Orophea sichaikhanii (Annonaceae), a new species from southern Thailand, with a key to the species of Orophea in Thailand and notes on some species

Background and aims – Recent botanical expeditions in Ranong Province, southern Thailand yielded unidentifiable collections of Orophea subgenus Sphaerocarpon (Annonaceae). To elucidate the taxonomic status of these collections, detailed morphological examinations and comparisons with morphologically similar species are made. Material and methods – This study followed standard practices of herbarium taxonomy. Specimens of Orophea spp. in BKF, CMUB, L, and QBG herbaria were studied. Digitised type specimens deposited in BM, E, G, K, and L herbaria were accessed. A stereo microscope was used for morphological observations and measurements. Key results – A new species Orophea sichaikhanii is described and illustrated. The Peninsular Malaysian O. hastata and O. kingiana are the species most similar to the new species. Orophea sichaikhanii is different from O. hastata in several traits: indumentum on ovaries and young twigs; length of pedicels, inner petals, and inner petal claw; and inner petal colour and tip. The new species differs from O. kingiana by having dissimilar colour and tip of inner petals; lower number of stamens and carpels per flower; and glabrous ovaries. Additionally, a key to the species of Orophea in Thailand and notes on certain species are provided.

The genus Orophea Blume, along with 24 other genera, constitutes the Miliuseae, the largest tribe of subfamily Malmeoideae (Chatrou et al. 2012;Chaowasku et al. 2014Chaowasku et al. , 2020Guo et al. 2017). The monophyly of the genus has been demonstrated (Guo et al. 2017), but its sister group remains obscure (Chaowasku et al. 2014Guo et al. 2017). Orophea is principally recognised by the combination of (1) dissimilar petal whorls, the inner one being clawed towards the base and generally connivent at anthesis, (2) a reduced number of stamens and carpels per flower, and (3) loosely arranged stamens with a minute connective apex not covering the thecae (Keßler 1988;Leonardía & Keßler 2001). In addition, glands of various shapes are generally present on the adaxial surface of the inner petals (Keßler 1988;Leonardía & Keßler 2001). The genus, with ca 61 species (Turner 2018), is distributed from the Indian subcontinent through mainland Asia to the Southeast Asian islands; the Moluccas mark the eastern boundary of the genus (Keßler 1988). Two subgenera have been differentiated: subgenus Orophea with ellipsoid-cylindrical to cylindrical monocarps, which are moniliform when containing multiple seeds, and subgenus Sphaerocarpon Kessler with globose (rarely shortly oblongate) monocarps (Keßler 1988;Leonardía & Keßler 2001). Each subgenus has been shown to be monophyletic, but this is only based on a limited number of species per subgenus (Guo et al. 2017). However, when more species in each subgenus are added in a molecular phylogeny, each subgenus remains monophyletic with strong support (Anissara Damthongdee et al. unpubl. res.).
In the course of revising the genus for the flora of Thailand, unidentifiable specimens of Orophea subgenus Sphaerocarpon from Ranong Province, southern Thailand were collected. Comparisons with the morphologically most similar species reveal that these collections represent a new species, which is herein described. Furthermore, a key to the species of Orophea in Thailand and a discussion on additional species with doubtful identification (i.

MATERIAL AND METHODS
Practices of standard herbarium taxonomy were conducted. Specimens studied were from BKF, CMUB, L, and QBG herbaria. Descriptions (and drawings) of Orophea spp. in the following publications were consulted: Hooker & Thomson (1872), King (1892), Craib (1922Craib ( , 1925, Sinclair (1955), Keßler (1988Keßler ( , 1990, and Leonardía & Keßler (2001). Type specimens of relevant names were accessed via online platforms of BM, E, G, K, and L herbaria. The indumentum terminology of Hewson (1988)  Diagnosis -Orophea sichaikhanii belongs to subgenus Sphaerocarpon, owing to the possession of a ± reticulate tertiary venation and a globose monocarp. By possessing similar leaf shape and size, inner petal shape, and nectary glands (number and shape) on the inner petal adaxial side, the Peninsular Malaysian O. hastata King and O. kingiana Leonardía & Kessler are morphologically most similar to the new species. Orophea sichaikhanii differs from O. hastata by having glabrous to subglabrous young twigs (vs sparsely to densely hairy), longer pedicels (10.5-15 mm vs ca 2 mm) and inner petals (11.5-13.5 mm vs 8-9 mm), shorter inner petal claw (1.5-2 mm vs ca 4 mm), dissimilar colour of inner petals (greenish yellow, tip yellow vs dark red with dark yellow point), and glabrous ovaries (vs sparsely hairy). Compared to O. kingiana, the new species is also different in several features: inner petal colour (greenish yellow, tip yellow vs dark red), number of stamens (9 vs 12) and carpels (11 or 14 vs 18) per flower, and indumentum on ovaries (absent vs sparse). In addition, the inner petal tip (both surfaces) of O. sichaikhanii is smooth, while it is slightly warty in O. hastata and O. kingiana. Description -Treelets ca 2.5 m tall. Young twigs glabrous to subglabrous. Petioles 5-8 mm, grooved on upper surface, glabrous on both surfaces; leaves subcoriaceous, elliptic, larger blades 8.7-21.5 × 3.5-9 cm, base cuneate to broadly cuneate, apex caudate-acuminate (acumen usually 15-20 mm long), seldom ± acute, glabrous to subglabrous on lower surface, glabrous on upper surface; midribs raised on lower surface (less so towards apex), puberulous with appressed hairs, slightly sunken (flatter towards apex) on upper surface, glabrous; secondary veins 8-10 per side, apical end of adjacent ones usually not joining into loops, rather prominent on lower surface, angle with midrib 35-40° (at middle part of leaf blade); tertiary veins reticulate, seldom reticulatepercurrent. Inflorescences 3-to 5-flowered, axillary or in axils of fallen leaves; flower buds ovoid; peduncles 3-8 mm long, glabrous; rachis up to 13 mm long, glabrous, bracts present; pedicels 10.5-15 mm long, glabrous, each bearing a single triangular bract, placed at a bit higher than midpoint of pedicels. Sepals connate at base, broadly ovate-triangular, 1.6-2.1 × 1.6-2 mm, adaxial side glabrous, abaxial side subglabrous, margin puberulous-tomentose with erect and appressed hairs. Outer petals broadly ovate, 3-5 × 3-4.5 mm, greenish yellow, glabrous on both surfaces, margin puberulous-tomentose with erect and appressed hairs; inner petals ± elongated rhombic, 11.5-13.5 × 4-5 mm, 2-2.5 mm thick (at midpoint of blade), connivent at anthesis, greenish yellow, tips (ca 1/3 of blade) yellow and separated at anthesis, smooth on both surfaces, apex obtuse, claw 1.5-2 mm long, adaxial side of inner petals glabrous on claw and lower half of blade, tomentose with erect and appressed hairs on midline of upper half of blade only, abaxial side subglabrous, margin puberulous with erect and appressed hairs, nectary glands on adaxial side located on lower half of blade, paired slits. Stamens 9 per flower, 1-1.5 mm long, connective apex obtuse. Carpels 11 or 14 per flower, 1-1.5 mm long, stigmas globose, ovaries glabrous, ovules 2 per ovary, lateral, uniseriate. Torus ± hemispherical, pilosevillous. Fruit only 1 monocarp found, globose, ca 14 mm in diameter, smooth and glabrous, base contracted into a stipe ca 3.5 mm long, smooth and glabrous, fruiting pedicel ca 17 mm long; seed not observed. Distribution (including conservation status), phenology, and ecology -Critically Endangered: CR B2ab(iii). Orophea sichaikhanii is so far only known from Ranong Province, southern Thailand. According to a recent expedition to Ban Khlong Ngoen forest area (ca 7 km northeast of Ban Khlong Ngoen School) with a distance of ca 5 km travelled, around 20 individuals were observed; based on our observations we assume that the AOO of the species is below 10 km 2 . The species occurs in a single location, which has been considerably disturbed by agricultural expansion. In  addition, other human activities such as mushroom or fruit foraging may cause damage to individuals. Although the new species may also occur in nearby protected forests (e.g. Prince of Chumphon Wildlife Sanctuary, South Side), it is under threat because deforestation in the Ban Khlong Ngoen forest area, particularly for plantations, happens nearly every single day (Geerawit Sichaikhan pers. comm.). On the basis of this information, we assess the species as critically endangered based on IUCN (2012) criterion "CR B2ab(iii)" . The flowering material was collected in November, whereas  Keßler (1988) and Leonardía & Keßler (2001) reported that the number of stamens and carpels per flower of Orophea is divisible by three, but our carpel counts (11 or 14) on three flowers of O. sichaikhanii and the stamen and/or carpel counts of King (1892) on some species of Orophea do not agree well with such a finding. Floral ontogenetic studies on various species of Orophea, including O. sichaikhanii, may shed light on the origin of this incongruence.
Although O. sichaikhanii and O. malayana Kessler belong to the same subgenus and exhibit ± green flowers, they differ in many aspects (e.g. leaf blade texture, petiole length, inner petal size and shape, arrangement of inner petal glands, and number of stamens and carpels per flower ;Keßler 1990) and are therefore unlikely to be confused. Although O. malayana has been claimed to occur in Thailand (Leonardía & Keßler 2001), it is absent from Thailand based on reinvestigations by the first author.

Key to the species of Orophea in Thailand
Orophea enterocarpa was reported to occur in Thailand (e.g. Keßler 1988;Chalermglin 2001;Gardner et al. 2015). However, comparisons of the lectotype and protologue of O. enterocarpa (Hooker & Thomson 1872) with the type specimens, protologue (Craib 1925), and several other collections of O. fusca (see appendix) led to the conclusion that the plant identified in Gardner et al. (2015: 140) as O. enterocarpa represents O. fusca, whereas the plant identified in Chalermglin (2001: 248-249) as O. enterocarpa possibly represents an undescribed species. Therefore, O. enterocarpa is excluded from the flora of Thailand and is considered to only occur in Peninsular Malaysia. It has smaller petals (both whorls: outer petals 5 × 3.5-4 mm vs 9-13.5 × 7-11 mm, inner petals 8-10 × 3-4 mm vs 13.5-17.5 × 4-6 mm, inner petal claw 3.5-4.5 mm long vs 8-10 mm long; fig. 4) than O. fusca. It is worth noting that Turner (2018) also lists the two species as distinct from each other.
According to Leonardía & Keßler (2001), Orophea siamensis occurs in southern Thailand and Nan Province of northern Thailand. Study of the type specimens and protologue of O. siamensis (Craib 1922)   It is worthwhile to mention that the northern part of southern Thailand is likely to be a centre of endemism for Annonaceae in Thailand, since, besides O. sichaikhanii, there are a number of species that are not found elsewhere outside this region, e.g. Artabotrys longipetalus J. Chen  awarded to the last author is gratefully acknowledged. The mentorship of Somran Suddee is appreciated. David Johnson and Lars Chatrou provided constructive comments for the improvement of an earlier draft of this article.