A New Species of Argophyllum (Argophyllaceae) with Notes on the Species from New Caledonia and Nickel Hyperaccumulation

The taxonomy of Argophyllum (Argophyllaceae) in New Caledonia is reviewed here. All names validly published in Argophyllum in this archipelago are discussed and lectotypified when necessary. A new species is described, Argophyllum riparium (The LSID for the name Argophyllum riparium is: 77216335-1) Pillon and Hequet sp. nov. Argophyllum grunowii and A. ellipticum are both species complexes in which several species previously recognized are included here as well. Seven species are recognized in New Caledonia: A. brevipetalum, A. ellipticum, A. grunowii, A. montanum, A. nitidum, A. riparium and A. vernicosum, all endemic. Leaf nickel content of A. riparium can exceed 1000 μg·g−1, which makes this species a nickel hyperaccumulator. Measurements with a handheld X-Ray Fluorescence (XRF) spectrometer confirmed that this was also the case for all other species from New Caledonia, except A. nitidum. An identification key of New Caledonian species is provided.

[1] (p. 29) is a genus of shrubs and small trees that has traditionally been placed in Saxifragaceae, or sometimes in Escalloniaceae or Grossulariaceae. Molecular phylogenetic studies [2] have indicated affinities with the genus Corokia A. Cunn. [3] (p. 248) and the two genera form the family Argophyllaceae [4] in the order Asterales [5] (APG I and subsequent others). Argophyllaceae are closely related to two other small families [2,6,7]: Phellinaceae, with a single genus of ten species all endemic to New Caledonia [8][9][10], and Alseuosmiaceae, with four to five genera and ten species from eastern Australia, New Guinea, New Zealand and New Caledonia [11][12][13]. The exact relationships between Argophyllaceae, Phellinaceae and Alseuosmiaceae, which form the "APA clade" or Alseuosmiineae Shipunov [14] (p. 63), are still not clear [15].
The genus Argophyllum was described by Johann and Georg Forster, two German botanists who travelled with James Cook during his second voyage around the world. They collected material in New Caledonia that they subsequently described as Argophyllum nitidum J. R. Forst. and G. Forst.
[1] (p. 29). A second species, A. ellipticum Labill. [16] (p. 39), was collected and described by Labillardière, a member of Bruni d'Entrecasteaux's voyage in search of the La Pérouse expedition. The genus was last revised in its entirety by Zemann [17], who recognized ten species in Australia and New Caledonia. Leaf cuticle attributed to Argophyllum have been described from the Miocene of New Zealand [18]. The species in Australia have recently been revised with a total of eleven endemic species [19,20]. Guillaumin [21] revised the New Caledonia species as part of his revision of New Caledonian "Saxifragaceae" and recognised seven species, to which he subsequently added three new species [22]. Here, we provide a taxonomic update of New Caledonian Argophyllum to complete work on the genera from New Caledonia previously included in "Saxifragaceae" [23,24].
High nickel content was previously reported in the leaves of New Caledonian Argophyllum [25,26]. However, because the taxonomy of the genus was not clear at that time, there was uncertainty regarding the number and the names of the species that can actually be considered as nickel hyperaccumulators [27,28]. Here, we provide new analyses of herbarium specimens [29] and field-collected material to update the list of nickel hyperaccumulators in the genus Argophyllum.

Materials and Methods
Measurements, shapes and colours of the different organs are based on the examination of herbarium material and field observations. All herbarium specimens of Argophyllum present at NOU and P were examined (for herbarium acronyms: [30]). Additional type material was viewed online.
Nickel content in herbarium specimens was measured with a handheld X-Ray Fluorescence (XRF) spectrometer [28,31]. For each species, we selected specimens to represent their morphological, geographical and ecological range. For a putative new species, one leaf from five individuals was collected in the field, dried, ground to powder, digested in HNO 3 /H 2 O 2 and analysed by Microwave Plasma-Atomic Emission Spectrometer (MP-AES) [32].

Results
Examination of herbarium specimens and observations in the field suggests that seven endemic species should be recognized in the genus. Some material resembling A. vernicosum Däniker [33] (p. 165) but with larger leaves, thicker twigs, sparser branching, sepals with a different shape, and a riparian ecology are placed in a new taxon described below, A. riparium Pillon and Hequet sp. nov.
Measurements with an XRF spectrometer indicate that the nickel content in dried leaves exceeds 1000 µg·g −1 in at least some individuals of six species: A. brevipetalum, A. ellipticum, A. grunowii, A. montanum, A. riparium sp. nov. and A. vernicosum (Table 1). The range of values observed between theses six species are similar. The median value for all species is below 1000 µg·g −1 and the highest value, 6670 µg·g −1 , is observed in A. ellipticum. The only exception is A. nitidum, where values were consistently below the level of detection. Values observed with MP-AES on five individuals of Argophyllum riparium sp. nov. confirmed that it is a nickel hyperaccumulator: 1498, 1520, 1678, 2111 and 2214 µg·g −1 nickel in dried leaves.

Discussion
Seven species of Argophyllum, all endemic, can be recognized in New Caledonia. Two of them are variable and appear as species complexes: A. ellipticum or A. grunowii. It is not clear if this complexity may be caused by, e.g., hybridization [34,35] or cryptic species [36,37].
The obscure taxonomy of the genus led to uncertainty and discrepancy regarding the number and identity of the nickel hyperaccumulating species in different publications: A. grunowii and A. laxum [25,26,38], A. ellipticum [28], A. brevipetalum, A. grunowii, A. latifolium, A. laxum, A. montanum and A. vernicosum [27]. Novel measurements associated with a new taxonomy indicate that the nickel content in dried leaves exceeds 1000 µg·g −1 in at least some specimens of six species that can, therefore, be considered as nickel hyperaccumulators [39]: A. brevipetalum, A. ellipticum, A. grunowii, A. montanum, A. riparium sp. nov. and A. vernicosum. These six species are ultramafic obligates, except A. ellipticum, which can also occur on other substrates. The seventh species of the genus in New Caledonia, A. nitidum, never occurs on ultramafic substrates and consistently has nickel content below the level of detection. The genus Argophyllum is therefore similar to Geissois Labill. [16] (p. 50) (Cunoniaceae): Out of the 13 species endemic to New Caledonia, seven of the eight species occurring on ultramafic substrates are nickel hyperaccumulators [40].
Although six species of Argophyllum qualify as nickel hyperaccumulators, their nickel content do not reach the high values observed in some other New Caledonian species [27]. The genus could nevertheless be used for phytoremediation, phytoextraction or ecological restoration, but its multiplication and cultivation has apparently not yet been tested in New Caledonia [41]. The cultivation of the Australian species is considered easy [42].

Taxonomic Section
• ARGOPHYLLUM J. R. Forst. and G. . Zemann distinguished the two sections based on the length of the petals: calyx much shorter than the corolla in section Brachycalyx, calyx half as long but generally equal to the corolla in sect. Dolichocalyx. These sections are not maintained. They may not be reciprocally monophyletic groups, or this would imply that Australian and New Caledonian Argophyllum are not monophyletic groups. Additionally, calyx length is a labile character, i.e., it is variable within A. ellipticum.

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Argophyllum brevipetalum Guillaumin [22] (p. 25). Type:-New Caledonia, cours moyen de la Tontouta, le long des rives, ± 50 m, 14 December 1940, Virot 375 (holo-: P00537620!) Argophyllum brevipetalum is very similar to A. ellipticum, with which it shares the combination of yellow petals and hairy reddish coriaceous leaves, with the entire margin and inflorescences covered with red hairs. However, it occurs at a much lower elevation and differs in its smaller proportions (smaller leaves, slender petiole and stem). Guillaumin [22] (p. 25) segregated A. brevipetalum from A. ellipticum because of its calyx that is as long as its corolla, but this character is variable within A. ellipticum and is therefore not a reliable character alone on which to separate them.

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Argophyllum ellipticum Labill. [ Argophyllum ellipticum is characterized by the combination of yellow petals, sepals much shorter or as long as petals, hairy coriaceous leaves (generally red and obovate in the most typical forms) with the entire margin and inflorescences generally covered with red hairs. Argophyllum ellipticum is widespread across the main island of New Caledonia, at mid-to high elevation, generally on ultramafic substrates, but occasionally on other, mostly poor, soils. It is a variable taxon, more robust in its dimension than the rare low-elevation A. brevipetalum. Specimens collected in the northeast of the island on non-ultramafic substrates, including the type of the species and of var. comptonii, have particularly short sepals (three times shorter than petals) and inflorescences with a long basal peduncle ramifying very distally. Most material collected on ultramafic substrates from Mont Humboldt and Mont Kouakoué, including the type of A. ellipticum var. rigidum, have leaves that are often revolute and whitish underneath, slender sepals (almost as long as petals), and shorter inflorescence ramified nearer the base. In locations in between these, generally on ultramafic substrates, the material displays a range of intermediate characters, which make this taxon difficult to divide further.
The name Argophyllum obovatum was published without a description and the specimen cited by Guillaumin [ Argophyllum grunowii is characterized by the combination of yellow petals, and leaves and inflorescences covered with greyish hairs. Plants from southwest New Caledonia (e.g., type of A. laxum) have large leaves and inflorescences much longer than the leaves. Plants from the eastern coast (e.g., type of A. latifolium) have large leaves but inflorescences shorter than the leaves. Plants from the northwest (e.g., the types of A. acinetochromum and A. brevistylum) have medium-sized leaves with a very dense indumentum underneath and inflorescences about the length of the leaves. In spite of these apparent geographical trends, there are many intermediate forms and variability found within populations. Although not entirely satisfactory, a broad concept is adopted here. Argophyllum grunowii is widespread across the main island of New Caledonia, at low to high elevation, only on ultramafic substrates.
The name Argophyllym amoenum (published as "amoenum") apparently never appeared in print except in combination with var. ovatum. The name was published without a description and the single specimen cited, Vieillard 2639 (P06309317!, P06309318!, P06309319!, P06309320!, P06309321!, P06309322!) belongs to A. grunowii. The collection number for the type collection of Argophyllum laxum var. subintegrifolium is 2095 on the herbarium label but was printed as 2905 in Rendle et al. [48]. The name A. splendens was published without a description and was treated by Guillaumin [52]  Argophyllum montanum is characterized by the combination of yellow petals, glabrous leaves and inflorescences covered by greyish hairs. Argophyllum montanum occurs in the southern part of New Caledonia, at low to mid elevation, only on ultramafic substrates.
The type specimen is unusual in having some toothed leaves. We follow Guillaumin [21] in treating A. schlechterianum as a synonym of A. montanum.

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Argophyllum nitidum J.R.Forst. and G. This species here is considered as endemic to New Caledonia, although some of the Australian species have sometimes been treated as varieties of A. nitidum. It is the only species from New Caledonia with white petals. Its calyx is particularly short (<2 mm). Its leaves are more chartaceous with a finer silvery indumentum than other species. Its inflorescences are silvery too, sometimes tinted with red. Argophyllum nitidum is widespread across the main island of New Caledonia, from Mount Koghis to Balade, and it is most common in the northern part, on non-ultramafic substrates and at low-to mid-elevation. Argophyllum ellipticum var. oblongifolium Brongn. and Gris ex Guillaumin [52] (p. 133). nom. nud.
Argophyllum vernicosum is characterized by the combination of yellow petals, short calyx, glabrous shiny leaves and inflorescences covered with reddish hairs. Argophyllum vernicosum occurs in the southern part of New Caledonia, at mid-to high elevation, on ultramafic substrates.

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Argophyllum riparium Pillon and Hequet, sp. nov (Figure 1) Diagnosis: A species most similar to Argophyllum vernicosum Däniker, from which it differs by its larger leaves (generally ≥ 10 cm long), thicker twigs (at least 3-4 mm in diameter) and sparse branching. Its sepals are narrowly attenuated, nearly as long as petals, rather than shortly acute and not more than half the size of petals as in A. vernicosum.
Ecology and conservation: This species has been found in two locations ( Figure 2). Most collections came from within the Rivière Bleue Provincial Park, where it grows near the rainforest edge on the banks of "rivière bleue". All these collections may have been made from the same population. It appears in small, scattered groups of individuals, and its population size may be small. It was found recently in a second location in Plaine des lacs, growing along a stream close to the Kuebini river, 35 km apart from the type locality. Its ecosystem is undisturbed, and one of the two sites occurs within a protected area where the most likely disturbance might be floods, which can be severe during tropical storms. Its conservation status will be evaluated by the Plant Red List Authority-New Caledonia, and it could qualify as Vulnerable (VU D2). Paratypes