Colocasia spongifolia P.J.Matthews, V.D.Nguyen, Q.Fang & C.L.Long, 2022

Matthews, Peter J., Nguyen, Van Du, Fang, Qiong & Long, Chun-Lin, 2022, Colocasia spongifolia sp. nov. (Araceae) in southern China and central Vietnam, Phytotaxa 541 (1), pp. 1-9 : 3-7

Colocasia spongifolia P.J.Matthews, V.D.Nguyen, Q.Fang & C.L.Long , sp. nov. (Figs. 2A, B & 5A, C–I type)

In contrast to Colocasia esculenta , C. formosana , C. gongii and other species, C. spongifolia has a thick, rubbery leaf blade with smooth underside (vs thin, not-rubbery, with raised interprimary veins on underside). It differs from C. esculenta , C. formosana , C. lihengiae and other species by having a stem without side-tubers or stolons (vs with side-tubers or stolons) and staminodes absent or rare at base of female zone (vs many or abundant throughout the male zone). It differs from C. lihengeae , C. yunnanensis , and some cultivars of C. esculenta by having a sterile appendix (vs much reduced or absent).

Type: — VIETNAM. Th ừa Thiên Hu ế, Phu Loc , Loc Tri , Bach Ma National Park , on the route from park headquarters to mountain top, and ca. 2 km before the hotel ‘ Do Quyen Villa’. Elevation 1,201 m. Lat. 16 o 11’39.32” N. Long. 107 o 50’57.54” E; 25th May, 2019, Nguyen V. D. & Nguyen S. K. 01 (holotype HN!), Nguyen V. D. & Nguyen S. K. 02 (isotype HN!). A single gathering; each specimen with stem, leaves, inflorescences (no fruit or seeds) .

Habit. Medium-sized herb, up to ca. 120 cm height (in open, sunny locations), perennial; usually single, never forming dense spreading colonies (clumps or patches) (Fig. 1). Stem and roots. Stem erect, or decumbent with erect apical part; up to 14 cm long and 5.5 cm diam.; adaxial buds single in each leaf axil, rarely forming new stem; side-tubers and stolons absent (Fig. 2). Outer skin surface white, or green on older stems where exposed to light, and covered by fibrous brown remnants of petiole tissue; leaf scars obvious, brown to light brown; inner skin (stem cortex) white; inner parenchyma white, not starchy, exuding clear gum (presumed glucomannan) when cut; roots white (occasionally purple where exposed to light), abundant, up to 25 cm long, 3–4 mm diameter, with many secondary rootlets of ca. 0.4 mm diameter emerging from the primary roots. Leaves. Two to several; blade ovate-cordate, peltate, with rounded posterior lobes, shallow sinus, and apiculate anterior lobe; mature blade size highly variable, with a maximum recorded length (from anterior tip to line defined by tips of posterior lobes) of ca. 58 cm, on ca. 116 cm tall petiole; blade waxy (not shining), darker green above, pale or milky green under, anterior lobe tilting down (Figs. 1 & 3). The central veins of the anterior and posterior lobes and the primary lateral veins (pinnate ribs) are raised and pale green on underside; submarginal collective and marginal veins are both distinct, separated by laminal tissue, the former ca. 2–4 mm from edge of blade, the latter close to edge (Fig. 3 A); margins of the posterior lobes fused between each lobe, forming a relatively shallow sinus (deeper in older or larger blades) (Figs 1, 3B). Interprimary (secondary to tertiary) veins are not raised on underside, which is distinctly smooth. Spongy mesophyll envelops the interprimary veins and is thicker next to primary lateral veins (Fig. 3 C, D). In reflected light, underside appears porous with “false pores” where epidermis is clear above mesophyll cavities (Fig. 3 C, D). The blade feels and looks rubbery when touched and bent by hand, forming axial wrinkles (Fig. 3C); milky-green colour of underside is most apparent when light is reflected by the white epidermal layer (Fig. 3C, D). Air cavities in spongy mesophyll (180–360 um diam.) are obvious to eye as lighter spots when viewed from below with light transmitted from above (and none reflecting off the lower surface) (Fig. 3E), or as dark cavities in reflected light when the lower epidermis is peeled away (Fig. 3F). Petiole bright green (pale green or white at base), slender, cylindrical; petiolar sheath extending to 1/2–2/3 of the petiole length; edge of sheath prominent after younger leaf has emerged, but not wide-flaring. Inflorescence. Peduncle, 25–30 cm long, cylindrical, upper cross-section sub-triangular (proximal face 8–12 mm across, lateral faces 12–14 mm), medium to light green, covered by one cataphyll up to half of length; erect when young, later bending towards ground, bringing the fruiting head close to ground or into contact (Fig. 4A). Spathe total length up to 19 cm; minimally constricted between lower tube and upper limb; lower tube conical, sub-triangular, 4.5–5.0 cm long, ca. 2.3 cm diam. at base, 1.5 cm diam. at apex, medium green; limb convoluted before opening; nearly oblong when fully open, 11.5– 14.0 cm long, 5.0– 5.5 cm wide, tip remaining convoluted and acuminate (3 cm long) after opening; yellow to light yellow, with papaya-like scent; upper spathe reflexes after anthesis, away from spadix (Fig. 5A). Spadix much shorter than spathe, 10–14 cm long; female (pistillate) zone of young spadix strongly conical, with sub-triangular crosssection (proximal face 10–12 mm across, lateral faces 15–17 mm; similar to upper peduncle), becoming cylindrical at upper end (4.0– 4.5 mm diam.); pistils dark green; staminodes absent, or rare at base of female zone; sterile interstice (between male and female zones) constricted or not, 8–15 mm long, upper part ca. 5 mm diam., covered by looselyspaced synandrodes; male (staminate) zone cylindrical, slightly narrower at upper end, 2.8–3.0 cm long, ca. 5.0– 5.2 mm diam., dull yellow; covered closely by male flowers joined in synandria; sterile appendix slender, tapering, 2–3 cm long, ca. 3 mm diam. at base, dull yellow, surface wrinkled (Fig. 5C, D). Ovary sub-globose to cylindrical, crosssection quadrilateral, apex rounded, 1.5 mm tall, 2 mm wide, dark green, glossy; stigma disciform, 4 blunt lobes, sessile, 1-locular; ovules abundant, parietal, orthotropous, ca. 0.3–0.4 mm long (Fig. 5D–G). Staminodes yellow, flask-shaped, smaller than pistil. Synandrodes porcelain-white, flattened, ca. 2 mm thick, outline mostly oblong (Fig. 5C). Synandrium cylindrical, 2 mm tall, 6–8 androus with connate stamens; apical pores 6–8; anther sacs in lateral pairs, each sac with an apical pore; pollen extruded as a loosely self-adhering mass; adjacent synandria appressed with lobes often interlocking (Fig. 5H, I). Infructescence (Fig. 5J) composed of lower spathe tube enclosing many green berries (up to ca. 200), berries firm and green when young and expanding, soft and yellow-green to orange at maturity; mature seeds ca. 1.25 mm long, uniform, light brown, oval, pointed at each end, with ca. 12 regular longitudinal ridges; average seed number per berry 40+/–(range 0–60+); total seed number per head up to ca. 8,000.

Etymology: —The specific epithet is derived from the distinct character of the leaf blade (visibly spongy appearance and rubbery to touch).

Ecology: —In contrast to other Colocasia species, dense spreading colonies (clumps or patches) were not seen. The persistent adaxial buds of C. spongifolia may support regeneration from fallen, decumbent stems. During ex situ cultivation, it has been possible to force buds to sprout by removing the stem apex and placing the stem horizontally on damp soil. At Bach Ma, immature fruiting heads were marked by the teeth of an unknown terrestrial frugivore (possibly a seed disperser, and likely a small mammal), and living larvae of a specialist pollinator ( Colocasiomyia sp. ) were found between berries on mature fruiting heads.

Phenology: —Initial field observations suggest seasonal patterns of flowering, fruiting and seedling growth in relation to rainfall and temperature. Rainfall is generally higher in Thua Thien-Hue than Mengla (Climate-Data.org 2021), peaking in autumn in Hue (600 mm /month, October–November) and summer in Mengla (300 mm /month, July). Maximum daytime temperatures are similar in each area, with hot summers (28– 300 C) in June–August. In Thua Thien-Hue, we saw early flowering in late May, and late fruiting with mature seeds in late September. These observations indicate a main phase of flowering during the months of peak temperature (May–August), with seed germination and early growth following in wet season. The months of peak temperature and rainfall coincide in Mengla, where young seedlings were seen in early May, at the start of the warm, wet summer. In both areas, wet conditions are likely to favour seedling establishment.

Distribution: —Southern China, Yunnan: Mengla County, Xishuangbanna Dai Autonomous Prefecture, Yunnan, China (several sites NW, NE and SE from Mengla Township). Elevation: 656–962 m; max. straight-line distance between sites: ca. 52 km. Central Vietnam, Thua Thien-Hue: (1) Bach Ma National Park, Loc Tri district, Thua (several sites), (2) vic. Hong Kim, A Luoi District (one site), (3) vic. Huong Nguyen, A Luoi district (several sites). Elevation: 675–1,201 m. Maximum straight-line distance between Thua Thien-Hue sites: ca. 70 km. Max. straight-line distance between northern and southern locations, Mengla to Thua Thien-Hue: ca. 500 km.

Habitat: —Isolated single plants or small populations; usually seen on roadsides but also found distant from road; on humus and colluvium or in rock crevices, in damp to wet positions on moderate to steep slopes with evergreen humid or cloud forest; at forest edge or under canopy gap, with limited direct exposure to sunshine (lower montane environments). The Thua Thien-Hue and Mengla populations occupy a boundary region between the temperate and tropical Köppen-Geiger climate zones defined by Beck et al. (2018), namely Cwa/Aw in Mengla, and Cwa/Am in Thua Thien-Hue (Cwa = temperate dry winter, hot summer; Aw = tropical savannah; Am = tropical monsoon).

Conservation: —Common but not abundant in each area of known distribution. Main threat may be loss of forest habitat near roads. Many locations are in conservation areas and appear well protected.

Notes: —The key below keeps the name C. lihengiae , though a previously reported Indian representative of this species ( Gogoi & Borah 2013) is now recognised as synonym for C. mannii Hooker (1893: 524) , and is an edible wild leaf vegetable in Assam ( Gogoi et al. 2019). C. lihengiae (c.f. C. mannii ) may have a complex history if economic use was widespread in the past. In our key, division of Colocasia into stoloniferous and non-stoloniferous groups may correspond to previously-suggested acaulescent (often tuberous) and caulescent (erect) sections ( Mayo 1997; Krause 1920). Stolons can be easily recognised as present or absent, but when stems are young, short, or often decumbent, it is difficult to classify them as erect or not. Stem position with respect to ground surface often reflects wind, water and soil conditions in sloping, semi-aquatic, or cultivated habitats. Tuberous, stout, erect, decumbent and rhizomatous habits are widespread in Araceae ( Mayo et al. 1997) , so taxa sharing one or more of these morphologies are not necessarily closely related. There might not be any general correlation between stem habit and other traits, though the present key suggests that erect species lack stolons and side-tubers, which in turn may reflect shy sprouting by adaxial buds. Erect stems with condensed nodes (where adaxial buds are located) are common in the sub-family Aroideae to which Colocasia belongs, and are found in Alocasia and Steudnera , for example ( Mayo et al. 1997).

Four characters described here for C. spongifolia have not been described in detail in previous reports of Colocasia species: margination of the leaf blade, spongy mesophyll cavity dimensions in the leaf blade, adaxial buds (including their abundance within an axil, and their development into lateral shoots), and peduncle orientation at the time of fruiting. All of these may be ecologically significant characters and require further study across the genus. Notes for each character follow.

(1) Leaf margination varies greatly in aroids generally ( Mayo et al. 1997). Among Colocasia species we see a marginal vein compression series (forms i-iv), with two marginal vein types located near or compressed at the edge: (i) No obvious marginal vein or submarginal collective vein (both types apparently fully merged into the blade edge). (ii) A submarginal collective vein near the edge, with no obvious marginal vein. (iii) A sub-marginal collective vein slightly separate from an inconspicuous marginal vein near the edge. (iv) A sub-marginal collective vein clearly separated by laminal tissue from an obvious marginal vein near the edge. Forms i–iii are found in other Colocasia species, while iv is typical in C. spongifolia , in wild plants and seedlings ex situ.

(2) Few data are available on spongy mesophyll cavity dimensions in Colocasia . Our own initial observation indicates an approximate range of 100–200 um diam. in planar view (after peeling off the lower epidermis) of a single, fresh sample of cultivated C. esculenta , and 180–360 um diam. in C. spongifolia . Keating (2002: 285) shows micrographs of similar, large substomatal air spaces in the spongy mesophyll of C. fallax , using thin sections of chemically-fixed tissue, but the dimensions in fresh and fixed tissues may not be comparable.

(3) In seedlings observed ex situ, over a period of three years, the adaxial buds in C. spongifolia have been consistently single and non-sprouting, with removal of the apical shoot needed to force sprouting and production of a new erect stem. Over the same period in the same location, and without removal of the apical shoot, C. formosana produced abundant long stolons, as it does in the wild. Bud behaviour (whether shy or readily sprouting) may be independent of whether or not adaxial buds are single or multiple in leaf axils. Further study is needed to understand the relationship between lateral bud abundance, lateral shoot absence or presence, and the overall growth habits of Colocasia species.

(4) Peduncles in C. spongifolia show strong downward bending (geotaxis) that begins during anthesis and continues as the upper spathe withers. In wild plants, mature fruiting heads were bent low to the ground or were missing (perhaps removed by terrestrial frugivores) (Fig. 4). The bending behaviour was also observed ex situ in potted seedlings that flowered and fruited after self-pollination. Peduncle orientation when fruit reach maturity has not been reported for most Colocasia species, but the mature fruit of C. esculenta are generally displayed on erect peduncles.