Amaranthus is a genus taxonomically complex because of its high morphological variability, which led to nomenclatural disorders, misapplication of names, and misidentifications. Floristic and taxonomic studies on this genus are still incomplete, and many questions remain open. Seed micromorphology has been shown to play an important role in the taxonomy of plants. Regarding Amaranthaceae and Amaranthus, investigations are rare, and they refer to one or a few species. With the primary aim to test if seed features are helpful in the taxonomy of Amaranthus, we here present a detailed SEM study on seed micromorphology in 25 Amaranthus taxa using morphometric methods. Seeds were collected from field surveys and herbarium specimens; 14 seed coat features (7 qualitative and 7 quantitative) were measured on 111 samples (up to 5 seeds per sample). The results obtained revealed that seeds micromorphology provides interesting new taxonomic data concerning some taxa (species and below ranks). In fact, we were able to distinguish a few seed types, including one or more taxa, i.e., blitum-type, crassipes-type, deflexus-type, tuberculatus-type, and viridis-type. On the other hand, seed features are not useful for other species, for example, those included in the deflexus-type (A. deflexus, A. vulgatissimus, A. cacciatoi, A. spinosus, A. dubius, and A. stadleyanus). A diagnostic key of the studied taxa is proposed. Subgenera cannot be distinguished using seed features, thus confirming the published molecular data. All these facts reveal, once again, the taxonomic complexity of the genus Amaranthus since, e.g., just a few seed types can be defined.

Six Amaranthus species (A. cruentus, A. hybridus, A. hypochondriacus, A. muricatus, A. tuberculatus, and A. viridis) were collected in Italy (wild habitats) from crops and roadsides. Amaranth seed oil was extracted to obtain fractions rich in squalene. Squalene, free fatty acid, tocopherol, and sterol composition and content were investigated in detail. An analysis of variance and principal components was performed. The oil content in the seed ranged from 5.17% (A. muricatus) to 12.20% (A. tuberculatus). The quantity of squalene in the oil varied from 3.43% (A. muricatus) to 6.09% (A. hypochondriacus). The primary sterols were beta-sitosterol, brassicasterol, campesterol, and stigmasterol. The main tocopherols in all the samples were alfa-tocopherol, beta-tocopherol, and delta-tocopherol. Our results exhibited that the smallest seeds (A. tuberculatus) have the highest percentages of oil and squalene, whereas the largest seeds size (A. muricatus) show the lowest percentages. There is also evidence that the samples growing at lower altitudes show the highest concentration of fatty acids. According to our results, the six wild Amaranthus species exhibited similar characteristics to commercial species. This study confirms that the site of the collection has an impact on the oil and squalene content of the Amaranthus species.

The occurrence and invasion status of Amaranthus hypochondriacus in Belarus, Estonia, Italy, the Netherlands, and the North Caucasus were discussed. For Italy, we change the status from casual to naturalized based on living populations which are able to sustain themselves for 5 and over 20 years. Concerning the other countries, we clarified the invasion status of the species (inconsistently reported in literature) indicating it as a casual alien in Belarus, Estonia, and the North Caucasus and naturalized in the Netherlands. Climatic data of the European stations in which Amaranthus hypochondriacus was found were compared with those referring to the native distribution area (Mexico and southeastern regions of the US). The occurrence of the species in Europe appears to be facilitated by the temperate climate (Dutch and Italian localities), which also characterizes the native distribution area (N-America). The occurrence of the species in Estonia, the Netherlands, and the North Caucasus is interesting. In fact, although the types of climates (“Cold, Dfb” and “Temperate, Cfb”) occur in America (northeastern US and central Mexico), there are some differences in precipitation and temperature values, i.e. lower mean precipitation [644.1 mm (Estonia) vs. 1119 mm (N-America); climate Dfb] and lower mean temperature [9.7–11.1 °C (Netherlands and North Caucasus) vs. 15.3 °C (Mexico); climate Cfb].

Amaranthus emarginatus belongs to the Amaranthus blitum complex, a critical taxonomic group. Within the variability of A. emarginatus, two taxa are often accepted, ‘emarginatus’ and ‘pseudogracilis’, which differ by habitus, synflorescence structure, and leaf size. However, their taxonomic value is debated, and A. emarginatus itself is sometimes included in A. blitum as subspecies. In this paper, molecular, morphological, phytogeographical, and ecological data are discussed to assess the most correct rank of these taxa. We conclude that A. emarginatus deserves specific distinctness from A. blitum, and A. pseudogracilis can be regarded as a subspecies of the former. This choice reflects both the molecular data and geographical distributions of A. blitum and A. emarginatus, i.e., Mediterranean area, Europe, and North Africa for the former vs. neotropics for the latter). Within A. emarginatus, in addition, to the different morphology, A. emarginatus var. pseudogracilis differs also on account of its ecological preferences, because it occurs in warm and humid areas, whereas A. emarginatus var. emarginatus is better adapted to cold and dry habitats. However, since molecular analyses cannot clearly separate the two taxa, we consider the subspecific rank as the most appropriate. We also found a further morphological difference in seed coat ornamentation, hitherto not pointed out: the shape of the peripheral cells is tetragonal in A. emarginatus subsp. pseudogracilis, but polygonal (cells with ≥ 5 sides) in subsp. emarginatus. Distributional maps and status assessment at regional level for the two taxa are provided. According to this revision, A. emarginatus subsp. emarginatus is to be regared as naturalized in central and southern Italy (Latium and Campania, respectively).