 2000) and the gametic counts of n = 17 (Koul et al., 1976; Behera & Patnaik, 1977; Behera & Patnik, 1982) have been reported. A. palmeri and A. spinosus may share a recent common ancestor (Wassom & Tranel, 2005; Riggins et al., 2010), as they have the same chromosome number (2n = 34), pollen morphological similarities (Grant, 1959b; Franssen et al., 2001b; Gaines et al, 2012), and similar genome sizes (Rayburn et al., 2005). Pal et al. (1982) reported that, within the Amaranthus genus, both n = 16 and n = 17 occasionally occur in the same species. The chromosome number of A. blitum and A. powellii is 2n = 34 (Weaver & McWilliams, 1980; Pal & Pandey, 1989; Greizerstein et al., 1997). The chromosome number for A. caudatus, A. hypochondriacus, A. cruentus, and A. hybridus is normally 2n = 32, but occasionally it is 34 (National Research Council, 1989), i.e. these species are diploids with a basic chromosome number of 16 or 17. There are no major chromosomal differences between the genomes of A. hypochondriacus, A. caudatus and A. edulis (Ramesh & Kumar, 2009).

Hybridization

Amaranthus is a predominantly self-pollinated plant (Murray, 1938), with varying amounts of outcrossing (Hauptli & Jain, 1985; Liu et al., 2012). Hybridization among different species has been widely reported within the genus Amaranthus (Saue.r 1950; Trucco et al, 2005b). The possibilities for interspecific hybridization among most of the Amaranthus species have been documented (Murray, 1938; Sauer, 1950; Pal & Khoshoo, 1973), and some have even referred to the genus as 'promiscuous' (Trucco et al., 2005b). The identification of three sources of cytoplasmic male sterility in Amaranthus paves the way for additional hybridization techniques (Peters & Jain, 1985, 1987; Gudu & Gupta, 1988).

A. retroflexus occasionally forms natural hybrids with other species of the same genus (Murray, 1938). It has been reported that hybridization between A. palmeri and A. tuberculatus following controlled pollinations yield practically no fertile offspring (Wetzel et al., 1999b; Franssen et al., 2001a; Steinau et al, 2003; Trucco et al., 2007). Hybridization between A. hybridus and A. tuberculatus produce some fertile F1 individuals (Murray, 1940; Trucco et al., 2007; Trucco et al., 2009), although genetic introgression between these species occurs only in one direction, from A. hybridus to A. tuberculatus (Trucco et al., 2005a, b; Trucco et al., 2009). Based on reported interspecific hybridizations in Amaranthus, equal chromosome number is not a prerequisite for hybridization; however, hybrid progeny appears to be more viable and fertile when their parental species have the same chromosome number, as in the case of hybridization between A. hybridus and A. tuberculatus (Trucco et al., 2009).

There is a concern that glyphosate resistance owing to EPSPS gene amplification could be introgressed from glyphosate resistant A. palmeri to other weedy Amaranthus species (Culpepper et al., 2006). The potential interspecific transfer of EPSPS gene amplification and glyphosate resistance within the genus Amaranthus has considerable evolutionary and agronomic significance (Gaines et al., 2012).

Seed Biology

The detailed understanding of seed biology helps in the development of effective integrated weed management systems (Bhowmik, 1997). Kigel (1994) has compiled much of the available information on seed biology of the genus Amaranthus, including research on effects of light (Oladiran & Mumford, 1985; Gutterman et al., 1992; Gallagher & Cardina, 1998a, b), temperature (Baskin & Baskin, 1977; Weaver, 1984; Oladiran & Mumford, 1985; Weaver & Thomas, 1986; Ghorbani et al., 1999), water availability, osmotic potential and salinity (Ghorbani et al., 1999), hormones (Holm & Miller, 1972; Weaver, 1984; Kepczyriski et al., 1996), soil types, burial depth (Webb et al., 1987; Oryokot et al, 1997a; Ghorbani et al., 1999), and other environmental parameters (Bibbey, 1935; Siriwardana & Zimdahl, 1984; Habib & Morton, 1987; Wiese & Binning, 1987; Forcella et al., 1997) on seed germination.

Seed Dormancy

Weed seed dormancy is often induced and regulated by a compliment of genetic, physiological, and environmental factors (Karssen, 1982; Baskin & Baskin, 1998), including day length and plant age (Gutterman & Genotypic, 1997; Castor et al, 2000), harvest date (Ghosh & Bruin, 1997) and temperature (Marayama et al. 1997). During seed development, A. retrojlexus seed dormancy is affected by various environmental conditions like parental photoperiod, temperature environments, fertilization of soil, competition with other plants (Kigel et al., 1977; Costea et al., 2004), and shading of the maternal plant or in fact by the aid of natural infrared light (Doroszeweski, 2001). Induction of secondary dormancy (thermodormancy) in seeds due to higher temperatures has been reported in A. retroflexus (Baskin & Baskin, 1977, 1985a, 1990; Egley, 1989), A. caudatus (Kepczynski & Bihun, 2002), A. quitensis (Faccini & Vitta, 2005), and A. palmeri seeds (Jha et al., 2010), indicating that this behaviour of seeds kept in soil is characteristic of the genus Amaranthus. The occurrence of such temporal changes in dormancy of buried Amaranthus seeds has been interpreted as an adaptation to enhance survival when seeds are in the soil, especially when they are deeply buried (Omami et al., 1999).

Effect of Temperature

The effect of temperature and light on seed germination of Amaranthus species have been studied by previous researchers (Baskin & Baskin, 1977, 1998; Gallagher & Cardina, 1998a, b; Leon & Owen, 2003; Steckel et al., 2004). The high temperature requirement for germination of Amaranthus species was reported by Bibbey (1935), Kadman-Zahavi (1960), McWilliams et al. (1968), Baskin and Baskin (1977), Washitani and Takenaka (1984), Habib and Morton (1987), Gutterman et al. (1992), Ghorbani et al. (1999), and Guo and Al-Khatib (2003) (Table 3).

Mature Amaranthus seeds remain dormant during autumn and winter (Baskin & Baskin. 1977, 1987; Jha et al., 2007, 2008c; Norsworthy & Oliveira, 2007), since temperatures during that period are below those required for germination. Following after-ripening during winter, seeds of Amaranthus species, like most summer annuals, require high fluctuating temperatures for germination and do not germinate in the field at lower temperatures until late spring to summer (Baskin & Baskin, 1977, 1987; Karssen, 1982; Benech-Arnold et al., 1990; Bouwmeester & Karssen, 1992; Faccini & Vitta, 2005), while as after-ripening for few more months after winter leads to decline in the minimum temperature requirements, or broadening of thermal range for seed germination (Baskin & Baskin, 1977, 1987; Benech-Arnold et al., 1990; Bouwmeester & Karssen, 1992). Thus, seeds gain the ability to germinate equally well at temperatures below the optimum range of 25 to 35[degrees]C (Wright et al, 1999; Guo & Al-Khatib, 2003).

Temperature fluctuations like alternating temperature regimes have been shown to reduce dormancy, decrease time to onset of germination and increase germination rates in various Amaranthus species, like A. cruentus, A. gangeticus, A. hybridus, A. palmeri, A. paniculatus, A. retroflexus, A. rudis, A. spinosus, and A. viridis (Santelmann & Evetts, 1971; Weaver, 1984; Oladiran & Mumford, 1985; Guo & Al-Khatib, 2003; Leon et al., 2004, 2007; Steckel et al., 2004; Thomas et al,. 2006; Chauhan and Johnson 2009; Jha et al., 2010) because alternating temperatures are most similar to diurnal temperature responses and can break seed coat-imposed physical dormancy of seeds (McKeon & Mott, 1982).

Minimum germination temperature for A. palmeri and A. retroflexus was >5[degrees]C, but the optimal temperature for maximum germination has been reported to be from 25 to 40[degrees]C (Evans, 1922; McWilliams et al., 1968; Baskin & Baskin, 1977; Habib & Morton, 1987; Wiese & Binning, 1987; Kcpczynski et al., 1996; Ghorbani et al, 1999; Wright et al., 1999; Kepczynski & Bihun, 2002; Guo & Al-Khatib, 2003). Dormant A. retroflexus seeds cannot germinate in darkness at 25[degrees]C and can either germinate partially at 35[degrees]C (Schonbeck & Egley, 1981a, b; Kcpczynski et al, 2003b), or fully at 35-40[degrees]C (Kepczynski et al., 1996). Optimum germination of A. caudatus and A. blitum has been reported at 35[degrees]C while as of A. hybridus seeds was reported to be between 32 and 34[degrees]C (Washitani & Takenaka, 1984; Teitz et al., 1990; Gutterman et al., 1992; Kcpczynski & Bihun, 2002). Oladiran and Mumford (1985) reported optimum germination between 30 and 35[degrees]C for A. cruentus, A. hybridus, A. paniculatus and A. gangeticus. High temperatures ([greater than or equal to] 25[degrees]C mean), thermal amplitudes ([greater than or equal to] 7.5[degrees]C) and high soil moisture favour germination of A. palmeri and other Amaranthus species (Hartzler et al., 1999; Wright et al., 1999; Guo & Al-Khatib, 2003; Steckel et al. 2004; Jha et al., 2008b, c, 2010).

A. spinosus seeds respond positively under an alternating temperature regime of 20, 25 and 30[degrees]C and adversely at 35[degrees]C and no germination occurs at 15[degrees]C but, when the temperatures were held constant, significant germination was recorded at 30[degrees]C and 35[degrees]C (Steckel et al., 2004). A. viridis seeds germinated over a range of 20 to 40[degrees]C with 30[degrees]C as the optimum temperature for germination (Thomas et al., 2006), while as no germination was observed at 10[degrees]C (Cristaudo et al., 2007).

Effect of Light

Kigel (1994) reported that most species of Amaranthus respond to light, but the response level varies among the species (Gallagher & Cardina, 1998a, b; Cristaudo et al., 2007), depending upon the dormancy level of seeds, which is further influenced by factors such as burial and temperature (Gallagher & Cardina, 1998a; Leon & Owen, 2003). Light requirement for germination has been reported in various Amaranthus species like A. blitum, A. caudatus, A. hybridus, A. retroflexus, A. rudis, A. spinosus, and A. viridis (Baskin & Baskin, 1977; Schonbeck & Egley, 1981a, b; Teitz et al., 1990; Gallagher & Cardina, 1998a, b; Leon & Owen, 2003; Cristaudo et al. 2007). Seeds of A. spinosus, A. retroflexus and A. viridis germinate better in light than in darkness, implying that the buried seeds will germinate following soil disturbance (Baskin & Baskin, 1977, 1985b, 1998; Omami & Medd, 1992; Gallagher & Cardina, 1998a, b; Benvenuti et al., 2001; Cristaiido et al., 2007). The ecological significance attributed to the light response in various species of Amaranthus is that light acts as a soil depth "indicator," or depth-sensing mechanism for seeds, allowing greater germination of surface seeds than seeds buried in soil (Ghorbani et al., 1999; Schutz et al., 2002).

Phytochrome-regulated seed germination of various Amaranthus species like A. arenicola, A. cauda tus, A. hybridus, A. palmeri, A. retroflexus, and A. rudis has been documented (Hendricks et al., 1968; Kendrick & Frankland, 1969; Kendrick et al., 1969; Taylorson & Hendricks, 1969, 1971; Gallagher & Cardina, 1998a, b; Leon & Owen, 2003; Jha et al., 2010). Exposure to red (R) light induces germination in donnant seeds (Jha et al., 2010), and this effect is more pronounced in chilled seeds (moist stratification at 4[degrees]C) compared to non-chilled seeds, suggesting an interaction of low temperatures with phytochrome in dormancy alleviation (Taylorson & Hendricks, 1969; Gallagher & Cardina, 1997, 1998a, b; Leon & Owen, 2003). However, high temperatures (30[degrees]C or above) during summer can reduce the photosensitivity of Amaranthus seeds and overcome the phytochrome mediated red light requirement for enhanced germination in various Amaranthus species like A. hybridus, A. retroflexus, and A. rudis (Gallagher & Cardina. 1998a; Hartzler et al, 1999; Leon & Owen, 2003). On the other hand FR light inhibits germination and induces dormancy (Kendrick & Frankland. 1969; Taylorson & Hendricks, 1971; Gallagher & Cardina, 1998a; Leon & Owen, 2003; Jha et al., 2010). This inhibitory effect of FR light is due to decrease in Pfr/Pr of the phytochrome, thus causing photo dormancy in seeds (Hendricks et al., 1968; Kendrick & Frankland, 1969; Taylorson & Hendricks, 1969, 1971).

Effect of Soil Types, Seed Burial Depth and Duration of Seed Burial

Percent emergence of A. retroflexus is greater in lighter soils (sandy clay loam, Loamy sand and Sandy loam) than heavier soils (silty clay and sandy clay). Maximum emergence occurs at 0.5 cm depth in the three lighter soils, between 0.5 and 2 cm in the sandy clay, and 3 cm deep in the silty clay (Ghorbani et al., 1999), possibly due to poor gas exchange, poor light, and lower temperature in heavier soils (Gallagher & Cardina, 1998a).

Buried weed seeds constitute an important part of the soil seed bank (Baskin & Baskin, 1985b, 1998; Benvenuti et al, 2001) and position, distribution and dormancy level of these seeds in the soil play an important role in their germination and subsequent emergence (Burnside et al., 1981; Benvenuti & Macchia, 1997; Benech-Arnold et al., 2000), which is further influenced by factors like soil temperature, soil moisture, and light availability. Small-seeded weeds such as Amaranthus species can germinate only from shallow soil-depths of 0.5 to 2.5 cm (Baskin & Baskin, 1977, 1998; Buhler et al., 1996; Oryokot et al., 1997a; Gallagher & Cardina, 1998a, b; Ghorbani et al., 1999; Benech-Arnold et al., 2000; Leon & Owen, 2003).

Santelmann and Evetts (1971) examined emergence for several Amaranthus species and found that germination decreases at depths below 1.9 cm. Increase in dormancy of seeds buried at a depth of 5 to 10 cm for 3 to 12 months has been reported in other Amaranthus species (Baskin & Baskin, 1977; Omami et al., 1999). Ghorbani et al. (1999) observed in-situ emergence patterns of A. retroflexus and concluded that the optimal burial depth was between 0.5 and 3 cm, with no emergence at 5 cm. Under field conditions also A. retroflexus seeds showed a decline in germination with an increase in burial depth from 0 to 10 cm (Wiese & Davis, 1967; Omami et al., 1999). Benvenuti et al. (2001) also reported a decrease in A. retroflexus seedling emergence with an increase in burial depth and the emergence was found to be less than 10% at a burial depth of 8 cm. Furthermore, the seeds of A. viridis on the soil surface had reduced emergence compared to the seeds placed just below the surface and germination was optimum from shallow soil depths of 0.5 to 2 cm, but some seedlings emerged from as deep as 6 cm (Thomas et al., 2006). Limited soil-to-seed contact, light conditions on the surface, and water availability or lower soil water potential close to the seed are some environmental conditions that may limit germination of seed on the soil surface (Ghorbani et al., 1999). Seedling emergence on the soil surface was lower than germination observed in Petri dishes in the light (Chauhan & Johnson, 2009). This difference could be due to poor soil-seed contact or more limited availability of moisture on the soil surface than on the filter papers (Ghorbani et al, 1999).

Seeds of A. spinosus and A. viridis emerge at the same rate from 0.5 cm to 2 cm but, as the burial depth increases, A. spinosus emergence declines more rapidly than that of A. viridis, with no emergence from 4 cm in the former and only 6% emergence in the later, while as no emergence was observed at a depth of 6 cm in either species (Chauhan & Johnson, 2009). Larger seeds with greater carbohydrate reserves have increased ability to emerge from greater burial depths compared to those with lower reserves (Baskin & Baskin, 1998). The greater seed mass of A. viridis (more than twice than that of A. spinosus) could explain its ability to emerge from deeper in the soil than A. spinosus (Chauhan & Johnson, 2009). Small-seeded broadleaf weed species, such as A. retroflexus, A. spinosus, A. viridis and several other Amaranthus species have a similar pattern of emergence due to limited carbohydrate reserves to support germination and seedling emergence, thus limiting the depth from which these seedlings can emerge (Webb et al., 1987; Santelmann & Evetts, 1971; Ghorbani et al, 1999; Thomas et al., 2006; Chauhan & Johnson, 2009) (Table 4).

The acquisition of depth-mediated dormancy of weed seeds (Milberg & Andersson, 1997; Benvenuti et al., 2001) is an important strategy that allows longevity and perpetuation of weed seeds in the soil seed bank (Thompson, 1987; Benvenuti et al., 2001), and is known to occur due to lack of light transmittance, decrease in thermal fluctuation, decrease in oxygen, increase in carbon dioxide, and low rates of gaseous diffusion with increasing soil depth (Holm, 1972; Woolley & Stoller, 1978; Baskin & Baskin, 1985b; Drew, 1990; Benvenuti & Macchia, 1997, 1998; Gallagher & Cardina, 1998a; Benvenuti et al., 2001; Benvenuti, 2003).

Furthermore, temperature fluctuations, which have been reported to have effect on seed dormancy alleviation of Amaranthus species (Baskin & Baskin, 1977, 1985b; Omami et al., 1999; Guo & Al-Khatib, 2003; Leon & Owen, 2003; Steckel et al., 2004; Cristaudo et al., 2007), decrease with increasing soil depth, thus acting as a depth-sensing mechanism for weed seeds (Ghersa et al., 1992; Baskin & Baskin, 1998; Kegode et al., 1998). Gutterman et al. (1992) and Kepczynski et al. (1996) reported that the non-dormant seeds of Amaranthus need at least 10% oxygen for germination. But with an increase in burial depth, there is a decrease in oxygen concentration, leading to hypoxia and germination inhibition in seeds of some weed species (Holm, 1972).

Milberg and Andersson (1997) reported that burial in soil induced a light requirement in some weed seeds. Burial induced red-light requirement for germination, resulted in a shift from low fluence response (LFR) to very low fluence response (VLFR) of the phytochrome in A. retroflexus and A. hybridus seeds (Scopel et al., 1991; Smith, 1995; Gallagher & Cardina, 1998a, b), which may also be expected in other species of Amaranthus.

Duration of seed burial also plays an important role in dormancy and germination of weed seeds like A.patulus (Baskin & Baskin, 1985b, 1998; Washitani, 1985) (Table 4). Washitani (1985) reported that only a negligible number of buried seeds of A. patulus maintained their germinability after 3 years of burial. Omami et al. (1999) reported cyclic changes in dormancy and germination of A. retroflexus seeds during a 12 month burial period. Seed germination of A. retroflexus and A. palmeri declined after 1 to 3 months of seed burial; peak germination occurred after 9 months and again declined at 12 months after seed burial (Omami et al., 1999; Jha et al., 2010). Buried seeds of A. retroflexus can remain viable for at least 6-10 years (Chepil, 1946a; Weaver & McWilliams, 1980; Burnside et al., 1981, 1996; Costea et al., 2004). In Beal's experiment in Michigan, 2% of seeds of A. retroflexus germinated after 40 years of burial (Telewski & Zeevaart, 2002).

Effect of Shade and Tillage

Subtle differences in the soil microclimate may have large aifects on seed germination of weeds including Amaranthus species (Mohler, 1993; Buhler et al., 1996; Oryokot et al., 1997a). Effects of shade and tillage on emergence characteristics of Amaranthus species such as A. retroflexus and A. rudis has been previously studied (Anderson & Nielsen, 1996; Oryokot et al, 1997a; Hartzleret al., 1999; Cardina et al., 2002; Leon & Owen, 2006).

Effect of Scarification and Stratification

Santelmann and Evetts (1971) observed that A. spinosus seeds germinated best when treated with sulfuric acid. Mechanical or chemical scarification with sulphuric acid for 1 to 5 min resulted in germination of the secondary dormant A. caudatus seeds at 25[degrees]C (Kepczynski & Bihun, 2002). Treatment with different concentrations of acetone, ethanol, ethylene, hydrogen peroxide, potassium cyanide, sodium azide, and sulfuric acid were very effective in breaking Amaranthus seed dormancy and promoting germination (Taylorson & Hendricks, 1973; Mahmudzadeh et al., 2003). Soomarin et al. (2010) treated the seeds of A. retroflexus with sulfuric acid and reported that with the increase in the duration of treatment the germination rate of Amaranthus seeds increased from 2% (in control) to 78.5% (in 25-min pre-treatment). Radicle length and weight, however, decreased (Evans, 1922; Mahmudzadeh et al., 2003; Soomarin et al., 2010).

Stratification, an effective way of alleviating seed dormancy (Bewley & Black, 1994), releases A. retroflexus and A. rudis seed dormancy gradually over time (Leon & Owen, 2003; Kepczynski & Sznigir, 2012) and increased seed germination percentage but the rate of germination at 35[degrees]C was higher than at 25[degrees]C after every period of stratification (Taylorson & Hendricks, 1969; Kcpczynski & Sznigir, 2012). Seeds of A. retroflexus grown under a short-day length had higher germination in response to cold stratification than those grown under long-day lengths (Kigel et al., 1977).

Stratification at a constant temperature was less effective in releasing dormancy than autumn-winter burial of seeds due to the reason that in the soil, seeds are exposed not only to fluctuating temperatures, but also to several other factors such as compounds solutions, gases and soil microorganisms, which may affect the dormancy state of Amaranthus seeds (Kcpczynski & Sznigir, 2012). Moore (1979) pointed out that chilling at 4[degrees]C increases endogenous GA and decreases ABA concentration, therefore, it is possible that dormancy alleviation in seeds of some species by stratification or partial burial is associated with changes in ABA/GA and ethylene balance and/or sensitivity to these hormones (Cadman et al., 2006; Rodriguez-Gacio et al., 2009).

Economic Importance of Genus Amaranthus

Composition and Nutritional Value of Amaranthus

The role of Amaranthus as an under-exploited plant with promising economic value was recognized by the National Academy of Sciences, USA (NAS, 1975), after which it's nutritional value has been extensively studied (Becker et al., 1981; Teutonico & Knorr, 1985; Petr et al., 2003).

Amaranthus leaves show significant energy value ranging from 27 to 53 kcal/100 g of fresh leaves and high nutrient value ranging from 4 to 6 g of protein, 0.2 to 0.6 g of fat, and 4 to 7 g of carbohydrates per 100 g of fresh leaves (Uusikua et al., 2010) and are known to be rich in micronutrients and vitamins particularly chlorine, copper, iron, manganese, sodium, vitamin A, vitamin C and vitamin B-12 (Mnkeni, 2005). Amaranthus leaves taste much like spinach but, are nutritionally superior as they contain 3 times more vitamin C, calcium and niacin than spinach (Mnkeni, 2005). As compared to lettuce, Amaranthus leaves contain 18 times more vitamin A, 13 times more vitamin C, 20 times more calcium and 7 times more iron (Mnkeni, 2005; Srivastava, 2011).

Amaranthus seeds have protein content of about 12.5 to 17.6% (Becker et al., 1981; Teutonico & Knorr, 1985), with significantly higher content of lysine (0.73 to 0.84% of the total protein content) and the sulphur-containing amino acids (methionine and cysteine) than other cereal grains (Becker et al., 1981; Saunders & Becker, 1984; Railey, 1993; Lehmann, 1996; Petr et al., 2003) except soybeans (Petr et al. 2003), thus having potential to improve world food situation as an alternative source of protein (Oliveira & de Carvalho, 1975).

Amaranthus seeds have an excellent amino acid profile which, when combined with maize or rice, would approximate the modern standard protein recommended by the FAO/ WHO (FAO, 1973; Senft, 1980; Teutonico & Knorr, 1985; Singhal & Kulkarni, 1988) and is useful in supplementing nutritive food and amelioration of protein deficiency strictly in the vegetarian diet people (Downtown, 1973; Senft, 1980; Railey, 1993).

The lipid content of Amaranthus seed is typically 6 to 20% (Lorenz & Hwang, 1985; Garcia et al, 1987a; Budin et al.. 1996). Both Amaranthus seeds as well as leaves are a good source of unsaturated fatty acids like palmitic, oleic, linoleic. and linolenic acids (Fernando & Bean, 1984; Jahaniaval et al., 2000; Leon-Camacho et al., 2001). Although Amaranthus seed is not considered a typical oilseed crop, it has been identified as a rich source (2.4 to 8%) of squalene (2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene) and tocotrienols (a form of vitamin E) (Budin et al., 1996; Lehmann, 1996; Sun et al., 1997). Squalene is an expensive terpenoid compound, derived primarily from liver of shark (Cantrophorus squamosus) and whale (Physeter macrocephalus) oils. Due to the concern for marine animal protection, attention has been focused on identifying crop sources of squalene (Sun et al., 1997). Squalene is an important ingredient in cosmetics, pharmaceuticals, and lubricants for computer disks (Sun et al., 1997; Budin et al., 1996). Both squalene and tocotrienols can play an important role in lowering LDL-cholesterol in blood (Railey, 1993; Budin et al., 1996; Lehmann, 1996) and thus acts as protective factors against cardiac infarction which is caused by isoproteronol (Farvin et al., 2006).

Amaranthus seeds are a rich source of calcium (1300 to 2850 mg/kg), iron (72 to 174 mg/kg), magnesium (2300 to 3360 mg/kg), sodium (160 to 480 mg/kg) and zinc (36.2 to 40 mg/kg); sterols (0.27-0.32 mg/g); as well as vitamin riboflavin (0.19-0.23 mg/100 g of flour) and ascorbic acid (4.5 mg/100 g of flour), niacin (1.17 to 1.45 mg/100 g of flour), thiamine (0.07 to 0.1 mg/100 g of flour) and other microelements (Becker et al, 1981; Plate and Areas, 2002).

Some Amaranthus species, when grown under conditions of stress, are known to accumulate toxic levels of oxalate and nitrate (Der Marderosian et al., 1980; Saunders & Becker, 1984; Wills et al., 1984) and these nitrates upon consumption may be chemically changed in the digestive tract into poisonous/carcinogenic nitrosamines.

Applications of Amaranthus

Amaranthus is one of the few highly nutritious multi-purpose crops and is used as vegetable, cereal, medicinal plant, dye plant, forage, fuel and as an ornamental. (Sauer ,1950; Oke 1983; Saunders & Becker, 1984; Railey, 1993; Mlakar et al., 2009; Sheikh & Singh, 2013).

The red dye from the leaves of various species of Amaranthus is used to color foods, alcoholic beverages and maize dough (Sauer, 1950). In Mexico, Amaranthus seeds are used chiefly for making alegria candies from popped seeds and molasses (Early, 1977) and for preparing atole, a drink from roasted and powdered seeds mixed with syrup and water (Oke, 1983). In India, A. hypochondriacus, commonly known as "rajgeera" (the King's grain), is extensively cultivated as subsidiary food crop from Kashmir to Arunachal Pradesh and is often popped to be used in confections called "laddoos," (Vietmeyer, 1978) which are very similar to Mexican 'alegria'. In Nepal, Amaranthus seeds are eaten as gruel called "sattoo" or milled into a flour to make chappatis (Vietmeyer, 1978; Singhal & Kulkami, 1988). Amaranthus leaves are used in custards, pastes, soups, stews, salad, boiled and mixed with a groundnut sauce (Oliveira & de Carvalho, 1975; National Research Council, 1984).

Various species of Amaranthus (A. blitum, A. caudatus and A. spinosus) are consumed as vegetable in Africa, Caribbean, China, Greece, India, Italy, Nepal and South Pacific Islands (Stallknecht & Schulz-Schaeffer, 1993). Based on utilisation of cultivated Amaranthus for human consumption, species can be divided into grain and vegetable Amaranthus. Grain Amaranthus is not a "true cereal" rather it belongs to a group of cereal-like grain crops or "pseudo-cereals" (O'Brien & Price, 1983). Pseudocereals are dicotyledonous species which are not closely related to each other or to the monocotyledonous true cereals (Shewiy, 2002).

The Amaranthus seeds can be ground and included as a flour ingredient in different mixtures for biscuits, bread, cookies, crackers, crepes, dumplings, muffins, noodles, pancakes, puddings, and other confectioneries (National Research Council, 1984; Mlakar et al., 2009; Sanz-Penella et al., 2013) and also the combination of amaranth and wheat flour increases the nutritional value of baked products (Saunders & Becker, 1984; Segura-Nieto et al., 1994; Mlakar et al., 2009).

Amaranthus has antiallergic, anticancer, antihypertensive and antioxidant properties (Conforti et al, 2005; Castelano-Sousa & Amaya-Farfan, 2012) and protects against several disorders such as bleeding tendencies, brain stroke, celiac disease, defective vision, diabetes, digestion disorder, functional sterility, haemorrhage, heart diseases, HIV/AIDS, hypertension, kwashiorkor, leucorrhoea, liver disease, marasmus, premature ageing, recurrent colds, respiratory infections, retarded growth, skin diseases, TB and wound healing (Thompson, 2001).

Phytoremediation Potential of Genus Amaranthus

The uptake of heavy metals by Amaranthus species has been recently studied in soils at refuse dump sites, animal waste dumpsites and other forms of contaminated soils (Adekunle et al, 2009; Adefila et al., 2010; Adefemi et al., 2012; Akubugwo et al., 2012; Shagal et al., 2012). A. caudatus plants grown on dump sites contain higher concentration of heavy metals like Fe, Cu, Pb, Zn, Mn, Cd (Adewuyi et al, 2010). A. hybridus grown on dumpsites possessed higher concentration of heavy metals like Fe, Zn, Cd, Cr, Cu, Ni, Pb, Mn and Hg (Akubugwo et al., 2012). The assessments of the heavy metal content of plants grown on dumpsites provide precious data on the heavy metals phytoaccumulation potential of such plants (McIntyre & Lewis, 1997). A. retroflexus has been identified as metal accumulator (Bigaliev et al., 2003; Mellem, 2008). A. spinosus is a potential agent for accumulation and translocation of heavy metal like Cu, Zn, Cr, Pb and Cd (Chinmayee et al., 2012). A. tricolor has high cadmium-accumulating ability (Watanabe et al., 2009). A. viridis has a method of concentrating heavy metals especially Pb and Cd in its tissues (Atayese et al., 2009). This suggests that these Amaranthus species can serve as phytoaccumulators of heavy metals and can be used for the purpose of Phytoremediation.

Amaranthus as a Weed

Genus Amaranthus consists of some of the worst [C.sub.4] weeds of the world (Holm et al., 1977), and its several species are consistently ranked among the top 10 most troublesome weeds in the southeast United States (Dowler, 1995; Webster, 2009), which have been causing problems for fanners since the mid-1990s. Nine Amaranthus species are listed as "invasive or noxious weeds" in the USDA Plants Database, and an additional 20 species are listed as "agricultural weeds" in the Global Compendium of Weeds (Randall, 2007; USDA, NRCS, 2010).

Several Amaranthus species such as A. retroflexus, A. spinosus and A. viridis (Yan et al., 2001), A. blitum (Costea & Tardif, 2003), A. albus, A. powellii and A. rudis (Ortiz Ribbing & Williams, 2006) and A. palmeri (Kendig, 2009), are known to compete with many economic crops including cereals and vegetables in different parts of the world and cause great yield losses (Holm et al., 1977; Menges, 1988; Monks & Oliver, 1988). The weedy Amaranthus species are referred to as "opportunists" (Sauer, 1955) because of the fact that they thrive in disturbed soils and tend to be associated with agricultural practices. They are able to compete with crops for water, nutrients and light, causing severe reductions in yield, quality and harvest efficiency (Vangessel & Renner, 1990) and their high competitiveness may be related to their prolific seed production, prolonged seed viability, seed dormancy, speed and timing of germination, long germination period, aggressive growth at higher temperatures due to its extensive root system, thermostability of the [C.sub.4] photosynthetic mechanism, high water use efficiency, and high density of infestation (Weaver & McWilliams, 1980; Knezevic & Horak, 1998; Horak & Loughin, 2000; Aguyoh & Masiunas, 2003; Massinga et al., 2003), which is further enhanced due to evolution of herbicide-resistant Amaranthus biotypes (Heap 2014). Besides, their competitiveness varies with species, density, and time of emergence relative to the crop (Klingaman & Oliver, 1994; Knezevic & Horak, 1998). Allelopathic effects may also interact with competition for resources between some Amaranthus species and the crop in which they are growing (Connick et al., 1987; Bradow & Connick, 1988; Menges, 1988).

A. blitum is listed by as a serious or principal weed in ten countries, mainly across Europe and Asia (Holm et al., 1979; Takabayashi & Nakayama, 1981 ; Walter & Dobes, 2004) and occurs in a wide range of field and horticultural crops, grassland, orchards, plantations and vineyards.

A. palmeri is the most troublesome weed in southeastern United States (Dowler, 1995; Norsworthy, 2003), and is known to cause severe interference and yield loss in various agronomic crops like Arachis hypogaea (Horak & Loughin, 2000; Burke et al., 2007), Glycine max (Monks & Oliver, 1988; Klingaman and Oliver 1994; Dieleman et al., 1995; Bensch et al., 2003; Norsworthy, 2003), Gossypium hirsutum (Keeley & Thullen, 1989; Dowler, 1995; Morgan et al., 1997; Rowland et al., 1999; Smith et al., 2000; Morgan et al., 2001), Ipomoea batatas (Meyers et al., 2010), Sorghum bicolor (Moore et al., 2004), and Zea mays (Massinga et al., 2001; Massinga & Currie, 2002; Massinga et al., 2003).

A. retroflexus, one of the ten weed species of greatest economic importance in Europe (Schroeder et al., 1993) is a common weed of cultivated fields in many agricultural areas of the world (Weaver & McWilliams 1980; Horak & Loughin, 2000), which is capable of infesting and reducing yields of crops such as Beta vulgaris, Brassica napus (Hendrick et al., 1974), Glycine max (Orwick & Schreiber, 1979; Dieleman et al., 1995; Cowan et al, 1998; Bensch et al., 2003), Gossypium hirsutum (Buchanan et al., 1980), Helianthus annus (Heidarian et al., 2012), Phaseolus sp. (Aguyoh & Masiunas, 2003), Solanum tuberosum (Vangessel & Renner, 1990), Zea mays (Knezevic et al., 1994). and other vegetables (Weaver & McWilliams, 1980).

A. spinosus has been reported as a weed in 28 crops and 44 countries in India, Southeast Asia and the west and south of Africa (Waterhouse, 1994; Chauhan and Johnson 2009). It occurs as a weed of varying significance in a variety of crops like Ananas comosus, Arachis hypogaea, Celosia argentia, Corchorus olitorius, Glycine max, Gossypium hirsutum, Oryza sativa, Saccharum ofjicinamm, Soighum bicolor, Zea mays (Ogunyemi et al., 2000; Ogunyemi et al., 2005; Chauhan & Johnson, 2009) and horticultural enterprises (Waterhouse, 1994). A remarkable example of recent weed invasion is that of A. tuberculatus in com and soybean fields in the central United States over the past two decades (Steckel, 2007). A. tuberculatus was first recorded as an agricultural weed in Illinois cornfields in the early 1950s (Sauer, 1957) and has become a weed of major concern since 1990s (Trucco et al. 2009). Yield losses caused by interference from various other species of Amaranthus have been reported for numerous crops (Moolani et al., 1964; Rushing et al., 1985; Bensch et al., 2003; Hartzler et al., 2004).

Allelopathy

Allelopathic effects of different Amaranthus species on various plants have been reported (Table 5). Dry residues of the aerial part of A. retroflexus reduced the germination of Beta vulgaris, Brassica oleracea, Brassica oleracea var. brotrytis, Capsicum annuum, Carthamus tinctorius, Cucumis sativus, Cucurbita ovifera, Daucus carota. Glycine max, Gossypium hirsutum, Helianthus annus, Hordeum vulgare. Lactuca sativa, Lycopersicon lycopersici, Phaseolus vidgaris, Solanum melongena. Sorghum bicolor and Zea mays (Munger et al., 1983; Qasem, 1995a, b; Alam et al., 2001; Aguyoh & Masiunas, 2003; Costea & Tardif, 2003; Dos Santos et al, 2004; Rezaie & Yarnia, 2009; Souza et al, 2011).

Shoot residues of A. retroflexus inhibited radicle and hypocotyl elongation, interfered with photosynthesis, reduced growth, nutrient uptake and productivity of Zea mays (Bhowmik & Doll, 1980, 1982, 1984) and decreased respiration, relative growth rate (RGR), net assimilation rate (NAR), root fresh weight (FWR), nitrogen fixation of nodules, chlorophyll content and biomass production in Glycine max (Bhowmik & Doll, 1980, 1982, 1984; Chaniago et al., 2006); reduced growth and yield in Nicotiana tabacum (Lolas, 1981; Lolas, 1986); affected common bean growth and establishment (Shimi & Termeh, 2004) and decreased safflower yield (Williams et al., 2005) through release of allelochemicals, but. the inhibition was not appreciably affected by temperature or light (Bhowmik & Doll, 1983). Dried shoot residues of A. retroflexus, A. blitoides and A. gracilis reduced germination, coleoptile length, root length, root dry weight, plant height, grain and straw yield of Triticum aestivum (Qasem, 1995b; Shahrokhi et al., 2012) and caused reductions in the productivity of Hordeum vulgare (Qasem, 1994, 1995a).

Allelopathic effects of A. palmeri have been reported on seedling emergence of several species (Menges, 1987, 1988). Extracts of A. palmeri inhibited seed germination and growth of Daucus carrota and Allium cepa (Altieri and Doll 1978; Bradow & Connick, 1987; Menges, 1987). Vapours of 2-heptanone and 2-heptanol, isolated from A. palmeri, inhibited the germination of onion, carrot, tomato, and palmer amaranth seeds (Connick et al., 1987). Menges, (1988) reported phytotoxicity of A. palmeri residues on growth of several crop species including Sorghum bicolor, Brassica oleracea var. Capitata, Daucus carrota and Allium cepa. Water-soluble extracts of A. palmeri were more phytotoxic than were extracts of A. retroflexus (Hicks et al., 1986).

Allelopathic effects of A. spinosus have been reported (Shrefler et al., 1996). Extract of leaves and inflorescence of A. spinosus drastically reduced the vegetative and reproductive phases of Sinapis alba and T. aestivum (Datta & Bandyopadhayay, 1981). Leached components of A. spinosus showed strong allelopathic effect over the growth and establishment of Parthenium hysterophorus (Chikkalingaiah & Mahadevappa, 1998) and also over the growth of some cultures (Suma, 1998).

Burned residues of A. viridis diminished growth and productivity of Pennisetum americanum (Singhal & Sen. 1981). Also the aqueous extracts of A. dubius were found inhibitory to seedling emergencz and growth of several plant species (Altieri & Doll, 1978). Dry shoot extracts of A. hybridus negatively affected the total chlorophyll content, number of developed leaves, stem length, and total plant dry matter of dry beans (Amini et al., 2013).

Allelochemicals such as aldehydes, alkaloids, apocarotenoids, flavonoids, steroids, xyloids, clerogenic acid and saponins (Anaya et al., 1987; Aim et al., 2002), secreted by aerial organs of Amaranthus plants were found to be released through washing by rain or irrigation water to the soil (Anaya et al., 1987; Al-Khatib, 1995; Khanh et al., 2005).

Fischer and Quijano (1985) isolated phytol, chondrillasterol, vanillin, 3-methoxy-4hydroxy nitrobenzene, and 2, 6-dimethoxy-benzoquinone from A. palmeri. Coumarins have been isolated from A. retroflexus (Rezaie & Yarnia, 2009). The principal allelochemicals present in A. spinosus were phenolic acids, alkaloids belonging to the quinolizidine class, steroid, indol and sesquiterpene lactones (Narwall 1994; Qasem 1995a; Velu & Ali, 1995; Suma, 1998). The quantity of phenolic acids in A. spinosus was greatest in plants growing in soils polluted with domestic, industrial and vehicle residues (Suma, 1998) while as the concentration of phenolic acids within the plant was highest in the leaves, followed by stems, inflorescence and the lowest in the roots (Souza et al., 2011).

Management

Weedy Amaranthus species are difficult to control due to their prolific seed production (200,000 to 600,000 seeds/female plant), small seed size, ability to cross successfully with other Amaranthus species, long germination period, high density of infestation, ability to rapidly evolve herbicide resistance, and difficulty in proper identification (Stevens, 1932; Keeley et al., 1987; Dillon et al., 1989; Horak, 1997; Wetzel et al., 1999a, b; Sellers et al., 2003; Vigueira et al., 2013). Although, proper identification of Amaranthus species can be difficult, it is important due to varying responses to herbicides and weed management practices (Pratt et al, 1999).

Shade can have a suppressive effect on weed seed gennination, as it dampens the soil thennal amplitude and alters the light quality perceived by seeds lying on the soil surface (Fortin & Pierce, 1990; Batlla et al., 2000; Norsworthy, 2004). Under a canopy, besides reductions in PAR, seeds experience a reduction in the red: far-red (R:FR) ratio as a result of an increase in far-red (FR) transmitted light (Taylorson &Borthwick, 1969; Thompson & Grime, 1983; Sattin et al.. 1994; Norsworthy 2004), which is inhibitory to germination of various Amaranthus species like A. hybridus, A. palmeri, A. retroflexus, and A. rudis (Taylorson & Borthwick, 1969; Gallagher & Cardina, 1998a, 1998b; Hartzler et al., 1999; Leon & Owen 2003; Jha et al., 2008c). Shading of the maternal plant in A. retroflexus resulted in reduced seed dormancy under short day (8 h light) and increased seed dormancy under long day (16 h light) conditions (Kigel et al., 1977). Washitani (1985) and Brainard et al. (2005) also reported a leaf canopy effect on increased seed dormancy of A. patulus. The germination percentage of A. powellii seeds was 50% lower for seeds maturing on plants grown under shade than in open sunlight (Brainard et al., 2005).

Tillage is a major mechanism for vertical movement of weed seeds in soil (Buhler et al., 1997, 2001). In a no-tillage system, weed seeds are concentrated in the upper 5 cm of the soil profile relative to conventional tillage systems (Cardina et al., 1991; Buhler, 1992; Clements et al., 1996), which allows small-seeded weeds such as pigweeds to emerge more easily from shallow depths (Webb et al., 1987; Buhler et al., 1996; Oryokot et al., 1997a; Ghorbani et al., 1999). Stimulation of germination and subsequent emergence following tillage was possibly due to increased soil aeration, improved soil-seed contact, and elevated soil temperatures (Litch and Al-Kaisi 2005; Leon and Owen 2006; Norsworthy and Oliveira 2007). Although high reduction of Amaranthus by the tillage system is promising, yet it is not sufficient. A control of approximately 90% is not satisfactory because in low densities the surviving Amaranthus plants produce more seeds per plant than in high densities (Burki et al., 1997).

Mowing is also not an effective option for control of various Amaranthus species because once mowed they bounces back and start growing prostrate, produces viable seed and complete their life cycle if mowing is not maintained. Therefore, herbicides can be a more effective option to control these species, than mowing.

Herbicides are cost-effective and efficient tools for weed control in modern agriculture. 2,4-D (Szmedra, 1997), Aminocyclopyrachlor (DuPont, 2009), Aminopyralid (Burch et al., 2005), Dicamba (Senseman et al., 2007), Pendimethalin (Malefyt and Duke, 1984), are some of the widely used herbicides for broadleaf weed control in the world. Early to midseason (early May to late June) herbicide applications (Dieleman et al. 1996) with an early crop canopy closure would be a promising strategy to manage different Amaranthus species (Jha et al., 2008a, 2008b). Steckel (2004) reported that Amaranthus species in various row crops can be best managed when a pre-applied herbicide is followed by a post applied herbicide or when a post applied herbicide is followed by another post applied herbicide, which contains a residual product. Although many currently labelled soil-applied herbicides like 2,4-D, Banvel[R] or Clarity[R] (dicamba), Cimarron[R] (metsulfiiron), Cimarron Max[R] (metsulfixron +2,4-D+ dicamba), Distinct[R] (diflufenzopyr + dicamba), Grazon P + D[R] (picloram +2,4-D), Milestone[R] (aminopyralid) or ForeFront R&P[R] (aminopyralid +2,4-D), Roundup[R] (glyphosate), Surmount[R] (picloram + fluroxypyr), and Weedmaster[R] (dicamba +2,4-D) provided good control of various Amaranthus species, difficulties in control have been reported (Fritz & Hartwig, 1986; Fuerst et al., 1986; Mayo et al, 1995; Grichar, 1997; Steckel, 2004; Green et al. 2006; Boyd, 2008). With proper herbicide selection, 90% control of A. albus, A. palmeri, A. retroflexus, and A. rudis is possible (Sweat et al., 1998). Besides, it may be necessary to make two to three herbicide applications to effectively control them all season long (Ferrell & Sellers, 2007).

Herbicide Resistance in Amaranthus Species

Sole reliance on herbicides has resulted in evolution of herbicide resistance in weeds (Heap, 2014). Herbicide-resistant biotypes of Amaranthus species have developed in many countries (Barralis & Gasquez, 1987). At least one Amaranthus weed species is reported to be resistant to one or more herbicide groups in twenty-nine U.S. States (Sellers et al., 2003). The presence of herbicide tolerance traits raises questions of how to address management issues at a grower, county, and regional level (Cardina et al., 1999). Evolution of resistance to glyphosate, the world's most widely used herbicide with effective broad-spectrum weed control, is a significant problem facing world agriculture (Powles, 2008; Webster & Sosnoskie, 2010).

Herbicide-resistant biotypes of A. blitum have been reported from North America, Europe and Asia which have developed resistance to different herbicides including ALS inhibitors, Atrazine and Paraquat (Itoh et al., 1992; Manley et al., 1996; Heap, 2014). A. palmeri has developed resistance to several different herbicides, including acetolactate synthase (ALS) inhibitors, dinitroaniline, glyphosate, imidazolinone, sulfonylurea, and triazine (Gossett et al, 1992; Horak & Peterson, 1995; Sprague et al., 1997; Peterson, 1999; Franssen et al. 2001a; Culpepper et al. 2006; Steckel et al., 2008; Vencill et al, 2008; Wise et al. 2009; Price et al., 2011; Heap, 2014). Also A. rudis and A. retroflexus biotypes with resistance to glyphosate, triazine or acetolactate synthase-inhibiting herbicides have been reported (Horak & Peterson, 1995; Peterson, 1999; Price et al, 2011). Prior to the recent occurrence of glyphosate resistance in A. palmeri (Culpepper et al, 2006; Mueller et al., 2006; York et al., 2007; Culpepper et al., 2008; Norsworthy et al. 2008; Steckel et al., 2008; Gaines et al., 2010; Price et al., 2011), glyphosate had been highly effective in controlling this weed (Scott et al., 2002; Norsworthy, 2004, 2005; Bond et al., 2006), which contributed to rapid adoption of glyphosate-resistant soybean and cotton throughout the southern United States. A survey of Georgia growers revealed that the presence of Glyphosate resistant A. palmeri in fields increased management costs by 58%, from $81 ha21 to $129 ha21 (Culpepper & Kichler, 2009).

A. tuberculatus have evolved resistance to five different chemical classes of herbicides: Acetolactate synthase (ALS) inhibitors (Horak & Peterson, 1995; Foes et al., 1998; Tranel & Wright, 2002; Patzoldt & Tranel, 2007; Nordby et al., 2010), glyphosate (Legleiter and Bradley 2008; Nandula et al. 2013), p-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors (Hausman et al. 2011), Photosystem II (PSII) inhibitors, also called triazines (Anderson et al., 1996), and Protoporphyrinogen oxidase (PPO) inhibitors (Shoup et al., 2003), furthermore its some populations have developed resistance to multiple herbicide classes (Tranel et al., 2004; Falk et al., 2005; Patzoldt et al., 2005; Mcmullan & Green, 2011; Bell et al., 2013). Simultaneously, ALS resistance appears to have no fitness cost in herbicide-free environments in some Amaranthus species (Sibony & Rubin, 2002; but see Tardif et al, 2006).

Biological Control of Amaranthus Species

Excessive use of herbicides has resulted in problems, including contamination of surface and groundwater resources (Guzzella et al., 2006; Spalding et al., 2003), and various human health risks (EPA, 2007). The growing public concerns about pesticide residues in our food and environment, and the increasing public pressure for more sustainable crop production methods have led to an increasing interest in integrated weed control strategies for Amaranthus species (Burki et al., 1997), based on mechanical, physical, or biological control. Numerous weedy Amaranthus species were chosen for biological control research within the framework of COST (European Cooperation in the Field of Scientific and Technical Research) Action (Muller-Scharer, 1993).

Various insects which are being promoted as biological control agents for various Amaranthus species (like A. caudatus, A. hybridus, A. retroflexus, A. spinosus, A. viridis) are: Disonycha glabrata (Garman, 1892; Vogt & Cordo, 1976; Balsbaugh et al., 1981; Tisler, 1990), Chaetocnema tibialis (Cagan et al., 2000), Cassida nigriventris and Coleophora versurella (Khan et al., 1978), Hypolixus truncatidus (Napompeth, 1982, 1989, 1992; Julien, 1992), Hypolixus nubilosus (Kolaib et al., 1986; Lopez et al., 2011; Torres et al., 2011 ; Kagali et al., 2013), Haplopeodes minutes (Spencer & Steyskal, 1986), Epicauta leopardina (Schuester, 1987), Melanagromyza amaranthi (Spencer & Havranek, 1989), Pellucidus Vittula (Cagan et al., 2000), and Hepertogramma bipunctalis (Lopez et al., 2011; Kagali et al., 2013).

Erwinia carotovora var. rhapontici (Kataryan, 1975; Mendoza & Rodriguez, 1990), Gliocladium virens (Howell & Stipanovie, 1984) are some examples of bacteria which can be used as bio control agents for various Amaranthus species. Promising pathogens that can be used as potential bioherbicide or mycoherbicide for Amaranthus include: Aposphaeria amaranthi (Mintz & Weidemann, 1992); Microsphaeropsis amaranthi (Ortiz Ribbing & Williams, 2006), Phomopsis amaranthicola (Charudattan, 1994; Ortiz Ribbing & Williams, 2006) and Alternaria alternata (Ghorbani et al., 2000).

Allelopathy may also be a useful means of biological weed control, especially when integrated into pest management systems, reducing dependence upon synthetic herbicides. A recent screening for inhibiting activity against A. spinosus identified several plants with herbicidal activity (Rizvi & Rizvi, 1992). Macharia and Peffiey (1995) reported the allelopathic effect of Allium fistidosum and A. cepa genotypes on plant growth and seed germination of A. spinosus. Aqueous extracts of tissues of some plants like Artemisia annua, Cirsium arvense, Fagopyrum esculentum, Helianthus annuus, Rumex crispus and Sorghum halepense decreases germination of A. retroflexus (Costea et al, 2004; Haramoto & Gallandt, 2005). Thus these plants can act as biological control agents.

The potential conflict of interest between the need for effective biological control of noxious Amaranthus species to prevent major crop losses and reduce herbicide application in systems of sustainable agriculture, on the one hand and the potential economic value of certain Amaranthus species as crops, on the other, needs to be discussed and resolved. The important point is that agents used in the inundative bio control of noxious Amaranthus species should not endanger crop Amaranthus (Burki et al., 1997).

Conclusion

The high nutritional value of Amaranthus seeds, functional potential, short lifecycle, rapid growth, adaptability to unfavourable climate and soil condition, drought tolerance and the food use of the entire plant is the reason for increasing research interest in this pseudocereal. This is a food for future crop for many developing countries, particularly in drought-prone areas of Africa and Asia. However, Genus Amaranthus consists of some of the troublesome invasive or noxious weeds of the world which are known to compete with many economic crops in different parts of the world resulting in great yield losses. Hence, extensive research is required to choose between the good and bad Amaranths.

https://doi.org/10.1007/s 12229-017-9194-1

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Appendix
Table 1 Accepted plant names of the species for the genus
Amaranthus with their synonyms

Name                       Synonyms

Amaranthus
  acanthobracteatus
  Henr.
Amaranthus acanthochiton   * Acanthochiton wrightii Torr.
  Sauer
Amaranthus x adulterinas
  Thell.
Amaranthus albus L.        * Amaranthus albus var. albus
                           * Amaranthus albus var. monosepalus Thell.
                           * Amaranthus albus var. parviflorus Moq.
                           * Amaranthus albus var. puberulus Thell.
                           * Amaranthus albus var. pubescens (Uline
                             & W.L.Bray) Fernald
                           * Amaranthus albus var. rubicundus Thell.
                           * Amaranthus gracilentus H.W.Kung
                           * Amaranthus graecizans Cutanda
                           * Amaranthus graecizans var. pubescens
                             Uline & W.L.Bray
                           * Amaranthus littoralis Homem.
                           * Amaranthus pubescens (Uline & W.L.Bray)
                             Rydb.
                           * Galliaria albida Bubani
                           * Glomeraria alba (L.) Cav.
Amaranthus anderssonii     * Scleropus urceolatus Andersson
  J.T.Howell
Amaranthus arenicola
  I.M.Johnst.
Amaranthus asplundii       * Amaranthus affinis Thell.
  Thell.                   * Amaranthus buchtienianus Thell.

Amaranthus atropurpureus
  Roxb.
Amaranthus aureus
  F.Dietr.
Amaranthus austratis       * Acnida alabamensis Standi.
  (A.Gray) Sauer
                           * Acnida australis A.Gray
                           * Acnida cannabina var. australis (A.Gray)
                             Uline & W.L.Bray
                           * Acnida cuspidate Bcrtcro ex Spreng.
Amaranthus bahiensis
  Mart.
Amaranthus bigelowii       * Amaranthus bigelowii var. emaiginatus
  Uline & W.L.Bray           (Torr.) Uline & W.L.Bray
                           * Satratia berlandieri var. emarginate
                             Torr.
Amaranthus blitoides       * Amaranthus blitoides var. crassius Jeps.
  S.Watson                 * Amaranthus blitoides var. densifolius
                             Uline & W.L.Bray
                           * Amaranthus blitoides var. halophilus
                             Aellen
                           * Amaranthus blitoides var. reverchonii
                             Uline & W.L.Bray
                           * Amaranthus reverchonii (Uline &
                             W.L.Bray) Kov.
                           * Galliaria blitoides Nicuwl.
Amaranthus blitum L.       * Albersia arenaria Schur
                           * Albersia ascendens Fourr.
                           * Albersia blitum Kunth
                           * Albersia livida Kunth
                           * Amaranthus adscendens auct.
                           * Amaranthus albus Rodschied ex F.Dietr.
                           * Amaranthus alius K.Krause
                           * Amaranthus ascendens Loisel.
                           * Amaranthus berchtoldii Scidl ex Opiz
                           * Amaranthus blitonius St.Lag.
                           * Amaranthus blitum var. ascendens
                             (Loisel.) DC.
                           * Amaranthus blitum var. blitum
                           * Amaranthus blitum var. polygonoides Moq.
                           * Amaranthus blitum subsp. Polygonoides
                             (Zoll. ex Moq.) Carretero
                           * Amaranthus diffusus Dulac.
                           * Amaranthus gangeticus Wall. [Invalid]
                           * Amaranthus graecizans var. blitum (L.)
                             Kuntze
                           * Amaranthus lividus Hook.f.
                             [Illegitimate]
                           * Amaranthus lividus subsp. ascendens
                             (Loisel.) Wacht.
                           * Amaranthus lividus subsp. Ascendens
                             (Loisel.) Heukels
                           * Amaranthus lividus var. ascendens
                             (Loisel.) Thell.
                           * Amaranthus lividus var. ascendens
                             (Loisel.) Hayw. & Drucc
                           * Amaranthus lividus subsp. lividus
                           * Amaranthus lividus var. polygonoides
                             (Moq.) Thell.
                           * Amaranthus lividus subsp. Polygonoides
                             (Moq.) Probst
                           * Amaranthus minor Gray
                           * Amaranthus mucronatus Poir.
                           * Amaranthus oleraceus Rodschied
                           * Amaranthus pallidas M.Bieb.
                           * Amaranthus polygonoides Zoll. Ex Moq.
                             [Invalid]
                           * Amaranthus prostratus T.Bastard
                             [Illegitimate]
                           * Amaranthus ruderalis Koch ex Moq.
                           * Amaranthus tenuiflorus Fisch. ex Moq.
                           * Amaranthus tenuifolius Roxb.
                           * Amaranthus viridis All. [Illegitimate]
                           * Blitum maius Scop.
                           * Euxolus alius (E.H.L.Krause)
                             E.H.L.Krause
                           * Euxolus ascendens (Loisel.) H.Hara
                           * Euxolus viridis var. ascendens (Loisel.)
                             Moq.
                           * Glomeraria blitum (L.) Cav.
                           * Pyxidium graecizans Moq.
Amaranthus blitum subsp.   * Albersia emarginate (A.Braun &
  emarginatus (Salzm.        C.D.Bouche) Asch. ex Hausskn.
  Ex Uline & Bary)         * Amaranthus ascendens var. polygonoides
  Carretero, Munoz Garm.   (Moq.) Thell.
  & Pedrol                 * Amaranthus ascendens subsp. polygonoides
                           (Moq.) Thell. ex Priszter
                           * Amaranthus blitum var. emaiginatus
                             (Moq. ex Uline & W.L.Bray) Lambinon
                           * Amaranthus emarginatus Salzm. ex Uline &
                             Bray
                           * Amaranthus emarginatus Salzm. ex Moq.
                           * Amaranthus lividus subsp. Ascendem
                             Heukels.
                           * Euxolus emarginatus A.Braun & C.D.Bouche
                           * Euxolus viridis var. polygonoides Moq.
Amaranthus blitum subsp.   * Albersia blinim var. oleraceus (L.)
  oleraceus (L.) Costea      Hook.f.
                           * Albersia oleraceo (L.) Kunth
                           * Amaranthus ascendens var. oleraceus (L.)
                             Thell. ex Priszter
                           * Amaranthus blitum var. oleraceus (L.)
                             Hook.f.
                           * Amaranthus circinnatus Poir.
                           * Amaranthus lividm L.
                           * Amaranthus lividm subsp. oleraceus (L.)
                             Soo
                           * Amaranthus lividus var. oleraceus (L.)
                             Thell.
                           * Amaranthus obtusiflorus (Mart.) Kov.
                           * Amaranthus officinalis Gromov ex Trautv.
                           * Amaranthus oleraceus L.
                           * Amaranthus olitorius Besscr
                           * Blitum lividum (L.) Mocnch
                           * Blitum oleraceum (L.) Mocnch
                           * Glomeraria livida (L.) Cav.
                           * Glomeraria oleraceo (L.) Cav.
                           * Pentrim oleraceus Raf.
Amaranthus blitum var.     * Amaranthus emarginatus subsp.
  pseuilogracilis            pseudogracilis (Thell.) Hugin
  (Thell.) Lambinon        * Amaranthm lividm f. pseudogracilis
                             Thell.
                           * Amaranthus pseudogracilis (Thell.)
                             G.H.Loos
Amaranthus brandegeei
  Standi.
Amaranthus brasiliensis
  Moq.
Amaranthus brownii
  Christoph. & Caum
Amaranthus x budensis
  Priszter

Amaranthus californiens    * Amaranthus albomarginatus Uline &
  (Moq.) S.Watson            W.L.Bray
                           * Mengea californica Moq.
Amaranthus campestiis
  Willd.
Amaranthus cannabinus      * Acnida cannabina L.
  (L.) Sauer               * Acnida cannabina var. concatenate Moq.
                           * Acnida cannabina var. cuspidate
                             (Bertero ex Spreng.) Moq.
                           * Acnida cannabina var. lanceolate Moq.
                           * Acnida cannabina var. salicifolia Moq.
                           * Acnida elliotii Raf.
                           * Acnida obtusifolia Raf.
                           * Acnida rhyssocarpa Spreng.
                           * Acnida ruscocarpa Willd.
                           * Acnida salicifolia Raf.
                           * Amaranthus macrocaulos Poir.
Amaranthus capensis        * Amaranthus capensis subsp. capensis
  Thell.                   * Amaranthus capensis subsp. Uncinatus
                             (Thell.) Brenan
                           * Amaranthus dinteri var. uncinatus Thell.
Amaranthus caracasanus     * Amaranthus coracanus Mart.
  Kunth
Amaranthus cardenasianus
  Hunz.
Amaranthus caturus Roxb.
Amaranthus caudatus L.     * Amaranthus abyssinicus L.H.Bailey
                           * Amaranthus alopecurus Hochst. ex A.Br. &
                             C.D.Bouche
                           * Amaranthus cararu Moq.
                           * Amaranthus caudatus var. albiflorus Moq.
                           * Amaranthus caudatus var. alopecurus Moq.
                           * Amaranthus caudatus subsp.
                             mantegazzianus (Pass.) Hanelt
                           * Amaranthus caudatus var. maximus (Mill.)
                             Moq.
                           * Amaranthus caudatus subsp. saueri
                             V.Jehlik
                           * Amaranthus dussii Sprengcr
                           * Amaranthus edulis Speg.
                           * Amaranthus edulis var. spadiceus Hunz.
                           * Amaranthus hybridus var. leucocarpus
                           (S.Watson) Hunz.
                           * Amaranthus leucocarpus S.Watson
                           * Amaranthus leucospermus S.Watson
                           * Amaranthus mantegazzianus Pass.
                           * Amaranthus maximus Mill.
                           * Amaranthus pendulinus Moq.
                           * Amaranthus pendulus Moq.
                           * Euxolus arvensis Rojas Acosta
Amaranthus celosioides     * Amaranthus hybridus var. pergaminensis
  Kunth                      Covas
Amaranthus centralis J.
  Palmer & Mo watt
Amaranthus chihuahensis
  S.Watson
Amaranthus clementii
  Domin
Amaranthus
  cochleitepalus Domin
Amaranthus commutatus
  A.Kern.

Amaranthus congestus
  C.C.Towns.
Amaranthus crassipes       * Amaranthus crassipes var. crassipes
  Schltdl.
Amaranthus crassipes       * Amaranthus warnockii I.M.Johnst.
  var. warnockii
  (I.M.Johnst.)
  Henrickson
Amaranthus crispus         * Albersia crispa Asch. Ex Hausskn.
  (Lesp. & Thevenau)       * Amaranthus cristidatus Speg.
  A.Terracc.               * Celosia crispus Lesp. & Thev.
                           * Euxolus crispus Lesp. & Thevenau
Amaranthus cruentus L.     * Amaranthus anacardana Hook.f.
                           * Amaranthus arardhanus Sweet
                           * Amaranthus carneus Moq.
                           * Amaranthus chlorostachys Moq.
                           * Amaranthus esculentus Besser ex Moq.
                           * Amaranthus farinaceous Roxb. ex Moq.
                           * Amaranthus guadeloupensis Voss
                           * Amaranthus guadelupensis Moq.
                           * Amaranthus hybridus subsp. cruentus (L.)
                             Thell.
                           * Amaranthus hybridus var. paniculatus
                             (L.) Uline & W.L.Bray
                           * Amaranthus hybridus var. patidus
                             (Bertol.) Thell.
                           * Amaranthus hybridus subsp. Patulus
                             (Bertol.) Carretero
                           * Amaranthus incarnates Moq.
                           * Amaranthus montevidensis Moq.
                           * Amaranthus paniculatus L.
                           * Amaranthus paniculatus var. cruentus
                             (L.) Moq.
                           * Amaranthus paniculatus var.
                             longispicatus Moq.
                           * Amaranthus paniculatus var. monstrosus
                             Moq.
                           * Amaranthus paniculatus var. sanguineus
                             (L.) Moq.
                           * Amaranthus paniculatus var. speciosus
                             L.H.Bailey
                           * Amaranthus paniculatus var. strictus
                             (Willd.) Moq.
                           * Amaranthus purgans Moq.
                           * Amaranthus rubescens Moq.
                           * Amaranthus sanguineus L.
                           * Amaranthus sanguinolentus Schrad. Ex
                             Moq.
                           * Amaranthus speciosus Sims
                           * Amaranthus spicatus Wirzen
                           * Amaranthus strictus Willd.
                           * Amaranthus violaceus Moq.
Amaranthus cuspidifolius
  Domin
Amaranthus deflexus L.     * Albersia deflexa (L.) Fourr.
                           * Albersia prostrate (Bastard) Kunth
                           * Amarantellus argentines Spcg.
                           * Amaranthus deflexus var. rufescens
                             (Godr.) Thell.
                           * Amaranthus deflexus f. rufescens (Godr.)
                             Thell. & Probst
                           * Amaranthus minor (Moq.) Senncn
                           * Amaranthus perennis Bellardi ex Colla
                           * Amaranthus prostrates Balb.
                           * Euxolus deflexus var. ascendens Moq.
                           * Euxolus deflexus var. major Moq.
                           * Euxolus deflexus var. minor Moq.
                           * Euxolus deflexus var. rufescens Godr.
                           * Galliaria prostrate (Bastard) Bubani
                           * Glomeraria deflexa (L.) Cav.
Amaranthus dinteri         * Amaranthus dinteri subsp.
  Schinz                     brevipetiolatus Brenan
                           * Amaranthus dinteri subsp. dinteri
Amaranthus dubius Mart,    * Amaranthus dubius var. flexuosus Thell.
  ex Thell.                * Amaranthus dubius var. leptostachys
                             Thell.
                           * Amaranthus dubius var. xanthostachys
                             Thell.
                           * Amaranthus tristis Willd.
                           * Amaranthus tristis vac. flexuosus Moq.
                           * Amaranthus tiistis var. xanthostachys
                             Moq.
Amaranthus fimbriatus      * Amblogyna flmbriata (Torr.) A.Gray
  (Torr.) Benth.           * Sarratia berlandieri var. denticulate
                             Torr.
                           * Sarratia berlandieri var. flmbriata
                             Torr.
Amaranthus floridanus      * Acnida floridana S.Watson
  (S.Watson) Sauer
Amaranthus furcatus
  J.T.Howell
Amaranthus globosa L.
Amaranthus graecizans L.   * Amaranthus angustifolius Lam.
                           * Amaranthus angustifolius M.Bicb. Ex
                             Willd.
                           * Amaranthus angustifolius subsp.
                             aschersonianus Thell.
                           * Amaranthus aschersonianus (Thell.)
                             Chiov.
                           * Amaranthus blitum Moq.
                           * Amaranthus blitum var. graecizans (L.)
                             Moq.
                           * Amaranthus blitum var. nanus Moq.
                           * Amaranthus graecizans subsp.
                             aschersonianus (Thell.) Costea. D.M.
                             Brenner & Tardif
                           * Amaranthus graecizans subsp. graecizans
                           * Amaranthus graecizans var. pachytepalus
                             Aellen
                           * Amaranthus graecizans subsp.
                             thellungianus (Nevski ex Vassilcz.)
                             Gusev
                           * Amaranthus hierichuntinus Vis.
                           * Amaranthus roxburgianus var.
                             aschersonianus (Thell.) N.C.Nair
                           * Amaranthus thellungianus Nevski ex
                             Vassilcz.
                           * Blitum graecizans (L.) Moench
                           * Galliaria graecizans (L.) Nieuwl.
                           * Glomeraria graecizans (L.) Cav.
Amaranthus graecizans      * Amaranthus angustifolius var. silvestris
  subsp. silvestris          (Villiers) Thell.
  (Vill.) Brenan           * Amaranthus angustifolius subsp.
                             silvestris (Vill.) Wacht.
                           * Amaranthus graecizans var. silvestris
                             (Vill.) Asch. & Schweinf.
                           * Amaranthus silvestris Vill.

Amaranthus grandiflorus
  (J.M.Black) J.M.Black

                           * Amaranthus mitchellii var. grandifloras
                             J.M.Black
Amaranthus greggii         * Amaranthus annectens S.F.Blake
  S.Watson                 * Amaranthus greggii var. muelleri Uline &
                             W.L.Bray
                           * Amaranthus muelleri (Uline & W.L.Bray)
                             Kov.
                           * Amaranthus myrianthus Standi.
Amaranthus haughtii
  Standl.
Amaranthus hunzikeri
  N.Bayon
Amaranthus hybridus L.     * Amaranthus aureus Moq.
                           * Amaranthus batalleri Sennen
                           * Amaranthus bellardii Moq.
                           * Amaranthus berehtholdii Moq.
                           * Amaranthus catechu Moq.
                           * Amaranthus chlorostachys Willd.
                           * Amaranthus chlorostachys var. hybridus
                             (L.) S.Watson
                           * Amaranthus cruentus var. patulus
                             (Bertol.) Lambinon
                           * Amaranthus eugenii Sennen
                           * Amaranthus flavescens Moq.
                           * Amaranthus flavescens Moq.
                           * Amaranthus hybridus f. aciculatus Thell.
                           * Amaranthus hybridus var. batalleri
                             (Sennen) Carretero
                           * Amaranthus hybridus var. bellardii Moq.
                           * Amaranthus hybridus var. chlorostachys
                             (Willd.) Beck
                           * Amaranthus hybridus var. chlorostachys
                             (Willd.) Thell.
                           * Amaranthus hybridus var. densus Farw.
                           * Amaranthus hybridus var. hecticus
                             (Willd.) Moq.
                           * Amaranthus hybridus subsp. hybridus
                           * Amaranthus hybridus var. hybridus
                           * Amaranthus hybridus subsp. Incurvatus
                             (Trimen ex Gren. & Gord.) Brcnan
                           * Amaranthus hybridus var. laetus (Willd.)
                             Moq.
                           * Amaranthus hybridus var. prostratus Moq.
                           * Amaranthus hybridus var. rubricaulis
                             Moq.
                           * Amaranthus hybridus var. sanguineus (L.)
                             Farw.
                           * Amaranthus incurvatus Trimen ex Gren. &
                             Gord.
                           * Amaranthus intermedins Guss. ex Moq.
                           * Amaranthus laetus Willd.
                           * Amaranthus laxiflorus Comclli ex Pollini
                           * Amaranthus neglectus Moq.
                           * Amaranthus nepalensis Moq.
                           * Amaranthus paniculatus var. sanguineus
                             Regel
                           * Amaranthus patulus Bertol.
                           * Amaranthus patulus f. multispicidatus
                             (Sennen) Priszter
                           * Amaranthus patulus var. multispiculatus
                             Sennen
                           * Amaranthus pseudoretroflexus (Thell.)
                             Almq.
                           * Amaranthus retroflexus var.
                             chlorostachys (Willd.) A.Gray
                           * Amaranthus retroflexus var. hybridus
                             (L.) A.Gray
                           * Amaranthus spicatus Rchb.
                           * Amaranthus timeroyi Jord. Ex Moq.
                           * Amaranthus trivialis Rota
                           * Galliaria hybrida (L.) Nieuwl.
                           * Galliaria patula Bubani
Amaranthus hybridus        * Amaranthus hybridus var. quitensis
  subsp. Quitensis           (Kunth) Covas
  (Kunth) Costea &         * Amaranthus quitensis Kunth
  Carretero                * Amaranthus quitensis f. rufescens Thell.
                           * Amaranthus quitensis var. stuckertianus
                             Thell.
                           * Amaranthus retroflexus subsp. Quitensis
                             (Kunth) O.Bolos & Vigo
Amaranthus                 * Amaranthus anardana Buch.Ham. ex Moq.
  hypochondriacus L.       * Amaranthus atrosanguineus Moq.
                           * Amaranthus aureus Besser
                           * Amaranthus bernhardii Moq.
                           * Amaranthus flavus L.
                           * Amaranthus frumentaceus Buch.Ham. Ex
                             Roxb.
                           * Amaranthus hybridus Veil.
                           * Amaranthus hybridus var. erythrostachys
                             Moq.
                           * Amaranthus hybridus f. hypochondiacus
                             (L.) B.L. Rob.
                           * Amaranthus hybridus f. hypochondriacus
                             (L.) H.Rob.
                           * Amaranthus hybridus var. hypochondriacus
                             (L.) H.Rob.
                           * Amaranthus hybridus subsp.
                             hypochondriacus (L.) Thell.
                           * Amaranthus hypochondriacus var.
                             macrostachys Moq.
                           * Amaranthus hypochondriacus var.
                             monstrosus Moq.
                           * Amaranthus hypochondriacus var.
                             racemosus Moq.
                           * Amaranthus hypochondriacus var.
                             tortuosus Moq.
                           * Amaranthus macrostachyus Merat ex Moq.
                           * Amaranthus monstrosus Moq.
Amaranthus                 * Amaranthus hybridus subsp. Powellii
  hypochondriacus var.       (S.Watson) Karlsson
  powellii (S.Watson)      * Amaranthus obovatus S.Watson
  Pedersen
Amaranthus induratus
  C.A.Gardner ex
  J.Palmer & Mowatt
Amaranthus interruptus     * Amaranthus lancifolius Delilc ex Moq.
  R.Br.                    * Amaranthus lineatus R.Br.
                           * Amaranthus rhombeus R.Br.
                           * Amaranthus spathulatus Desf. Ex Moq.
                           * Amaranthus undulates R.Br.
Amaranthus kloosianus
  Hunz.
Amaranthus leptostachyus
  Benth.
Amaranthus lepturus
  S.F.Blake
Amaranthus lomhardoi
  Hunz.
Amaranthus looseri
  Suess.
Amaranthus macmcarpus      * Amaranthus macrocarpus var. pallidus
  Benth.                     Benth.
Amaranthus minimus         * Goerziella minima (Standi.) Urb.
  Standi.
Amaranthus mitchellii      * Amaranthus mitchellii var. strictifolius
  Benth.                     Domin
Amaranthus muricatus       * Euxolus muricatus Gillies ex Moq.
  (Gillies ex Moq.)
  Hieron.
Amaranthus x ozanonii      * Amaranthus x ralletii Contre
  Piszter
Amaranthus pallidiflorus   * Amaranthus pallidiflorus var.
  F.Muell.                   viridiflorus Thell.
Amaranthus palmeri         * Amaranthus palmeri var. glomeratus
  S.Watson                   Uline & W.L.Bray
Amaranthus paolii Chiov.
Amaranthus paraguayensis
  Parodi
Amaranthus parvulus
  Peter
Amaranthus persimilis
  Hunz.
Amaranthus peruvianus      * Mengea peruviana Schauer
  (Schauer) Standi.
Amaranthus polygamus L.    * Albersia polygama Boiss.
                           * Amaranthus angustifolius subsp.
                             polygonoides polygonoides
                           * Amaranthus polygonoides Roxb.
                           * Amaranthus roxburgianus Nevski
                           * Amaranthus roxburgianus var.
                             angustifolius (Moq.) N.C.Nair
                           * Amaranthus tenuifolius Wall.
Amaranthus polygonoides    * Albersia polygonoides (L.) Kunth
  L.                       * Amaranthus berlandieri (Moq.) Uline &
                             W.L.Bray
                           * Amaranthus polygonoides subsp.
                             berlandieri (Moq.) Thell.
                           * Amaranthus taishanensis F.Z.Li & C.K.Ni
                           * Amaranthus verticillatus Pav. ex Moq.
                           * Amblogyna polygonoides (L.) Raf.
                           * Euxolus polygonoides Nakai
                           * Glomeraria polygonoides (L.) Cav.
                           * Roemeria polygonoides (L.) Mocnch
                           * Sarratia berlandieri Moq.
                           * Sarratia polygonoides (L.) Moq.
Amaranthus polystachyus    * Albersia polystachya Kunth
  Willd.
Amaranthus powellii        * Amaranthus chlorostachys var. powellii
  S.Watson                   (S.Watson) Priszter
                           * Amaranthus chlorostachys var.
                           pseudoretroflexus Thell.
                           * Amaranthus hybridus f. pseudoretroflexus
                             (Thell.) Thell.
                           * Amaranthus hybridus var.
                             pseudorettvflexus (Thell.) Carretero
                           * Amaranthus retroflexus var. powellii
                             (S.Watson) B.Boivin
                           * Amaranthus retroflexus var.
                             pseudoretroflexus (Thell.) B.Boivin
Amaranthus powellii        * Amaranthus bouchonii Thell.
  subsp. Bouchonii         * Amaranthus hybridus subsp. Bouchonii
  (Thell.) Costea &          (Thell.) O.Bolos & Vigo
  Carretero

Amaranthus praetermissus
  Brenan
Amaranthus pringlei
  S.Watson
Amaranthus pumilus Raf.
Amaranthus retroflexus     * Amaranthus bulgaricus Kov.
  L.                       * Amaranthus bullatus Besser ex Spreng.
                           * Amaranthus chlorostachys Willk.
                           * Amaranthus curvifolius Spreng.
                           * Amaranthus delilei Richt. & Loret
                           * Amaranthus johnstonii Kov.
                           * Amaranthus recurvatus Dcsf.
                           * Amaranthus retroflexus var. delilei
                             (Richt. & Loret) Thell.
                           * Amaranthus retroflexus subsp. delilei
                             (Richt. & Loret) Tzvelev
                           * Amaranthus retroflexus var. retroflexus
                           * Amaranthus retroflexus var. rubricaulis
                             Thell.
                           * Amaranthus retroflexus f. rubricaulis
                             Thell. Ex Probst
                           * Amaranthus retroflexus var. salicifolius
                             I.M.Johnst.
                           * Amaranthus rigidus Schult. Ex Steud.
                           * Amaranthus spicatus Lam.
                           * Amaranthus strictus Ten.
                           * Galliaria retroflexa (L.) Nieuwl.
                           * Galliaria scabra Bubani
Amaranthus rosengurttii
  Hunz.
Amaranthus roxburghianus   * Amaranthus blitum var. angustifolius
  H.W.Kung                   Moq.
Amaranthus scandens L.f.
Amaranthus scariosus       * Amaranthus floridus Benth.
  Benth.                   * Amblogyna scariosa (Benth.) A.Gray
                           * Sarratia scariosa (Benth.) Moq.

Amaranthus schinzianus
  Thell.
Amaranthus                 * Amaranthus scleranthoides f.
  scleranthoides             abingdonensis A.Stewart
  (Andersson) Andersson    * Amaranthus scleranthoides f.
                             albemarlensis A.Stewart
                           * Amaranthus scleranthoides f.
                             B.L.Rob. & Greenm.
                           * Amaranthus sclerantoides f.
                             abingdonensis Stewart
                           * Amaranthus sclerantoides f. hoodensis
                             B.L. Rob. & Greenm.
                           * Amaranthus sclerantoides f. rugulosus
                             Howell
                           * Euxolus scleranthoides Andersson
Amaranthus scleropoides    * Amaranthus blitoides var. scleropoides
  Uline & W.L.Bray           (Uline & W.L.Bray) Thell.

                           * Amaranthus blitoides f. scleropoides
                             (Uline & W.L.Bray) Thell. Ex Probst
Amaranthus x
  soproniensis Priszter
  & Karpati
Amaranthus
  sparganicephalus
  Thell.
Amaranthus spinosus L.     * Amaranthus spinosus var. basiscissus
                             Thell.
                           * Amaranthus spinosus var. circumscissus
                             Thell.
                           * Amaranthus spinosus var. indehiscens
                             Thell.
                           * Amaranthus spinosus f. inermis Lauterb.
                             & K.Schum.
                           * Amaranthus spinosus var. purpurascens
                             Moq.
                           * Amaranthus spinosus var. pygmaeus Hassk.
                           * Amaranthus spinosus var. rubricaulis
                             Hassk.
                           * Amaranthus spinosus var. viridicaulis
                             Hassk.
                           * Galliaria spitosa (L.) Nicuwl.
Amaranthus squamulahts     * Amaranthus squarrulosus (Andersson)
  (Andersson) B.L.Rob.       Uline & W.L.Bray
                           * Scleropus squamulatus Andersson
Amaranthus standleyanus    * Amaranthus parodii Standi.
  Parodi ex Covas          * Amaranthus vulgatissimus var.
                             sublanceolatus Thell.
Amaranthus tamariscinus    * Acnida tamariscina (Nutt.) Alph.Wood
  Nutt.
Amaranthus tamaulipensis
  Henrickson
Amaranthus tenuifolius     * Mengea tenuifolia Moq.
  Willd.
Amaranthus x texensis
  Henrickson
Amaranthus thellungianus
  Nevski
Amaranthus thunbergii      * Amaranthus albus Thunb.
  Moq.
Amaranthus tricolor L.     * Amaranthus amboinicus Buch.Ham. ex Wall.
                           * Amaranthus bicolor Nocca ex Willd.
                           " Amaranthus cuspidatus Vis.
                           * Amaranthus dubius Mart.
                           * Amaranthus flexuosus Moq.
                           * Amaranthus gangeticus L.
                           * Amaranthus gangeticus var. angustior
                             L.H.Bailey
                           * Amaranthus gangeticus var. angustior
                             Bailey
                           * Amaranthus inamoenus Willd.
                           * Amaranthus incomptus Willd.
                           * Amaranthus japonicas Houtt. Ex Willd.
                           * Amaranthus japonicas Houtt. Ex Stcud.
                           * Amaranthus lanceolatus Roxb.
                           * Amaranthus lancifolius Roxb.
                           * Amaranthus lividus Roxb.
                           * Amaranthus mangostanus L.
                           * Amaranthus mangostanus Blanco
                           * Amaranthus melancholicus L.
                           * Amaranthus melancholicus var. obovatus
                             Moq.
                           * Amaranthus melancholicus var.
                             parvifolius Moq.
                           * Amaranthus melancholicus var. tricolor
                             (L.) Lam. ex Moq.
                           * Amaranthus mucronatus Hook.f.
                           * Amaranthus oleraceus Roxb.
                           * Amaranthus polygamus Roxb.
                           * Amaranthus polygamus Thwaites
                           * Amaranthus rotundifolius Moq.
                           * Amaranthus salicifolius H.J.Veitch
                           * Amaranthus tricolor var. gangeticus
                             (L.) Fiori
                           * Amaranthus tricolor var. mangostanus
                             (L.) Aellen
                           * Amaranthus tricolor var. melancholicus
                             (L.) Lam.
                           * Amaranthus tricolor var. tristis
                             (Willd.) Mchrotra, Aswal & Bisht
                           * Amaranthus tricolor var. tristis (L.)
                             Thell.
                           * Amaranthus tristis L.
                           * Amaranthus tristis Wall.
                           * Amaranthus tristis var. leptostachys
                             Moq.
                           * Blitum gangeticum Moench
                           * Blitum melancholicum Moench
                           * Glomeraria bicolor Cav. Ex Moq.
                           * Glomeraria tricolor (L.) Cav.
                           * Pyxidium gangeticum Moq.
                           * Pyxidium melancholicum Moq.
Amaranthus tuberculatus    * Acnida altissima Moq.
  (Moq.) Sauer             * Acnida altissima var. prostrate (Uline &
                             W.L.Bray) Fcmald
                           * Acnida altissima var. subnuda (S.Watson
                             ex A.Gray) Fcmald
                           * Acnida cannabina var. prostrate (Uline
                             & W.L. Bray) Fcrnald
                           * Acnida cannabina var. subnuda (S.
                             Watson) Fernald
                           * Acnida concatenate (Moq.) Small
                           * Acnida subnuda (S.Watson) Standi.
                           * Acnida tamariscina var. concatenate
                             (Moq.) Uline & W.L.Bray
                           * Acnida tamariscina var. prostrate Uline
                             & W.L.Bray
                           * Acnida tamariscina var. subnuda
                             (S.Watson) J.M.Coult.
                           * Acnida tamariscina var. tuberculata
                             (Moq.) Uline & W.L.Bray
                           * Acnida tuberculata Moq.
                           * Acnida tuberculata var. prostrate
                             (Uline & W.L.Bray) Lunell
                           * Acnida tuberculata var. prostrate
                             (Uline & W.L. Bray) B.L. Rob.
                           * Acnida tuberculata var. subnuda
                             S.Watson
                           * Amaranthus altissimus Riddell
                           * Amaranthus ambigens Standi.
                           * Amaranthus cannabinus var. concatenates
                             Moq.
                           * Amaranthus miamiensis Riddell
                           * Amaranthus rudis J.D.Sauer
                           * Amaranthus tuberculatus var. prosfrates
                             (Uline & W.L.Bray) Mohlenbr.
                           * Amaranthus tubercidatus var. rudis
                             (J.D.Sauer) Costea & Tardif
                           * Amaranthus tuberculatus var. subnudus
                             (S.Watson) Mohlenbr.
                           * Montelia tamariscina (Nutt.) A. Gray
                           * Montelia tamariscina var. concatenate
                             (Moq.) A. Gray

Amaranthus tucsonensis
  Henrickson
Amaranthus urceolatus      * Amaranthus jonesii (Uline & W.L.Bray)
  Benth.                     Kov.
                           * Amaranthus urceolatus var. jonesii
                             Uline & W.L.Bray
                           * Amaranthus urceolatus var. obcordatus
                             Uline & W.L.Bray
                           * Amblogyna urceolata (Benth.) Andersson
                           * Amblogyna urceolata (Benth.) A.Gray
                           * Amblogyna urceolata var. obcordata
                             A.Gray
                           * Euphorbia gracilis Pav.ex Moq.
                           * Sarratia urceolata Moq.
Amaranthus venulosus       * Amaranthusfimbriatus var. denticulatus
  S.Watson                   Uline & W.L.Bray
Amaranthus viridis L.      * Albersia caudate (Jacq.) Boiss.
                           * Albersia gracilis (Dcsf.) Webb &
                             Berthel.
                           * Amaranthus acutilobus Uline & W.L.Bray
                           * Amaranths fasciatus Roxb.
                           * Amaranthus gracilis Dcsf.
                           * Amaranthus littoralis Bernh. Ex Moq.
                           * Amaranthus polystachyus Buch.Ham. Ex
                             Wall.
                           * Chenopodium caudatum Jacq.
                           * Euxolus caudatus (Jacq.) Moq.
                           * Euxolus caudatus var. gracilis Moq.
                           * Euxolus caudatus var. maximus Moq.
                           * Galliaria adscendens Bubani
                           * Glomeraria viridis (L.) Cav.
                           * Pyxidium viride (L.) Moq.
Amaranthus vulgatissimus   * Amaranthus ataco Thell.
  Speg.
Amaranthus watsonii        * Amaranthus torreyi (A.Gray) Benth. ex
  Standl.                    S.Watson
                           * Amaranthus torreyi f. prostrates Farw.
                           * Amaranthus torreyi var. suffruticosus
                             Uline & W.L.Bray
                           * Amblogyna torreyi A.Gray

Amaranthus wriglitii
  S.Watson

Source: http://www.theplantist.org/browse/A/Amaranthaceae/
Amaranthus/ (Accessed on November 20, 2016)

Table 2 Chromosome number(s) of various Amaranthus species

Amaranthus        Chromosome     Ploidy     Reference
species             number

A. albus              32        Diploid     Heiser and Whitaker
                                              (1948)
                      34                    Grant (1959b)
                                            Sharmaand Banik (1965)
                                            Mulligan (1984)
                                            Song et al. (2002)
A. arenicola          32        Diploid     Grant(1959b)
A. asplundii          34        Diploid     Grant(1959b)
A.                    34        Diploid     Sharma and Banik (1965)
  atropurpureus
A. aureus             32        Diploid     Grant(1959b)
                      34                    Milan (2008)
A. australis          32        Diploid     Murray (1940)
                                            Grant(1959b)
A. blitoides          32        Diploid     Song et al. (2002)
                                            Sheidai and Mohammadzadeh
                                              (2008)
A. blitum             28        Diploid     Takagi(1933)
                      34                    Murray (1940)
                                            Grant (1959b)
                                            Pal et al. (2000)
                                            Srivastava and Roy (2012)
A. caturus            64       Tetraploid   Behera and Patnaik (1974)
A. caudatus           30        Diploid     Takagi (1933)
                      32                    Grant (1959b)
                      34                    Behera and Patnaik (1974)
                                            National Research Council
                                              (1989)
                                            Greizerstein and Poggio
                                              (1995)
                                            Song et al. (2002)
                                            Ramesh and Kumar (2009)
                                            Bonasora et al. (2013)
A. crispus            34        Diploid     Grant (1959b)
A. cruentus           32        Diploid     Takagi (1933)
                      34                    Grant (1959b)
                                            National Research Council
                                              (1989)
                                            Greizerstein and Poggio
                                              (1995)
                                            Song et al. (2002)
                                            Lanta et al. (2003)
                                            Milan (2008)
                                            Ramesh and Kumar (2009)
                                            Bonasora et al. (2013)
A. deflexus           34        Diploid     Grant(1959b)
A. dubius             64       Tetraploid   Grant (1959a) Pal (1971)
                                            Madhusoodanan and Nazeer
                                              (1983)
                                            Greizerstein and Poggio
                                              (1992)
                                            Greizerstein et al.
                                              (1997)
A. fimbriatus         34        Diploid     Ward and Spellenberg
                                              (1986)
A. graecizans         32        Diploid     Heiser and Whitaker
                                              (1948)
                      34                    Grant(1959b)
                                            Pal (1972)
A. hybridus           32        Diploid     Covas and Schnack (1946)
                      34                    Grant (1959b)
                                            Behera and Patnaik (1974)
                                            Weaver and McWilliams
                                              (1980)
                                            Pal et al. (1982)
                                            National Research Council
                                              (1989)
                                            Song et al. (2002)
                                            Ramesh and Kumar (2009)
A.                    32        Diploid     Grant(1959b)
  hypochon-
  driacus
                      34                    Pal and Khoshoo (1974)
                                            Pal et al. (1982)
                                            National Research Council
                                              (1989)
                                            Greizerstein and Poggio
                                              (1995)
                                            Song et al. (2002)
                                            Milan (2008)
                                            Ramesh and Kumar (2009)
                                            Bonasora et al. (2013)
A. mangostanus        32        Diploid     Takagi(1933)
                      34                    Behera and Patnaik (1974)
A. muricatus          34        Diploid     Covas and Hunziker (1954)
A. palmeri            32        Diploid     Heiser and Whitaker (1948)
                      34                    Grant (1959a)
                                            Grant (1959b)
                                            Pal et al. (1982)
                                            Mulligan (1984) Raybum et
                                              al. (2005)
                                            Gaines et al. (2012)
A. polygonoides       32        Diploid     Song et al. (2002)
                      34
A. powellii           32        Diploid     Murray (1940)
                      34                    Grant (1959b)
                                            Pal and Khoshoo (1974)
                                            Weaver and McWilliams
                                              (1980)
A. retroflexus        32        Diploid     Murray (1940)
                      34                    Heiser and Whitaker
                                              (1948)
                                            Grant(1959b)
                                            Weaver and McWilliams
                                              (1980)
                                            Mulligan (1984)
                                            Song et al. (2002)
                                            Lanta et al. (2003)
A.                    34        Diploid     Song et al. (2002)
  rvxburghianus
A. spinosus         n= 17                   Koul et al. 1976
                                            Behera and Patnaik 1977
                                            Behera and Patnik 1982
                      34        Diploid     Takagi(1933)
                                            Murray (1940)
                                            Grant (1959b)
                                            Behera and Patnaik (1974)
                                            Baquarand Oiusi (1988)
                                            Greizerstein and Poggio
                                              (1995)
                                            Al-Turki et al. (2000)
                                            Song et al. (2002)
                                            Raybum et al. (2005)
                                            Srivastava and Roy (2012)
A. standleyanus       34        Diploid     Grant(1959b)
                                            Covas and Hunziker (1954)
A. tamariscinus       32        Diploid     Grant(1959b)
A. tenuifolius        28        Diploid     Pal et al. (2000)
A. tricolor           34        Diploid     Takagi (1933)
                                            Grant (1959b)
                                            Sharma and Banik (1965)
                                            Behera and Patnaik (1974)
                                            Madhusoodanan and Nazeer
                                              (1983)
                                            Song et al. (2002)
                                            Srivastava and Roy (2012)
A. tuberculatus       32        Diploid     Murray (1940)
                                            Grant (1959b)
                                            Wetzel et al. (1999b)
                                            Franssen et al. (2001b)
                                            Steinau et al. (2003)
                                            Trucco et al. (2007)
A. viridis            32        Diploid     Covas and Hunziker (1954)
                      34                    Grant (1959b)
                                            Sharma and Banik (1965)
                                            Behera and Patnaik (1974)
                                            Madhusoodanan and Nazeer
                                              (1983)
                                            Greizerstein and Poggio
                                              (1995)
                                            Song et al. (2002)

Table 3 Effect of temperature on seed germination of Amaranthus
species

Parameters    Amaranthus       Results (Optimum     Reference
              species          germination)

Effect of     A. blitum        35[degrees]C         Teitzetal. (1990)
temperature   A. caudatus      35[degrees]C         Gutterman et al.
                                                      (1992)
                                                    Kepczynski and
                                                      Bihun (2002)
              A. cruentus      30 to 35[degrees]C   Oladiran and
                                                      Mumford (1985)
              A. gangeticus    30 to 35[degrees]C   Oladiran and
                                                      Mumford (1985)
              A. hybridus      32 to 34[degrees]C   Washitani and
                                                      Takenaka (1984)
                                                    Oladiran and
                                                      Mumford (1985)
              A. palmeri       25 to 40[degrees]C   Wright et al.
                                                      (1999)
                                                    Guo and Al-Khatib
                                                      (2003)
                                                    Jha et al.
                                                      (2008c)
                                                    Jha et al. (2010)
              A. paniculatus   30 to 35[degrees]C   Oladiran and
                                                      Mumford (1985)
              A. retroflexus   25 to 40[degrees]C   Evans (1922)
                                                    McWilliams et al.
                                                      (1968)
                                                    Baskin and Baskin
                                                      (1977)
                                                    Habib and Morton
                                                      (1987)
                                                    Kepczynski et al.
                                                      (1996)
                                                    Ghorbani et al.
                                                      (1999)
                                                    Guo and Al-Khatib
                                                      (2003)
              A. rudis         25 to 40[degrees]C   Guo and Al-Khatib
                                                      (2003)
              A. spinosus      30 to 35[degrees]C   Steckel et al.
                                                      (2004)
                                                    Chauhan and
                                                      Johnson (2009)
              A. viridis       30[degrees]C         Thomas et al.
                                                      (2006)
                                                    Chauhan and
                                                      Johnson (2009)

Table 4 Effect of various environmental parameters on seed
germination of Amaranthus species

Parameters    Amaranthus species   Results (Optimum germination)

Effect of     A. albus             1.9 cm
seed burial   A. graecizans
depth         A. hybridus
              A. palmeri
              A. retroflexus       0.5 cm to 4 cm

              A. spinosus          0.5 to 2 cm

              A. viridis           0.5 to 2 cm

Effect of     A. palmeri           peak germination occurred
duration                             after 9 months of burial

of seed       A. patidus           remain viable up to 3 years
burial        A. retroflexus       peak germination occurred

                                     after 9 months of burial

              A. retroflexus       remain viable for 6-10 years

              A. retroflexus       germinated even after 40 years
                                     of burial

Parameters    Amaranthus species   Reference

Effect of     A. albus             Santelmann and Evetts (1971)
seed burial   A. graecizans
depth         A. hybridus
              A. palmeri
              A. retroflexus       Wiese and Davis (1967)
                                   Baskin and Baskin (1977)
                                   Ghorbani et al. (1999)
                                   Omami et al. (1999)
                                   Benvenuti et al. (2001)

              A. spinosus          Santelmann and Evetts (1971)
                                   Chauhan and Johnson (2009)

              A. viridis           Thomas et al. (2006)
                                   Chauhan and Johnson (2009)

Effect of     A. palmeri           Jha et al. (2010)
duration

of seed       A. patidus           Washitani (1985)
burial        A. retroflexus       Baskin and Baskin (1985b)

                                   Baskin and Baskin (1998)
                                   Omami et al. (1999)

              A. retroflexus       Chepil (1946)
                                   Burnside et al. (1981)
                                   Burnside et al. (1996)

              A. retroflexus       Telewski and Zeevaart (2002)

Table 5 Allelopathic effect of Amaranthus species on other
plants

Amaranthus       Allelopathic               Effect
Species          effect against

A. blitoides     Triticum aestivum          * Reduced germination,
A. gracilis                                 coleoptile length, root
                                            length, root dry weight.

                                            plant height, grain and
                                            straw yield.

A. hybridus      Phaseolus spp.             * Negative effect on
                                            total chlorophyll
                                            content, number of
                                            developed leaves, stem
                                            length, and total plant
                                            dry matter.

A. palmeri       A. palmeri seeds           * Inhibit growth and seed
                 Allium cepa                germination.
                 Brassica oleracea var.
                 Capitata

                 Daucus carrota
                 Lycopersicon lycopersici
                 Sorghum bicolour

A. retroflexus   Beta vulgaris              * Decrease chlorophyll
                                              content.
                 Brassica oleracea var.     * Decrease nitrogen
                 brotrytis                    fixation of nodules.
                 Capsicum annuum            * Decrease respiration.
                 Cucumis sativus            * Decrease RGR, NAR and
                                              FWR.
                 Cucurbita ovifera          * Inhibit radicle and
                 Daucus carota                hypocotyl elongation.
                 Helianthus annus           * Interfere in
                                              photosynthesis.
                 Lycopersicon lycopersici   * Reduce coleoptile
                 Solanum melongena            length, root length,
                 Carthamus tinctorius         root dry weight, plant
                                              height, grain and straw
                                              yield.
                                            * Reduce germination,
                                              nutrient uptake, growth
                                              and yield.
                 Glycine max

                 Gossypium hirsutum
                 Hordeum vulgare

                 Lactuca sativa
                 Nicotiana tabacum

                 Phaseolus vulgaris

                 Soighum bicolour
                 Triticum aestivum

                 Zea mays

A. spinosus      Cultures                   * Negative impact on
                                              growth.
                 Lactuca sativa             * Reduce time duration of
                 Parthenium                   vegetative and
                 hysterophorus                reproductive phases.
                 Sinapis alba
                 Triticum aestivum

A. viridis       Pennisetum americanum      * Decrease growth and
                                              productivity

Amaranthus       Allelopathic               Reference
Species          effect against

A. blitoides     Triticum aestivum          Qasem (1995b)
A. gracilis

A. hybridus      Phaseolus spp.             Amini et al. (2013)

A. palmeri       A. palmeri seeds           Altieri and Doll (1978)
                 Allium cepa                Bradow and Connick (1987)
                 Brassica oleracea var.     Connick et al. (1987)
                 Capitata                   Menges (1987)

                 Daucus carrota             Menges (1988)
                 Lycopersicon lycopersici
                 Sorghum bicolour

A. retroflexus   Beta vulgaris              Bhowmik and Doll (1980)

                 Brassica oleracea var.     Qasem (1995a)
                 brotrytis                  Alam et al. (2001)
                 Capsicum annuum            Souza et al. (2011)
                 Cucumis sativus

                 Cucurbita ovifera
                 Daucus carota
                 Helianthus annus

                 Lycopersicon lycopersici
                 Solanum melongena
                 Carthamus tinctorius       Williams et al. (2005)

                                            Rezaie and Yamia (2009)

                 Glycine max                Bhowmik and Doll (1982)
                                            Bhowmik and Doll (1984)
                                            Chaniago et al. (2006)
                 Gossypium hirsutum         Mungeret al. (1983)
                 Hordeum vulgare            Lolas (1981)
                                            Lolas (1986)
                                            Qasem (1994)
                 Lactuca sativa             Dos Santos et al. (2004)
                 Nicotiana tabacum          Lolas (1981)
                                            Lolas (1986)
                 Phaseolus vulgaris         Aguyoh and Masiunas
                                              (2003)
                 Soighum bicolour           Bhowmik and Doll (1983)
                 Triticum aestivum          Qasem (1995b)
                                            Shahrokhi et al. (2012)
                 Zea mays                   Bhowmik and Doll (1982)
                                            Bhowmik and Doll (1984)

A. spinosus      Cultures                   Suma (1998)

                 Lactuca sativa             Shrefleret al. (1996)
                 Parthenium                 Chikkalingaiah and
                 hysterophorus                Mahadevappa (1998)
                 Sinapis alba               Datta and Bandyopadhayay
                 Triticum aestivum            (1981)

A. viridis       Pennisetum americanum      Singhal and Sen (1981)

Web-Links

http://www.theplantlist.org/1.1/browse/A/Amaranthaceae/
Amaransthus/ (Accessed on November 20, 2016).


Rezwana Assad (1) * Zafar A. Reshi (1) * Snober Jan (1) * Irfan Rashid (1,2)

(1) Department of Botany, University of Kashmir, Srinagar. J&K 190 006, India

(2) Author for Correspondence: e-mail: ecoirfan@yahoo.co.in Published online: 26 October 2017
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