Rhizanthella johnstonii K.W.Dixon & Christenh., 2018

Dixon, Kingsley W. & Christenhusz, Maarten J. M., 2018, Flowering in darkness: a new species of subterranean orchid Rhizanthella (Orchidaceae; Orchidoideae; Diurideae) from Western Australia, Phytotaxa 334 (1), pp. 75-79 : 75-79

Rhizanthella johnstonii K.W.Dixon & Christenh. , sp. nov. ( Figs. 2 , 3 )

Type:— AUSTRALIA . Western Australia : Ravensthorpe, Munglinup, Oldfield location 1002, on tributary of Dallinup Creek, which is a tributary of the Oldfield River, 1 June 1979, George 15717 (holotype: PERTH 288586!; isotypes: K!, NSW, US).

This species differs from R. gardneri in having fewer flowers in a smaller capitulum that are white or suffused with a pink blush, much paler than R. gardneri . The bracts are also lighter, white, occasionally tinged purple pink on the margins and apex ( Fig. 2 ). The species is associated with Melaleuca hamata Fielding & Gardner and M. uncinata R.Br. It has a disjunct distribution from R. gardneri , which has a different Melaleuca host preference.

Holomycotrophic, achlorophyllous, fully subterranean plants. Rhizomes more or less horizontal, upturned to the peduncle, 3.0–4.0 × 0.5 – 1.3 cm, 6–12 cm below the soil surface, swollen at the peduncle base, sparsely hairy, the hairs fine. Bracts few, appressed, broadly triangular, 0.2–0.6 mm, obtuse, but the tip thickened-canaliculate. Lateral rhizome branches smaller, arising from a thickened part at the base of the peduncle. Peduncles erect, to 60 mm long, c. 0.3–0.6 mm thick, narrowest below the capitulum, glabrous. Bracts in two spirals, widely spaced, the lowermost appressed, broadly triangular, 0.3–0.4 × c. 10.0 mm, becoming narrowly triangular towards the apex, and there slightly divergent from the peduncle, 10–25 × 3–10 mm, concave, amplexicaul, obtuse with a thick central vein, margins thinner. Inflorescences a terminal capitulum, the receptacle 10–25 mm in diameter. Involucral bracts six to eight, rarely ten, imbricate, forming a cup surrounding the flowers, narrowly obovate, 25–35 mm × c. 10 mm, concave at base, apex obtuse but with an apiculum, cream to pale pinkish-cream, with 3–7 simple or singly forked veins. Flowers up to c. 60 per capitulum in 8–12 spiral rows, each row with 6–8 flowers, the innermost not fully developing, all inwardly facing, the outermost flowers opening first. Perianth white but pink suffused, 4–6 mm long, tubular for the lower half, sepals and petals free near the apex but closely imbricate. Lateral sepals broadly triangular, curved forwards, obtuse to acute. Dorsal sepal broadly triangular, cucullate, the margins overlapped by those of the lateral sepals, which are slightly longer. Petals more or less triangular, obtuse, free in the upper part, about 1 mm shorter than the sepals, apices slightly crenulate with the margins flaring. Labellum motile on slender claw of 1–3 mm, erect at the base, strongly curved forward and protruding. More or less obliquely semi-circular with a recurved tip when seen from the side, 1.5–2.0 × 0.7–1.5 mm, deeply channeled, smooth, entire. Column more or less terete, 2.0– 2.5 mm. Anther terminal, 0.8–0.9 mm, shortly apiculate, persistent. Stigma 1 mm, cuneate with curved, lateral margins and a small projection at the base. On each side of the column at the upper edge of the stigma there is a small, narrow, obtuse lobe. Ovary elongate, smooth, 5–6 × 4 mm, smaller in inner flowers, enlarging to elongate-ovoid if pollinated ( Figs. 2A , 3 ).

Distribution:— Found in the area around Munglinup, east of Ravensthorpe in southern Western Australia , where it is only found in thickets of Melaleuca hamata , M. uncinata and an unnamed Melaleuca species of the M. uncinata complex ( Brown et al. 2013). This differs from the other Western Australian species, R. gardneri , which is only found in thickets of Melaleuca scalena Craven & Lepschi between Babakin and Corrigin in the central Wheatbelt ( Fig. 4).

Ecology:— Plants are holomycotrophic and acquire nutrients from a shared mycorrhizal fungus ( Ceratobasidium sp. ) association with the fine root systems of Melaleuca hamata and M. uncinata and grow in coarse sandy clay or sandy loam with a thin layer of surface leaf litter. The specialised holomycotrophic association with a fungus that links the orchid to the root system of a specific autotrophic host is key to explaining why Rhizanthella johnstonii is restricted to only a few sites. In this tripartite relationship, the autotrophic host provides nutrients and carbon ( Bougoure et al. 2010), and thus conservation of the orchid is reliant on ensuring the health of the host species.

Specimens examined:— AUSTRALIA . Western Australia : Ravensthorpe, Munglinup, 25 June 2002, Braun & Brundrett 3740a (PERTH 6873626!); 26 June 2002, Braun & Brundrett 3740b (PERTH 6873618!); December 1979, Chapman s.n. (PERTH 998575!); 1 August 1979, George 15733 (PERTH 3218910!); 13 October 1979, George 15779 (PERTH 7862016!); 26 May 1979, McGuiness s.n. (PERTH 288594!, US).

Etymology:— We name this species in recognition of Lionel Johnston, who has worked tirelessly and without reward on the many field trips to resolve the distributional status of this new taxon. His unwavering support of orchid conservation and his life-long support of KWD’s programme of orchid research is also acknowledged with the publication of this new taxon.

Conservation:— We recommend this species to be listed as rare flora as the species is highly restricted to a narrow band of vegetation along drainage systems on sandy loams between the 350 mm and 400 mm isohyets. It is likely the species may have occurred in higher rainfall zones, but the vegetation has been extensively cleared or is unsuitable habitat (clays, sands or rocky soils). Despite extensive surveys for the species east and west along the 350–400 mm isohyet band and in drier climes north of the current distribution range the species was not located due to lack of suitable Melaleuca thickets, unfavourable soil or unfavourable hydrology (e. g. swamps). Because of its subterranean habit, it is difficult to make a complete IUCN red list assessment, but we expect this species to fall in one of the categories of threat, most likely Critically Endangered (CR B1a,b(i,iii), B2a,b(ii,iii); IUCN 2012), because the species has a single location, a small number of known individuals and a projected continued decline due to the extensive clearing of the habitat, impacts of climate change and continued habitat alteration due to feral animal grazing and weed impacts.

Figures

FIGURE 2. Western Australian Rhizanthella species. A. Rhizanthella johnstonii. Munglinup, Western Australia. Photo by A. Brown. B. Rhizanthella gardneri. Babakin, Western Australia. Photo by K. Dixon.

FIGURE 3. Sketch of Rhizanthella johnstonii. Adapted by M. Christenhusz from an anonymous sketch in the herbarium of the Royal Botanic Gardens, Kew.