Amaranthus emarginatus s.lat. (Amaranthaceae) in Italy

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).