Chenopodium ambiguum, R. Brown, 1810

Mosyakin, Sergei L. & De Lange, Peter J., 2018, New combinations for three taxa of the Oxybasis glauca aggregate (Chenopodiaceae) from Australasia, East Asia, and South America, Phytotaxa 350 (3), pp. 259-273 : 261-263

Chenopodium ambiguum from Australasia ( Australia, New Zealand) and Oceania

Brown (1810: 407) in his original description of Chenopodim ambiguum reported the following characters: “ C. ambiguum , foliis rhombeo-lanceolatis dentatis acutis subtus glauco-pulverulentis, spicis foliatis, perianthiis glabris ecarinatis 3–4-partitis monandris. (D.) v. v.”. In this paragraph, the abbreviation “ v. v.” (vidi vivo) indicates that Brown observed living plants during his travel onboard HMS Investigator in 1801–1803 and/or stay in Australia until 1805 ( Stearn 1960, Chapman et al. 2001 –onward, Vallance et al. 2001, Barker 2007, etc.), and the letter (D.) refers to Tasmania (“[designat] Insulam Van Diemen, cujus regiones septentrionales et praecipue Australes visitavi”; Brown 1810: vii).

Moquin-Tandon (1840: 31, 1849: 67) accepted Chenopodim ambiguum and provided its updated description in which, in particular, additionally indicated that seeds of that species are obtusate at margins (“...semine laevigato nitido margine obtusiusculo”), while in C. glaucum the seed margins are acute (but not always because vertical and horizontal seed may differ in that respect; see Sukhorukov 2014). However, Hooker (1857: 313) in his Flora Tasmaniae , while acknowledging the differences mentioned by Moquin-Tandon, nevertheless concluded that “the two plants [ C. ambiguum and C. glaucum —S.M. & P.dL.] agree so entirely in habit, foliage, general appearance, and in all other characters <...> that I should hesitate before regarding the above differences, even if constant, as of specific value”, and proposed the variety-rank combination. That was typical of Hooker, who was definitely one of the “founding fathers” of the Southern Hemisphere botany, but often tried to ‘shoehorn’ distinct species and genera from Australia and New Zealand into Northern Hemisphere names (see Gillbank 2014 for a typical example).

Thellung (1912: 196) proposed the subspecies rank under Chenopodium glaucum for C. ambiguum and applied the name to plants introduced in Montpellier, France. Aellen (1929, 1960–1961) cited the authorship of the combination C. glaucum subsp. ambiguum as “(R. Br.) Thellung et Aellen” [not as “(R. Br.) Murr & Thellung”] because he concluded that Thellung (1912: 196) misapplied that name to the introduced plants, which actually belonged to C. macrospermum (now accepted as Oxybasis macrosperma ). However, according to Art. 7.3 of the ICN ( McNeill et al. 2012), a nomenclatural combination is typified by the type of its basionym even though it may have been applied erroneously to a taxon now considered not to include that type, and because of that the authorship of the subspecies-rank combination should be attributed to Murr & Thellung, despite the initial misapplication of the name by Thellung. The occurrence of this Australasian taxon in Europe as a rare wool alien (see Aellen 1960–1961: 613) needs confirmation.

Additional morphological differences were summarized by Aellen (1960–1961: 612–613), who also mentioned the difference in the leaf shape and fruit/seed diameter (0.5–0.8 [–1.0] mm in subsp. glaucum and 1–1.2 mm in subsp. ambiguum ), but further comparative morphological studies are needed to reveal the most reliable diagnostic characters (see below).

Chenopodium ambiguum was accepted at species rank in some floras of Australia and New Zealand (e.g., Allan 1961). It was also reported from Easter Island ( Skottsberg 1922, 1935, 1956, Guillamin et al. 1936, Flenley et al. 1991, as C. ambiguum ; Zizka 1991, as C. glaucum ). The plants from Easter Island were also described as C. glaucum f. paschale Fuentes (1913: 332) , reportedly differing from the typical form in being of smaller size, much branched, with smaller leaves, larger tissue cells filled with saline water (?), and racemose loose spicate inflorescences [“Esta planta constituye una forma nueva ; diferente del tipo por su talla menor, mui ramificada, con hojas mas pequeñas, tejido fundamental con células mas grandes llenas de agua salina, i sus espículas racemosas mui flojas. Puede llamarse Ch. g. for. nov. paschale ”]. Aellen (1929: 47) confirmed that the Easter Island specimens correspond well to the Australian plants [“Die Exemplare der chilenischen, ozeanischen Inseln stimmen mit Belegen aus Australien gut überein”] and listed C. glaucum f. paschale in synonymy of C. glaucum subsp. ambiguum . Zizka (1991: 39) placed the species (which he recognized as C. glaucum s. l., following Wilson 1984) in the group of idiochores (“plants whose occurrence in a region is not due to the activity of man”) and suggested that it was probably introduced to Easter Island by birds.

Later this Australasian taxon was often considered a variety or subspecies, or sometimes as a mere synonym of C. glaucum . For example, Cheeseman (1906: 581) mentioned in a note under C. glaucum that the “ New Zealand and Australian plant is sometimes kept as a separate variety or species ( C. ambiguum, R. Br. ), but the differences appear to be slight and inconstant”. Wilson (1983: 142) provided the following noteworthy comment: “ Chenopodium glaucum is a polymorphic species in which numerous infraspecific taxa have been described. The ‘Australia’ plant is by some authors distinguished from the European as a distinct species ( C. ambiguum ) and by others as subspecies or variety of C. glaucum . Within Australia many variants are found, some of which may be of very recent introduction while others could be indigenous. The typical variant of C. ambiguum has deltoid 3-lobed leaves which are sinuate-dentate on the margin whereas in the typical variant of C. glaucum the leaves are narrow-ovate to narrow-elliptic with sinuate margins; Aellen (1960–1961) noted differences in the flower morphology but this varies considerably between populations which have similar leaf types ”. Later Wilson confirmed his opinion and stated that C. glaucum is a “polymorphic species in Australia which may include an endemic element ( subsp. ambiguum ) as well as an introduced element ( subsp. glaucum )” ( Wilson 1984: 137). Sykes (in Webb et al. 1988) reported under C. glaucum that “N.Z. subsp. also indigenous to temperate Australia and Easter Id. N.Z. plants are referable to subsp. ambiguum (R. Br.) Thell. sometimes accorded specific rank as C. ambiguum R. Br. ”. Walsh & Stajsic (2015–onward) commented that “ Chenopodium glaucum is a variable species, and it is probable that some forms of the species are native and other forms are naturalised. Among the native elements include subsp. ambiguum , supposedly differing in having flowers in leafy racemes, and the seed with an obtuse margin. However, these features appear to be inconsistent in both European and native Australian specimens”. As we see, many authors of recent floras of Australia and New Zealand usually commented that the presumably native plants of the C. glaucum group seem to differ from introduced Eurasian plants belonging to C. glaucum s. str. in their general habit and in some not quite evident morphological characters, but most of those authors were usually reluctant to accept the species status for the Australasian taxon (see also Duretto & Morris 2011, Wilson & Chinnock 2013, etc.). In recent checklists of New Zealand vascular plants that taxon was accepted as a species, C. ambiguum ( de Lange et al. 2006, 2010, 2011, 2013), or as a subspecies, Oxybasis glauca subsp. ambigua (de Lange 2013, Schönberger et al. 2017).

It should be also noted that C. ambiguum is, especially in New Zealand, mostly known from natural and undisturbed (mainly coastal) habitats with only a few introduced species present, especially offshore islands and island groups ( Johnson 1982, Whinray 1972, Fineran 1973, de Lange & Sawyer 2008, de Lange et al. 2011). Indeed, the species is now in decline on the main islands of New Zealand, and as such has been listed as threatened (de Lange et al., in press).

de Lange & Murray (2002) reported that three New Zealand accessions of the Australasian endemic taxon, which they accepted as Chenopodium glaucum subsp. ambiguum , had chromosome counts of n = ca. 16–18 II and 2 n = ca. 32– 36 (most probably a tetraploid with 2 n = 36), on the basis of which they suggested it may be more appropriate to treat this plant at species rank. In contrast, the diploid counts 2 n = 18 were reported for C. glaucum s. str. in various parts of its vast range by many researchers ( Fedorov 1969, Uotila 1973, Goldblatt & Johnson 1979 –onward, Schwarzová 1980, Takhtajan & Agapova 1993, Grozeva 2007, Probatova et al. 2009, 2013, Chung et al. 2016, Skaptsov et al. 2017, and references therein). This is also the same number (n = 9 or 2 n = 18) reported for C. glaucum subsp. salinum (Standl.) Aellen (1929: 46) by Löve & Löve (1982) and Bassett & Crompton (1982), and for C. amurense by Probatova & Sokolovskaya (1990). Considering that, de Lange & Murray (2002) advocated for C. glaucum subsp. ambiguum its reinstatement at species rank, as C. ambiguum . Later de Lange (2013: 13) confirmed his opinion regarding the species status of that taxon but noted that “a combination at species rank for Chenopodium ambiguum R. Br. in Oxybasis is unavailable”.

It should be noted that in the continuously updated Index to Plant Chromosome Numbers (IPCN: Goldblatt & Johnson 1979 –onward) and in some publications (e.g., Chung et al. 2016) it was mentioned that Schwarzová (1980) reported for Chenopodium glaucum the tetraploid chromosome number 2 n = 36. However, Schwarzová (1980: 728) in her publication definitely reported for that species the diploid number 2 n = 18. Thus, the IPCN record was based on some misunderstanding or a technical error, which was, unfortunately, uncritically reproduced in several other sources without checking the original publication. Thus, probably the first reliable record of the tetraploid chromosome number 2 n = 36 in Eurasian members of the Oxybasis glauca aggregate has been published by Lomonosova & Krasnikov (2006) for a voucher from Irkutsk Region (southeastern Siberia, Russia) identified as C. glaucum . However, considering the data on morphological and genetic diversity in the group (see Sukhorukov 2014, Feodorova 2017, etc.), that Siberian plant might in fact belong to a hybrid or to some yet unrecognized species of allopolyploid hybrid origin.

Morphologically Chenopodium ambiguum (here recognized as Oxybasis ambigua , see below) is primarily distinguished by its rather large fruits (which is consistent with its polyploid nature), and by leaf characters (see Aellen 1960–1961, and discussion above).The leaves in this species are distinctly succulent, typically broadly deltoid to deltoidtriangular, with the lamina margins more or less distinctly 3-lobed, with sinuate-dentate margins (or rarely subentire). In some populations the upper stem leaves may be narrowly deltoid and 3-lobed, or ovate-elliptic to elliptic-lanceolate; in such cases the lamina margins are either deeply dentate or entire. In contrast, the leaves of typical representatives of O. glauca are usually narrowly ovate to elliptic [also, unusual forms with narrowly lanceolate to linear leaves, known as Chenopodium wolffii Simonkai ( Simkovics 1879: 164) , occur very rarely in southeastern Europe: see Iljin 1952: 292, Mosyakin 1996: 36–37], with sinuate or sinuate-dentate margins, and only scarcely fleshy. However, plants of O. glauca occurring in saline habitats can be also rather succulent. The lower sides of leaves in O. ambigua also tend to be less prominently farinose than those in typical O. glauca . Trilobate leaves morphologically somewhat similar to those of O. ambigua may also occur in O. × schulzeana (Murr) Mosyakin (2013: 5) (≡ Chenopodium × schulzeanum Murr 1906: 110 ) considered to be a hybrid of O. glauca and O. rubra (see Mosyakin 2013: 5), or probably a species of hybrid origin ( Sukhorukov 2014: 63).

It would seem that Fuentes-Bazan et al. (2012) and Mosyakin (2013) were unaware of the chromosome counts reported by de Lange & Murray (2002) for Chenopodium ambiguum and the latter authors’ preference to treat that plant at species rank. Those chromosome counts would render the allopatric Oxybasis ambigua reproductively isolated from other taxa of the O. glauca aggregate (subsp. glauca, subsp. salina , etc.) because if they were sympatric and hybridized their progeny would be most probably sterile. As such, a new combination at species rank is here made (see below).

Regarding the possible origin and pathways of migration of Oxybasis ambigua or its ancestor to Australasia, we may consider two options. This species may be related to Asian members of the O. glauca aggregate, especially alluvial or coastal ones (such as O. amurensis , see below). Another option is its possible links with North and/or South American members of Oxybasis , such as O. macrosperma or O. parodii (see below). Both these historical biogeographic patterns occur in other Australian taxa of Chenopodiaceae (see Kadereit et al. 2005, Mosyakin & Iamonico 2017, and references therein) and in other groups of plants (e.g., Winkworth et al. 2015, Murphy & Crayn 2017). Considering the tetraploid chromosome number of O. ambigua , its allopolyploid origin due to hybridization of two different taxa of Oxybasis is quite possible. However, to solve the problem of its geographic and evolutionary origins, special morphological and molecular studies in comparison to presumably related Eurasian and American taxa are needed.