Lecanorchis tabugawaensis Suetsugu & Fukunaga, 2016

Suetsugu, Kenji & Fukunaga, Hirokazu, 2016, Lecanorchis tabugawaensis (Orchidaceae, Vanilloideae), a new mycoheterotrophic plant from Yakushima Island, Japan, PhytoKeys 73, pp. 125-135 : 126-131

Lecanorchis tabugawaensis Suetsugu & Fukunaga sp. nov. Figs 1 , 2 , 3

Diagnosis.

Lecanorchis tabugawaensis differs from its close relative Lecanorchis taiwaniana in having a straight column, a narrow and almost entire labellum and the glabrous apical part of the adaxial labellum surface.

Type.

JAPAN. Kyushu: Kagoshima Pref., Yakushima Island, Yakushima Town , Koseda , along Tabu River , alt. 170m, 16 July 2015, H. Yamashita s.n. (holotype KYO; isotype OSA) .

Additional specimen examined.

JAPAN. Kyushu: Kagoshima Pref., Yakushima Island, Yakushima Town, Koseda, along Tabu River, alt. 170m, 9 October 2015, K. Suetsugu s.n. (OSA).

Description.

Terrestrial, mycoheterotrophic herb. Inflorescence 15-45 cm tall, unbranched or branched at lower half, yellowish white at flowering, brownish black at fruiting, glabrous, ca. 1.0 mm in diam., with membranaceous scale-like sheaths. Ra chis 6-15 cm, 4-15 flowered, internodes 5-15 mm apart. Floral bracts deltoid, ca. 2.0 mm long, ca. 1.0 mm wide. Pedicellate ovary ascending, 15-20 mm long. Sepals and lateral petals widely spreading, ca. 2.5 cm in diameter. Sepals yellowish white, linear, slightly narrower in lower half, 14-17 mm long, ca. 1.8-2.5 mm wide, apex obtuse, 3-nerved. Petals yellowish white, linear, slightly oblique, 14-17 mm long, ca. 2.0-2.5 mm wide, apex obtuse, 3-nerved. Labellum white tinged with purple toward apex, glabrous, 14-15 mm long, ca. 5 mm wide when flattened, entire. Column 12-13 mm long, straight, fused with labellum for about 2/5-1/2 its length, glabrous; anther whitish, ca. 1.5 mm wide. Capsule 20-30 mm long, bright brown, ascending at 20-45° angle from axis.

Conservation.

IUCN red list category: Critically Endangered, [CR D1]. To date, the distribution of Lecanorchis tabugawaensis appears to be restricted to two locations, separated by ca. 1.5 kilometers, along the Tabu and Onna Rivers at an elevation of ca. 100-180 m on the lower slopes of Mt. Aiko on the eastern Yakushima Island. The two known locations are located in humid evergreen broadleaved forests dominated by Castanopsis sieboldii (Makino) Hatus. ex T.Yamaz. & Mashiba and Distylium racemosum Siebold & Zucc. Lecanorchis tabugawaensis flowers in mid-July to early-August, and each location consists of only dozens of flowering individuals. The population of Lecanorchis tabugawaensis contains less than 50 mature plants, and at present we are not aware of any other locality where this species persists. Therefore, Lecanorchis tabugawaensis is classified as CR under Criterion D1 ( IUCN 2014).

Although the lowland humid evergreen forests flanking the rivers in Yakushima Island have previously been identified as hotspots for endemic plant species, only a small proportion of the area is currently under protection ( Yahara et al. 1987). This is in spite of the fact that 61% (20989 ha) of Yakushima Island is designated as a National Park, and 21% (10747 ha) is a World Natural Heritage site ( Yahara and Tsukaya 2008). Consequently, one of the two locations of Lecanorchis tabugawaensis remains unprotected in an evergreen forest along the Tabu River adjacent to a Cryptomeria japonica (L.f.) plantation that has recently been cut. The area also harbors rare mycoheterotrophic plants, such as Gastrodia uraiensis T.C.Hsu & C.M.Kuo, Gastrodia takeshimensis Suetsugu, and Gastrodia albida T.C.Hsu & C.M.Kuo. Considering these mycoheterotrophs are completely dependent on their unique host fungi (e.g. Suetsugu et al. 2014b), it is important to conserve the entire ecosystem of their surrounding habitat. Further regulations restricting forest logging and construction are therefore required to conserve the flora, fauna, and the numerous endemic species restricted to low altitude habitats on Yakushima Island.

Etymology.

The specific epithet is derived from “Tabugawa”, which is the Japanese name for the Tabu River, the type locality that also harbors other rare mycoheterotrophic plants.

Taxonomic notes.

Lecanorchis tabugawaensis is similar to Lecanorchis nigricans Honda and Lecanorchis taiwaniana S.S. Ying. Lecanorchis taiwaniana has often been treated as a synonym of Lecanorchis nigricans Honda, a species known to be found in Japan, China, and Taiwan ( Su 2000; Chen et al. 2009; Govaerts et al. 2016). However, this is based on the ambigu ity of the original description by Ying (1987). Suetsugu et al. (2016b) reported that Lecanorchis taiwaniana can be easily distinguished from Lecanorchis nigricans by a combination of several characters (Table 1 ), including the longer peduncles, the longer rachis, the longer internodes, the narrower sepals and petals, the slightly 3-lobed labellum, the bright brown ascending capsules, the column that is more than half-fused with the labellum, the pubescence at the base of the column and the paler rachis coloration, based on the knowledge obtained by the newly discovered specimens. In addition, the differences of Lecanorchis taiwaniana (junior synonym: Lecanorchis amethystea ) and Lecanorchis nigricans have been clearly stated by not only Suetsugu et al. (2016b) but also Sawa et al. (2006), Hsu and Chung (2010) and Lin et al. (2016).

When comparing Lecanorchis tabugawaensis to Lecanorchis nigricans , Lecanorchis tabugawaensis has the taller inflorescences; the longer and lighter colored rachis; the yellowish-white, narrower sepals and petals; and the brighter brown suberect capsules. These characteristics of Lecanorchis tabugawaensis are shared with Lecanorchis taiwaniana (Table 1 ). However, Lecanorchis tabugawaensis can be distinguished from Lecanorchis taiwaniana by column morphology (straight vs. slightly curved), the pubescence at the base of the column (none vs. interspersed), labellum morphology (almost entire vs. 3-lobed), the width of the labellum when flattened (ca.5 mm vs. 6-7 mm), the proportion of the column fused with the labellum (2/5~1/2 vs. 1/2~2/3), the colored area of the labellum (2/3~1 vs. 1/4~1/5), and the apical part of the adaxial labellum surface (glabrous vs. puberulent for Lecanorchis tabugawaensis and Lecanorchis taiwaniana , respectively; see also Table 1 ).

Ecology.

Investigation on the column morphology suggested that the rostellum of Lecanorchis tabugawaensis is not very developed, as it does not function as a physical barrier between the stigma and the pollinia. As such, columns excised from flowers about one day after anthesis exhibit contact between the pollinia and the stigma because the pollinia begins to drop downward onto the stigma from the clinandrium. However, autonomous self-pollination in a bud stage is unlikely to occur because columns from the buds picked about a day before flower opening showed that pollinia are usually compacted within the clinandrium and basally inserted behind the apex of the stigma.

Autonomous self-pollination in Orchidaceae has previously been reported in various species, including the Vanilloideae subfamily, under which Lecanorchis belongs (e.g. Suetsugu 2013b, 2015b). Autonomous self-pollination has been proposed as an evolutionary response to ensure reproductive success given a lack of pollinators when the frequency of pollination is regularly quite low ( Baker 1955). Mycoheterotrophic plants are often found growing on the dense forest floor, shaded by woodland or scrub. It can be theorized that mycoheterotrophy developed as an adaptation for these species to survive in such low-light conditions ( Bidartondo et al. 2004). At the same time, pollinators may not be particularly suited to such low-light environments ( Herrera 1995, 1997), thus indirectly creating a problem for plant reproduction if pollinators are unlikely to visit these areas. It appears that most of the mycoheterotrophic species investigated to date (especially nectarless species) have indeed abandoned an insect-mediated pollination system in favor of self-pollination (e.g. Suetsugu 2013a, b; Suetsugu 2014 a; Suetsugu 2015c). Thus, autogamy in Lecanorchis tabugawaensis can also be considered a reproductive assurance to compensate for pollinator limitation due to their lack of nectar and pollinators’ habitat preferences.

Figures

Figure 1. Photographs of Lecanorchis tabugawaensis in Taiwan. A Flowering habit B-C Flower D Fruiting habit E-F Lip and column G Lip H Column. Photos by Hiroaki Yamashita (A-B), Takuto Shitara (C, E-H) and Kenji Suetsugu (D).

Figure 2. Lecanorchis tabugawaensis. A Flower B Flowering habit C Fruit D Fruiting habit. Bars: 5 mm (A, C); 3 cm (D). Line drawings by Kumi Hamasaki.

Figure 3. Lecanorchis tabugawaensis. A Flower B Anther cap C Column D Lip and column E Dorsal sepal F Lateral petal G Lateral sepal H Flattened lip. Bars: 5 mm (A, C, D, E, F, G, H); 1 mm (B). Line drawings by Kumi Hamasaki.