Allium
Allium is a large genus of monocotyledonous flowering plants with around 1000 accepted species, making Allium the largest genus in the family Amaryllidaceae and among the largest plant genera in the world. Many of the species are edible, and some have a long history of cultivation and human consumption as a vegetable including the onion, garlic, scallions, shallots, leeks, and chives.
Allium species occur in temperate climates of the Northern Hemisphere, except for a few species occurring in Chile, Brazil, and tropical Africa. They vary in height between. The flowers form an umbel at the top of a leafless stalk. The bulbs vary in size between species, from small to rather large. Some species develop thickened leaf-bases rather than forming bulbs as such.
Carl Linnaeus first described the genus Allium in 1753. The generic name Allium is the Latin word for garlic, and the type species for the genus is Allium sativum which means "cultivated garlic". The decision to include a species in the genus Allium is taxonomically difficult, and species boundaries are unclear. Estimates of the number of species are as low as 260, and as high as 979. In the APG III classification system, Allium is placed in the family Amaryllidaceae, subfamily Allioideae. In some of the older classification systems, Allium was placed in Liliaceae. Molecular phylogenetic studies have shown this circumscription of Liliaceae is not monophyletic.
Various Allium species have been cultivated from the earliest times. About a dozen species are economically important as crops, or garden vegetables, and an increasing number of species are important as ornamental plants. Plants of the genus produce chemical compounds, mostly derived from cysteine sulfoxides, that give them a characteristic onion or garlic taste and odor. Many are used as food plants, though not all members of the genus are equally flavorful. In most cases, both bulb and leaves are edible. The characteristic Allium flavor depends on the sulfate content of the soil the plant grows in. In the rare occurrence of sulfur-free growth conditions, all Allium species completely lose their usual pungency.
Description
The genus Allium is characterised by herbaceous geophyte perennials with true bulbs, some of which are borne on rhizomes, and an onion or garlic odor and flavor.The bulbs are solitary or clustered and tunicate and the plants are perennialized by the bulbs reforming annually from the base of the old bulbs, or are produced on the ends of rhizomes or, in a few species, at the ends of stolons. A small number of species have tuberous roots. The bulbs' outer coats are commonly brown or grey, with a smooth texture, and are fibrous, or with cellular reticulation. The inner coats of the bulbs are membranous.
Many alliums have basal leaves that commonly wither away from the tips downward before or while the plants flower, but some species have persistent foliage. Plants produce from one to 12 leaves, most species having linear, channeled or flat leaf blades. The leaf blades are straight or variously coiled, but some species have broad leaves, including A. victorialis and A. tricoccum. The leaves are sessile, and very rarely narrowed into a petiole.
The flowers, which are produced on scapes are erect or in some species pendent, having six petal-like tepals produced in two whorls. The flowers have one style and six epipetalous stamens; the anthers and pollen can vary in color depending on the species. The ovaries are superior, and three-lobed with three locules.
The fruits are capsules that open longitudinally along the capsule wall between the partitions of the locule. The seeds are black, and have a rounded shape.
The terete or flattened flowering scapes are normally persistent. The inflorescences are umbels, in which the outside flowers bloom first and flowering progresses to the inside. Some species produce bulbils within the umbels, and in some species, such as Allium paradoxum, the bulbils replace some or all the flowers. The umbels are subtended by noticeable spathe bracts, which are commonly fused and normally have around three veins.
Some bulbous alliums increase by forming little bulbs or "offsets" around the old one, as well as by seed. Several species can form many bulbils in the flowerhead; in the so-called "tree onion" or Egyptian onion the bulbils are few, but large enough to be pickled.
Many of the species of Allium have been used as food items throughout their ranges. There are several unrelated species that are somewhat similar in appearance to Alliums but are poisonous, but none of these has the distinctive scent of onions or garlic.
Taxonomy
With over 850 species Allium is the sole genus in the Allieae, one of four tribes of subfamily Allioideae. New species continue to be described and Allium is one of the largest monocotyledonous genera, but the precise taxonomy of Allium is poorly understood, with incorrect descriptions being widespread. The difficulties arise from the fact that the genus displays considerable polymorphism and has adapted to a wide variety of habitats. Furthermore, traditional classifications had been based on homoplasious characteristics. However, the genus has been shown to be monophyletic, containing three major clades, although some proposed subgenera are not. Some progress is being made using molecular phylogenetic methods, and the internal transcribed spacer region, including the 5.8S rDNA and the two spacers ITS1 and ITS2, is one of the more commonly used markers in the study of the differentiation of the Allium species.Allium includes a number of taxonomic groupings previously considered separate genera Allium spicatum had been treated by many authors as Milula spicata, the only species in the monospecific genus Milula. In 2000, it was shown to be embedded in Allium.
Phylogeny
History
When Linnaeus formerly described the genus Allium in his Species Plantarum, there were thirty species with this name. He placed Allium in a grouping he referred to as Hexandria monogynia containing 51 genera in all.Subdivision
Linnaeus originally grouped his 30 species into three alliances, e.g. ''Foliis caulinis planis. Since then, many attempts have been made to divide the growing number of recognised species into infrageneric subgroupings, initially as sections, and then as subgenera further divided into sections. For a brief history, see Li et al. The modern era of phylogenetic analysis dates to 1996. In 2006 Friesen, Fritsch, and Blattner described a new classification with 15 subgenera, 56 sections, and about 780 species based on the nuclear ribosomal gene internal transcribed spacers. Some of the subgenera correspond to the once separate genera included in the Gilliesieae. The terminology has varied with some authors subdividing subgenera into Sections and others Alliances. The term Alliance has also been used for subgroupings within species, e.g. Allium nigrum, and for subsections.Subsequent molecular phylogenetic studies have shown the 2006 classification is a considerable improvement over previous classifications, but some of its subgenera and sections are probably not monophyletic. Meanwhile, the number of new species continued to increase, reaching 800 by 2009, and the pace of discovery has not decreased. Detailed studies have focused on a number of subgenera, including Amerallium. Amerallium is strongly supported as monophyletic. Subgenus Melanocrommyum has also been the subject of considerable study, while work on subgenus Allium has focussed on section Allium, including Allium ampeloprasum'', although sampling was not sufficient to test the monophyly of the section.
The major evolutionary lineages or lines correspond to the three major clades. Line one with three subgenera is predominantly bulbous, the second, with five subgenera and the third with seven subgenera contain both bulbous and rhizomatous taxa.
Evolutionary lines and subgenera
The three evolutionary lineages and 15 subgenera here represent the classification schemes of Friesen et al. and Li, and subsequent additional species and revisions.;Evolutionary lines and subgenera
- First evolutionary line
- # Nectaroscordum Asch. et Graebn Type: Allium siculum Mediterranean bells, Sicilian honey garlic
- # Microscordum N. Friesen Type: Allium monanthum
- # Amerallium Traub Type: Allium canadense
- Second evolutionary line
- # Caloscordum R. M. Fritsch Type: Allium neriniflorum
- # Anguinum N. Friesen Type: Allium victorialis
- # Porphyroprason R. M. Fritsch Type: Allium oreophilum
- # Vvedenskya R. M. Fritsch Type: Allium kujukense
- # Melanocrommyum Rouy Type: Allium nigrum
- Third evolutionary line
- # Butomissa N. Friesen Type: Allium ramosum fragrant garlic
- # Cyathophora R. M. Fritsch Type: Allium cyathophorum
- # Rhizirideum Wendelbo s.s Type: Allium senescens
- # Allium L. Type: Allium sativum garlic
- # Reticulatobulbosa N. Friesen Type: Allium lineare
- # Polyprason Radic Type: Allium moschatum
- # Cepa Radic ́ Type: Allium cepa onion, garden onion, bulb onion, common onion
First evolutionary line