Gastrodia kuroshimensis Suetsugu, 2016

Suetsugu, Kenji, 2016, Gastrodia kuroshimensis (Orchidaceae: Epidendroideae: Gastrodieae), a new mycoheterotrophic and complete cleistogamous plant from Japan, Phytotaxa 278 (3), pp. 265-272 : 266-270

Gastrodia kuroshimensis Suetsugu , sp. nov. ( Figs. 1–3 )

Type: — JAPAN. Ryukyu. Kagoshima Pref., Kuroshima Island, Osato 16 April 2016, K. Suetsugu s.n. (holotype KYO).

Additional specimens examined: — JAPAN. Ryukyu. Kagoshima Pref., Kuroshima Island , 24 April 1985, H. Umata s.n. ( KAG) ; JAPAN. Ryukyu. Kagoshima Pref., Akusekijima Island , 25-30 March 1966, S. Sako 6311 ( KAG) ; JAPAN. Ryukyu. Kagoshima Pref., Yakushima Island, Shiratani, 11 May , 2016, H. Yamashita s.n. ( OSA) .

Diagnosis:— Gastrodia kuroshimensis differs from its close relative G. fontinalis T.C. Hsu, S. W. Chung & C.M. Kuo (2012: 271) in that it has the cleistogamous flowers, the smaller perianth tube and the anther cap joined with the column.

Terrestrial, mycoheterotrophic herb. Roots few, slender, often germinating from the junction between rhizome and inflorescence after flowering season. Rhizome tuberous, fusiform or cylindrical, 3–11 cm long, 4–14 mm in diameter, yellowish brown, covered with numerous scales and root-hair-like unicellular hairs. Inflorescence erect, pale brown, 8–17 cm long, 2.5–5 mm in diameter, 3–4 nodes, with tubular, membranous sheaths. Bracts ovate, up to 6 mm long, 4 mm wide. Pedicel and ovary up to 15 mm long. Flowers 1–4, tubular, slightly upwards, resupinate, 11–13 mm long, ca. 7 mm in diameter. Sepals and petals united forming a five-lobed perianth tube. Perianth tube enclosed or hardly opening. Sepals subsimilar, 11–13 mm long, connate ca. 2/3 their length with petals, lateral ones connate ca. 3/5 their length with each other, outer surface dark greenish brown, densely verruculose, inner surfaces pale greenish brown, smooth; margins entire; free portion of dorsal sepal, ovate-triangular, retuse, ca. 5 mm long, 5 mm wide; free portions of lateral sepals spreading, obtuse at apex. Free portions of petals ovate or ellipse, ca. 4–5 mm long, 2 mm wide. Lip joined with perianth tube, ca. 10 mm long, hypochile reddish brown with two white degenerated smooth calli; epichile reddish brown, ovate-ellipse, base contracted, disc 2–4 ridged with a longitudinal keel extending toward apex, margin slightly undulate. Column straight, semi-cylindrical, ca. 7 mm long, 2–2.5 mm wide, white tinged with grayish brown at base; lateral wings (stelidia) brown, narrow, the edges parallel to column, base incurved, apex acute; rostellum degenerated; stigma located slightly above middle.Anther cap joined with column, hemispheric, ca. 1 mm in diameter, pollinia 2. Capsule cylindrical, ca. 3 cm long, pedicel elongating to ca. 30 cm long in fruit. Seeds fusiform, ca. 2 mm long.

Distribution: —To date, the distribution of G. kuroshimensis was discovered to cross three islands (Kuroshima Island, Akusekijima Island and Yakushima Island). Flowering was observed from mid April to mid May, and fruiting from mid May to late May.

Taxonomic notes:— G.kuroshimensis is similar to G.fontinalis T.P. Lin (1987: 129) due to the longer inflorescences and the keeled lip. However, G. kuroshimensis can be distinguished by the floral condition (cleistogamous vs. chasmogamous), small perianth tube size (11–13 mm long vs. 17–21 mm long), the lip morphology (epichile with 2–4 ridged vs. epichile with 6–8 ridged), the rostellum condition (degenerated vs. ca. 0.7 mm long), the stigma positon (slightly above middle vs. base) and the anther cap morphology (joined with column vs. independent).

Its outline floral morphology,such as the dark greenish-brown colored, densely verruculose,complete cleistogamous flower, is also similar to G. clausa T.C. Hsu, S. W. Chung & C.M. Kuo (2012: 271) . However, G. kuroshimensis can be distinguished by the larger stature during flowering (8–17 cm vs. 2–4 cm), the lip morphology (well-developed, ca. 8–10 mm long vs. peloric, less than 5 mm long), the column morphology (semi-cylindrical, ca. 7 mm long without any ventral appendage vs. ca. clavate, less than 7 mm long with a prominent ventral appendage) and the anther cap morphology (joined with column vs. independent). For a detailed comparison of morphological characters between G. kuroshimensis and its related species, see Table 1.

In addition, the dissected flower of G. kuroshimensis is similar to that of G. uraiensis T.C. Hsu & C.M. Kuo (2010: 244) . However, G. kuroshimensis can be distinguished by the larger stature during flowering (8–17 cm vs. 1–4 cm), the floral condition (cleistogamous vs. chasmogamous), the lip morphology (hypochile with two white degenerated smooth calli vs. hypochile with 2 orange globose calli), the rostellum condition (degenerated vs. ca. 0.7mm long), the stigma positon (slightly above middle vs. base) and the anther cap morphology (joined with column vs. independent).

Reproductive biology:—Although a slit was occasionally observed between the lateral sepals, most of the G. kuroshimensis flowers studied remained completely closed throughout the entire flowering period ( Fig. 2 ). Although cleistogamous plants usually adopt a mixed pollination strategy, also bearing chasmogamous flowers for open pollination (reviewed by Culley & Klooster 2007), the completely cleistogamous nature of G. kuroshimensis indicates that it is an obligate self-pollinating species. Although complete cleistogamy is extremely rare, it is relatively common in the genus Gastrodia , having been observed in at least three species: G. clausa , G. takeshimensis Suetsugu (2013: 375) and G. flexistyloides Suetsugu (2014: 270) . Careful dissection of the G. kuroshimensis flowers at different stages revealed that their pollinia fragment into massulae before the flowers matured. The flowers were also found to have degenerated rostellums, which allowed the massulae to drop directly onto the surface of the stigma, confirming the obligate self-pollinating nature of this species.

Although orchids have evolved many mechanisms to reduce or prevent incidents of self-pollination and enhance outcrossing, reports of autonomous self-pollination are still fairly common, and according to the review of Catling (1990), occurs in all orchid subfamilies. The most comprehensive estimate, based on the reports of peer-reviewed articles, indicates that self-pollination occurs in approximately 400 species of orchids, which corresponds to 31% of all species for which pollination systems are known ( Peter 2009). The most common mechanism of self-pollination in orchids, occurring in approximately half of the self-pollinating species studied to date, results from a reduction in the size of the rostellum that allows contact between the pollinia and the stigma (Catling 1990). This is the same mechanism observed in the newly discovered species G. kuroshimensis .

It has been suggested that autonomous self-pollination represents an evolutionary response to ensure reproductive success when there is a lack of pollinators, and the frequency of pollination is regularly quite low ( Baker 1955). Given that mycoheterotrophic plants commonly inhabit forest floors, where they are shaded by dense woodland or scrub, it is likely that they frequently experience such conditions. These environments have extremely low levels of light intensity in the dim to dark range, which have been linked to the development of mycoheterotrophy as an adaptation to survive these conditions ( Bidartondo et al. 2004). However, such low-light environments are not suited to species of insects commonly associated with pollination, which could limit plant reproduction ( Herrera 1995, 1997). Indeed, most of the mycoheterotrophic species investigated to date (especially nectarless species) seem to have abandoned insect pollinators in favor of self-pollination (e.g. Suetsugu 2013a, b, Suetsugu 2014, Suetsugu 2015b). The complete cleistogamy observed in the mycoheterotrophic genus Gastrodia can therefore be considered a mechanism that provides reproductive assurance to compensate for pollinator limitation due to their lack of nectar and pollinatorhostile habitat.

Figures

FIGURE 1. Flowering plants of Gastrodia kuroshimensis (from the type locality; Bar = 5 cm).

FIGURE 2. Gastrodia kuroshimensis (from the type locality). A–B. Flower (A, front view; B, back view). C. Flower with slit between the lateral sepals, back view). D. Flattened perianth tube. E. Lip and column. F. Lip. G. Flattened lip. H–I. Column (H, back view; I, front view). Bar = 5 mm.

FIGURE 3. Gastrodia kuroshimensis (from the holotype).A. Habit. B–D. Flower.(B, side view; C, front view; D, back view). E. Flattened perianth tube. F. Flattened lip.G–I. Column (G, lateral view; H, back view; I, front view).A. Bar = 3 cm. B–I. Bar = 5 mm.