Arisaema melanostomum Z.X.Ma, Xiao Yun Wang & Wen Yan Du, 2019

Ma, Zheng-Xu, Du, Wen-Yan & Wang, Xiao-Yun, 2019, A new species and a new combination of the genus Arisaema (Araceae) from China, Phytotaxa 395 (4), pp. 265-276 : 266-274

Arisaema melanostomum Z.X.Ma, Xiao Yun Wang & Wen Yan Du , sp. nov. ( Figs. 1–3 , 4A )

The new species is closely related to Arisaema kiushianum Makino , but differs from it in having a glossily atropurpureus subterranean stem producing glabrous, short cylindric and nodular tuberlets with multiple lateral buds (vs. creamy to whitish yellow subterranean stem covered with fibrous, applanate-globose tuberlets without lateral buds), a stably 5–7-foliolate leaf blade (vs. 7–13(–15)-foliolate leaf blade), a flat spathe-limb with a glossily atropurpureus ring on adaxially (vs. a fornicate-orbiculate spathe-limb with sub-circular atropurpureus lines on adaxially) and a spathe-tube with thick T-shaped white area adaxially, viridescent-colored at middle part (vs. a spathe-tube with thin T-shaped white area adaxially, never viridescent-colored at middle part).

Type: — CHINA. Guangdong Province: Shenzhen City, Yantian District, Yantian Sub-district, Sanzhoutian, ca. 450 m, 14 April 2018, Zhengxu Ma 0021 (holotype PE!, isotypes HK!, PE!).

Perennial herb, seasonally dormant, paradioecious. Subterranean stem tuberous, glossily atropurpureus to purplish-brown at outside, subglobose to slightly depressed globose, to 4.0 cm in diam., bearing several tubercles at apex. Tubercles concolorous with subterranean stem, short cylindric, nodular and bearing multiple lateral buds. Cataphylls 3, very thinly membranous. Foliage leaf solitary; petiole green to purplish-green, glossy; proximally sheathing and forming subterranean pseudostem, leaf blade pedate, forming obscure rachis, 5–7-foliolate; veins abaxially raised, lateral numerous; leaflets olive-green, waxy, thick membranous, sessile, oblong, 17.0–21.0 cm × 5.0–7.0 cm, base broadly cuneate, apex acute to slightly acuminate. Inflorescence solitary, adhere to ground. Peduncle creamy, glossy, 2.3–2.5 cm in length, nearly subterranean. Spathe basically creamy, waxy and very thickly membranaceous; tube cylindric, 4.0– 4.5 cm in length, 1.6–2.0 cm in diam., abaxially creamy, obscurely darkenedly mottled, with a white area at base, adaxially glossily atropurpureus, with white longitudinal lines, presenting a thick white “T” shaped area (attenuate to base and abruptly expending at apex forming a semicircle distally, 4.1–5.2 cm in length, 0.9–1.3 cm in width) viridescent at middle; mouth broadly auriculate, auricle horizontal or bending, to 1.3 cm in width, creamy, mottled purplely; limb bending and flat, acuminate-ovate, ca. 4.5 cm × 5.0– 5.3 cm, slightly caudate at apex, broadly rounded-auriculate at base, abaxially creamy, sometimes slightly purplish, with the mid rib raised and arcing at base, adaxially creamy, with a thick atropurpureus ring, but never presenting reticulations or anastomosed strips. Spadix unisexual. Female zone short cylindric, 1.3–1.4 cm in length; gynoecium dense; ovary green, barrel-shaped, rugose; stigma very short, nearly invisible; style a white spot, puberulent. Male zone cylindric, ca. 1.5 cm in length; androecium lax; synandria pale yellow, stipitate, consistingca. 2 anthers of each; thecae globose, dehiscing by an apical pore. Spadix-appendix long-flagelliform, sigmoid, to ca. 49.0 cm in length; proximal part reddish-white with purple mottles, erect, sub-cylindric, 2.0– 2.7 cm in length, sessile and attenuate to base, to ca. 0.4 cm in diam. at base; middle part dark reddish-purple, bending down and reaching out of spathe-mouth, obviously swelling, to ca. 7.6 cm in length, to ca. 0.6 cm in diam.; distil part greenish white, upright, flagelliform, to ca. 39 cm in length, ca. 0.1 cm in diam., attenuate to apex. Infructescence cylindric, erect, ca. 2.9 cm in diam., ca. 3.5 cm in length. Fruit a bacca, densely arranged, orange-red when ripened, ellipsoidal, 0.5–0.7(–1.0) cm in diam., 0.6–1.2 cm in length, rugose to slightly echinate and with obvious irregular ridges on the upper side, apex acute, with a short tip, each comprising 1–2(–3) seeds. Seed creamy, globose or semi-globose, 0.5–0.6 cm in diam.

Additional specimen examined (paratype):— CHINA. Guangdong Province: Shenzhen City, Yantian District, Yantian Subdistrict, Sanzhoutian, ca. 450 m, 30 March 2018, Jun-Rong Xie 1803001 ( SZG!).

Phenology:— Flowering from mid-March to mid-April. Fruiting from August to October.

Distribution and habitat:— Arisaema melanostomum is only known to a nameless stream valley covering dense subtropical forests in Shenzhen, Guangdong Province, China ( Fig. 6 ).

Conservation Status:— Arisaema melanostomum is so far only recorded from a single locality near Shenzhen City, Guangdong Province, China, with fewer than 250 individuals. Potential threats of the species include illegal collecting, illegal land clearing, illegal lumbering, and road construction. The inefficiency of the legislature and frequent nonfeasance of the government and local botanical gardens are also serious problems for the conservation of A. melanostomum as well as other rare local species. According to our field surveys and empirical evidence provided by the local hobbyists, the extent of occurrence (EOO) of the species is estimated to be 0.010 km 2, whereas its area of occupancy (AOO) is estimated to be 5 km 2. Consequently, A. melanostomum is supposed to be assigned a preliminary conservation status of “Critically Endangered” [CR B1a+2a+C] (IUCN, 2001).

Eponymy: —The epithet of the new species, melanostomum , is the combination of two greek morpheme, “melano- ” and “-stomum”, derived from the words melanos (black, dark colored) and stoma (mouth), respectively, referring the obvious atropurpureus ring at spathe-limb and spathe-mouth. The Chinese name of this new species is recommended as “ǟfiffifl”, a translation of the epithet.

Key to the Arisaema sect. Flagellarisaema (partly revised from Li et al. (2010) and Murata (2011))

1. Spathe basically green; spadix bisexual or unisexual.........................................................................................................................2

– Spathe multi-colored, usually with purple strips or reticulations; spadix unisexual..........................................................................4

2. Peduncle longer than petiole; spathe flat, bending. Eastern Asian............................................ A. heterophyllum Blume (1836: 110)

– Peduncle shorter than petiole; spathe sub-erect. Northern American.................................................................................................3

3. Leaf blade 5–7(–9)-foliolate; spathe-limb widened, expending at base or slightly auriculate, slightly warping spadix-appendix at apex......................................................................................................................................... A. macrospathum Bentham (1840: 52)

– Leaf blade (5–)7–13(–21)-foliolate; spathe-limb small, slightly involute, margin truncate ................................................................ ...................................................................................................................... A. dracontium ( Linnaeus 1753: 964) Schott (1832: 17)

4. Spathe-limb expanded at base, united with expanded part of spathe-mouth and showing a single cordate appearance ..................... ....................................................................................................................................................................... A. cordatum N.E.Brown

– Spathe-limb not united with auricles at base......................................................................................................................................5

5. Spathe-tube without a T-shaped area adaxially..................................................................................................................................6

– Spathe-tube with a T-shaped area adaxially.......................................................................................................................................9

6. Spathe with distinctly longitudinal striation on both surfaces, spathe-limb flat; anthesis at autumn .................................................. ........................................................................................... A. thunbergii Blume subsp. autumnale J.C.Wang, J.Murata & H.Ohashi

– Spathe without distinct longitudinal striation on both surfaces, spathe-limb fornicate; anthesis at spring........................................7

7. Spadix-appendix abruptly inflate at and bright yellow, crisped and verrucose at spathe-mouth, obviously attenuate to the base...... ................................................................................................................................................ A. thunbergii Blume subsp. thunbergii

– Spadix-appendix inflated dark to light purple proximally, slightly attenuate to the base..................................................................8

8. Spadix-appendix smooth entirely ......................................................................................................................................................... ...................................... A. thunbergii Blume (1836: 105) subsp. urashima (H. Hara 1935: 822) H.Ohashi & J.Murata (1980: 307)

– Spadix-appendix rugose at base .................................................... A. thungbergii Blume subsp. geomundoense S.C. Ko (2006: 213)

9. Subterranean stem bearing subglobose to depressed-globose tubercles; T-shaped area at spathe-tube pure white; spathe-limb orbiculate-fornicate, with sub-circular atropurpureus lines, never ringed adxially......................................... A. kiushianum Makino

– Subterranean stem bearing nodular tubercles; T-shaped area at spathe-tube viridescent at middle; spathe-limb flat, with a thick atropurpureus ring adaxially.................................................................. A. melanostomum Z.X.Ma, Xiao Yun Wang & Wen Yan Du

Discussion:— The morphological characteristics of the new species A. melanostomum well fit in the A. sect. Flagellarisaema, consisting i) a tuberous subterannean stem; ii) a quincuncial phyllotaxy; iii) an often pedate leaf blade; iv) apically dehiscing thecaes and v) a flagelliform and sigmoid spadix-appendix, sessile at base, without neuter flower and largely elongate at apex. The morphological evidences reveal close relationships among the novelty, A. thunbergii subsp. autumnale , A. cordatum and A. kiushianum , while meticulous morphologic comparisons are shown in Table 1 and detailed discussions are presented below.

The novelty, A. melanostomum , was previously misrecognized as A. cordatum since they share a similar silhouette in general. However, according to our observation of living material as well as examination of the desiccated specimens, the diagnostic characters are readily distinguishable in the two species, and the detailed comparison is present in Table 1. Here we would like to discuss on several details, which are often illegible or neglected in observation.

First, the spathe structure of the 2 species are completely different ( Figs. 2–3 ). The spathe structure of A. cordatum is unique in the genus Arisaema for its almost united spathe-limb and spathe-mouth showing a single triangular appearance. The spathe-limb of this species is always noticeably sub-erect for its correlation with the spathe-mouth. On the contrary, the one in A. melanostomum is much simpler and well differentiated into spathe-limb and spathe-mouth: its spathe-limb is flat and bending, with a mid-rib rising abaxially and its spathe-mouth is simply auriculate and independent.

Second, the structural discrepancy in gynoecium between the two species is also conspicuous. Arisaema melanostomum has a slightly rugose and dense gynoecium, while A. cordatum exhibits a smooth and lax one. Furthermore, when the pistil develops into immature fruit, the recognizable differences between the two taxa even increase: berry in the former presents a verrucose to slightly echinate appearance with a little tip at apex ( Fig. 4A ), but it is completely glossy and slightly concaved in A. cordatum ( Fig. 4B ). However, the morphology of gynoecium and fruit are still seldom studied or often completely neglected in Arisaema taxonomy, and we are looking forward to more precise comparative studies on these characters.

In addition, from a geological perspective, though A. melanostomum shares a sub-sympatric distribution with A. cordatum , the two are never really observed mixed in the same population. Our field works suggested that the former inhabits the understory of the dense sub-tropical forests in a lower elevation at about 200–300 m, whereas the latter is always found in a higher elevation generally above 650 m, in lax forests, sometimes mixing with bamboo, or gravel slops. As a result, though the two species are closely related and even sharing the adjacent distribution, the genetic exchange could be minimal due to geographical isolation.

On the other side, the utility of morphological characters in classification should be examined. In general, the taxonomic significance of subterranean stem morphology and leaflets’ number (vegetative organ), as well as coloration of spathe (reproductive organ), are highly debated ( Gusman & Gusman 2002, Gusman & Gusman 2006, Li et al. 2010, Ma & Li 2017). Therefore, based on our empirical observations and measurements, we hope to give a clear morphological evaluation of A. melanostomum .

The morphology of subterranean stem is often regarded as an important character for Arisaema taxonomy and it can sometimes exhibit an enormous variation among closely allied species ( Gusman & Gusman 2002, Gusman & Gusman 2006, Murata 2011, Ma & Li 2017). A. melanostomum has a sub-globose subterranean stem, atropurpureus to purplish-brown at outside, bearing several glossily atropurpureus, short cylindric and nodular tubercles, which is often distributed with many multiple lateral buds at apex. According to our observations of the ex-situ collections, structure of the subterranean stem of the novelty is morphologically conservative and it is readily distinguishable from all its supposed relatives, A. cordatum , A. kiushianum and A. thunbergii subsp. autumnale . Hitherto, the short cylindric and nodular tuberlet with multiple lateral buds has never been reported to any species in the A. sect. Flagellarisaema and its discovery in A. melanostomum reveals the high morphological diversity of stem structure in the genus Arisaema .

Besides, the number of leaflets is also worth of discussing for its controversy. This characteristic is often abandoned in Arisaema taxonomy due to its wide range of variation—even in a single population—profoundly affected by the precipitation as well as other environmental factors. However, according to our measurements A. melanostomum always has a 5–7-foliolate leaf blade, thus differing from A. kiushianum (7–13(–15)-foliolate) and A. thunbergii subsp. autumnale (11–15-foliolate). Therefore, it is suggested that this character could be diagnostic among the three taxa. Nonetheless, due to our deficiency in the cultivation of the latter two taxa, as well as the controversies in the stability of the leaflets’ number, we would still like to hold a cautious attitude toward the utility of this character.

As for the reproductive organ, the coloration of spathe, a part of inflorescence, is sometimes taxonomically confusing as well. The coloration of the spathe-mouth-limb in the 4 allied species is distinctive and detailed comparison of this characteristic among them is given below in Table 1. Especially, the presence of the white area in the spathe-tube (here as the T-shaped area) is believed to be crucial diagnostic characters for certain species in the genus, considering its importance in the pollination, a vital process for genetic flow, with obvious discrepancies potentially influencing the process and possibly even generating reproductive isolation between the populations. Consequently, we presume that because of the significantly distinct spathe morphology the gene flow between A. melanostomum and its relatives should be minimal and, thus, it is suggested that the spathe coloration is capable of distinguishing the 4 morphologically allied species.

In conclusion, based on the morphological studies comparing A. melanostomum with its relatives, as well as evaluating the characters used in Arisaema taxonomy, we consider that A. melanostomum should be confirmed as a distinct species based on sufficient evidence.

Figures

FIGURE 1. Arisaema melanostomum. A. subterranean stem; B. mature individual; C. abaxial surface of spathe (side view); D. adaxial surface of spathe; E. female spadix; F. male spadix; G. infrutescence. The painting is drawn by Yi-fan Li.

FIGURE 2. Arisaema melanostomum. A. individual in the habitat; B. inflorescence (front view); C. inflorescence (side view); D. adaxial surface of spathe; E. male spadix with in spathe; F. gynoecium; G. subterranean stem. Images A, C & E © Xiao-Yun Wang; B, D, F & G © Zheng-Xu Ma. All plant images were taken from the type population of A. melanostomum, and B, F & G are from the holotype.

FIGURE 3. Arisaema cordatum. A. individual in the habitat; B. inflorescence (front view); C. inflorescence (side view); D. adaxial surface of spathe; E. male spadix with in spathe; F. gynoecium; G. subterranean stem. Images A, C & E © Xiao-Yun Wang; B, D, F & G © Zheng-Xu Ma.

FIGURE 4. Immature infructescences of Arisaema melanostomum (A) & A. cordatum (B). Images A © Jun-Rong Xie; B © Liu. Image A was taken from the type population of A. melanostomum.

FIGURE 6. Distribution map of Arisaema cordatum (triangle), A. kiushianum (square). A. melanostomum (solid circle) and A. thunbergii subsp. autumnale (pentagram). Scale 1: 25 000 000. China’s border in this map is derived from Topographic Map of the People’s Republic of China (scale 1: 4 000 000) (1989) published by Sinomaps Press.