Curcuma sumonii sp. nov., and C. wanchaii sp. nov. (Zingiberaceae), two new taxa of Curcuma subgen. Curcuma from Thailand

Abstract

This research presents the discovery of two novel taxa, Curcuma sumonii Saensouk, P.Saensouk, Boonma & Techa, and C. wanchaii Saensouk, P.Saensouk, Maknoi & Boonma, belonging to the Curcuma subgenus Curcuma. Curcuma sumonii was collected in Sukhothai Province, Northern Thailand, while C. wanchaii was found in Kanchanaburi Province, Southwestern Thailand. Our study provides a comprehensive documentation of their morphological attributes, including detailed descriptions supported by visual aids. In addition to morphological data, we offer insights into vernacular names, ecological characteristics, phenology, traditional uses, conservation status, and distribution maps. Morphological comparison Tables 1 and 2 are provided to differentiate between these newly discovered taxa and their allied species. Furthermore, we present an identification key to facilitate the differentiation of these taxa from their allied species. The findings of this study contribute to the understanding of biodiversity within the Curcuma genus and offer valuable information for further conservation efforts and taxonomic research.

Introduction

The Zingiberaceae family is renowned for its rich diversity of plant species with over 50 genera, and over 1400 species worldwide, distributed throughout tropical and subtropical regions¹, which are esteemed for their ornamental allure, culinary versatility, and medicinal properties, as well as their cultural significance in rituals and socio-religious practices. Among the esteemed taxa in this family, the genus Curcuma L. stands out for its captivating flower displays. Significantly, C. longa L., which is the representative species of the genus, has had a significant influence on different facets of Indochina’s everyday existence, despite not being native to the region^1–3.

The genus Curcuma is now classified into three subgenera: Curcuma L., Ecomata Škorničk. & Šida, and Hitcheniopsis (Baker) K.Schum^4,5. , with over 160 species worldwide. In Thailand consisting of approximately 80 species distributed throughout the entire country⁶. The subgenus Hitcheniopsis is characterized by an absence of epigynous glands, unless the remaining two subgenera have epigynous glands. More specifically, the subgenus Curcuma is recognized by its distinctively well-developed coma bracts and usually produces closed-form flowers, whereas the subgenus Ecomata generally does not have coma bracts and usually produces open-form flowers.

While conducting an investigation into the diversity of Zingiberaceae in Thailand, two previously unidentified species have been discovered. The first plant was found in Sukhothai province, Northern Thailand, bearing resemblance to C. angustifolia Roxb^7–9. Meanwhile, the second undescribed taxon was found in Kanchanaburi province, Southwestern Thailand, with morphological characteristics most similar to Curcuma nepalensis M. Sabu, Kasaju & S. Subedi¹⁰.

In order to verify their distinctiveness, the recently identified taxa were thoroughly compared to other species in the Curcuma L. genus that have similar morphological features. Thorough morphological investigations have been carried out and measurements were collected to evaluate important diagnostic characteristics. Furthermore, electronic images and data from verified herbarium specimens were thoroughly examined to ensure precise species identification. The results presented emphasize the continuous exploration endeavors in Thailand’s botanical geography and emphasize the significance of documenting and researching the variety of plants in the area. The recent identification of these two new taxa in the subgenus Curcuma L. not only expands the existing roster of known species, but also offers prospects for more investigation into their ecological functions, conservation status, and potential purposes. These discoveries enhance our understanding of the evolutionary relations within the Curcuma genus and contribute to greater conservation efforts by deepening our knowledge of Curcuma diversity in Thailand.

Results

Two novel taxa, Curcuma sumonii Saensouk, P.Saensouk, Boonma & Techa, and C. wanchaii Saensouk, P.Saensouk, Maknoi & Boonma, both categorized under the Curcuma subgenus Curcuma. The first species was encountered in Sukhothai Province, Northern Thailand, while the latter was found in Kanchanaburi Province, Southwestern Thailand (Fig. 1). Our documentation encompasses in-depth details regarding their morphological attributes, accompanied by visual aids. Furthermore, we furnish vernacular names, ecological characteristics, phenology, traditional uses, conservation status, distribution maps, and morphological comparison Tables 1 and 2 are provided to differentiate between these newly discovered taxa and their allied species. Moreover, we provide a key to identifying them and their allied species.

Fig. 1.

Distribution map of two new taxa: Curcuma sumonii Saensouk, P.Saensouk, Boonma & Techa sp. nov. from Sukhothai Province, and C. wanchaii Saensouk, P.Saensouk, Maknoi & Boonma sp. nov. from Kanchanaburi Province. (Map created with “Pixelmator Pro,” designed by Thawatphong Boonma).

Table 1.

Morphological comparison of Curcuma sumonii and C. angustifolia Roxb^7–9.

Characters	C. sumonii	C. angustifolia
Rhizome	Ovoid, without branches, white internally 1.8–2.2 × 1.2–1.8 cm	Oblong, with short branches, pale brown to white 3–5 × 3–4 cm
Tuberous root	Place closed to the rhizome (0.05–0.3 cm apart)	Place not close to the rhizome (more than 3.0 cm apart)
Leaf sheaths	6.5–10 cm long	10–40 cm long
Ligule	1.0–1.5 mm long	2–3 mm long
Petiole	1.0–1.5 cm long	4–10 cm long
Lamina	12.0–23.5 × 0.9–1.8 cm	25–60 × 3–6.5 cm
Inflorescence	Terminal	Lateral and terminal
Thyrse	4.5–6 cm long, 2–2.5 cm in diam.	10–12 cm long, 3.5–5 cm in diam.
Coma bract	Lanceolate, 2.0–2.5 × 0.5–0.8 cm	Elliptic, 3.0–4.0 × 0.7–1.0 cm
Fertile bracts	2.5–3 × 0.7–1.0 cm Lanceolate, apex acute	2.5–4.0 × 1.5–2.5 cm Ovate to obovate, apex obtuse to rounded
Bracteoles	Whitish	White with pink tinge toward the apex
Flowers	3–3.4 cm long	3.5–4.1 cm long
Floral tube	1.0–1.4 cm long	2.0–2.5 cm long
Corolla lobes	Pale yellow	White with dense pink to red tinge
Lateral staminodes	14–15 × 10–11 mm	10–12 × 4–7 mm
Labellum	16–18 × 15–17 mm	10–13 × 12–13 mm
Anther spurs	c. 2.5 mm long	1–2 mm long
Epigynous glands	c. 3 mm long	c. 5 mm long

Table 2.

Comparison of morphological characteristic of C. wanchaii and C. nepalensis¹⁰.

Characters	C. wanchaii	C. nepalensis
Rhizome	2–2.5 × 3–3.5 cm, pale yellow	4–5 × 2.5–3 cm, pale yellowish brown
Ligule	6–8 mm long	10–13 mm long
Thyrse	18–24 cm long, 7–9 cm in diam.	12–16 cm long, 5–6 cm in diam.
Peduncle	14–16 cm long, 0.9–1.2 cm in diam., sparsely hairy	11–13 cm long, 0.8–1.0 cm in diam., glabrous
Coma bract	Whitish or tip with a pinkish tinge, pubescent	Light green or white with a purple tinge, glabrous
Fertile bracts	20–36 in number, uniquely obovate, pubescent, White to creamy white with various levels of orange tinge	14–28 in number, ovate–elliptical, glabrous, Red with light greenish red towards the tip
Cincinnus	4–5 flowers	3–4 flowers
Bracteoles	Sparsely hairy	Glabrous
Flowers	5.2–5.5 cm long	4.5–5 cm long
Floral tube	3.7–3.9 cm long, sparse hairs Yellow	2.7–3 cm long, glabrous White, sometimes tinged with pink or yellow
Dorsal corolla lobe	17–18 mm long, pale yellow	13–15 mm long, white with a pink tinge towards tip
Lateral staminodes	Asymmetrical obovate, 13–14 × 6–7 mm	Oblong-lanceolate, 20–21 × 10–12 mm
Labellum	16–17 mm long, apex with incision 3–3.5 mm long	20–23 mm long, apex with incision 4–6 mm long
Filament	c. 2 mm long c. 2 mm broad at the connection point c. 4 mm wide at the base Yellow	5–7 mm long c. 3 mm broad at the connection point c. 6 mm broad at the base White
Anther	Vertical rectangle in the side view, yellow	Triangle in the side view, white
Ovary	c. 2.0 × 1.8 mm	3–3.5 × 2.5–3 mm

Taxonomic treatments

1. Curcuma sumonii Saensouk, P.Saensouk, Boonma & Techa sp. nov.—Figs. 1, 2, 3, Table 1

Fig. 2.

Curcuma sumonii Saensouk, P.Saensouk, Boonma & Techa sp. nov. (A) habit in natural habitat, and (B) side view of inflorescence with flowers. (Photographed by Surapon Saensouk).

Fig. 3.

Curcuma sumonii Saensouk, P.Saensouk, Boonma & Techa sp. nov. (A) side view of anther with filament, floral tube, calyx and ovary, (B) front view of anther with stigma, (C) dorsal corolla lobe, (D) lateral corolla lobe, (E) lateral staminodes, (F) labellum, (G) habit, (H) calyx with ovary, (I) side view of inflorescence, and (J) ovary with epigynous glands (drawn by Thawatphong Boonma).

–Subgen. Curcuma.

Diagnosis: This new species is similar to Curcuma angustifolia Roxb. but differs. Curcuma sumonii has an ovoid rhizome without branches, white internally (vs. C. angustifolia having an oblong rhizome with short branches, pale brown to white internally); Tuberous root place close to the rhizome, measuring approximately 0.05–0.3 cm apart (vs. tuberous root place not close to the rhizome, more than 3.0 cm apart); Leaf sheaths short, 6.5–10 cm long (vs. 10–40 cm long); Ligule 1.0–1.5 mm long (vs. 2–3 mm long); Petiole 1.0–1.5 cm long (vs. 4–10 cm long); Lamina measuring 12.0–23.5 × 0.9–1.8 cm (vs. 25–60 × 3–6.5 cm); Inflorescence terminal (vs. both lateral and terminal); Thyrse 4.5–6 × 2–2.5 cm (vs. 10–12 × 3.5–5 cm); Coma bracts lanceolate (vs. elliptic); Fertile bracts lanceolate, apex acute (vs. ovate to obovate, apex obtuse to rounded); Bracteoles whitish (vs. white with a pink tinge toward the apex); Floral tube 1.0–1.4 cm long (vs. 2.0–2.5 cm long); Corolla lobes pale yellow (vs. white with a dense pink to red tinge); Lateral staminodes 14–15 × 10–11 mm (vs. 10–12 × 4–7 mm); Labellum 16–18 × 15–17 mm (vs. 10–13 × 12–13 mm); Anther spurs c. 2.5 mm long (vs. 1–2 mm long); Epigynous glands c. 3 mm long (vs. c. 5 mm long).

Type: Thailand, Northern, Sukhothai Province, Si Satchanalai District, c. 300 m a.s.l., Surapon SKT-001, 22 August 2022 (holotype KKU!, isotype FOF!).

Description: Perennial herb, Primary rhizome ovoid, brownish externally, white internally, 1.8–2.2 × 1.2–1.8 cm, aromatic, monopodial, new shoot arises from the buds of the old rhizome and growing close to the old rhizomes, without branched rhizome. Root fibrous with tuberous roots. Tuber ovoid, 2–4 × 1.2–1.5 cm, white internally, tuberous roots placed closed to rhizome (0.5–3 mm apart). Leafy shoot 30–38 cm tall. Sheathing bracts 1–3 in number, 2–9 cm long, green and become brown while growing, white underground part, apex mucronulate, papery, pubescent. Leaf-sheaths 4–5 in number, 6.5–10 cm long, distichous, green, pubescent; ligule membranous, 1.0–1.5 mm long, green, tend to wither and turn brown easily, apex rounded, ciliate (hairs c. 0.5 mm long); petiole canaliculate, short, 1.0–1.5 cm long, green, pubescent. Lamina narrowly elliptic, 12.0–23.5 × 0.9–1.8 cm, apex attenuate, base attenuate, margin entire, green margin, adaxially dark green, midrib usually green, not prominent veins, abaxially pale green, pubescent on both surfaces. Inflorescence terminal; thyrse 4.5–6 cm long, 2–2.5 cm in diam. in the broadest part; peduncle 10–15 cm long, 2–2.5 mm in diameter, pale green, pubescent. Coma bract 4–5 in numbers, lanceolate, 2.0–2.5 × 0.5–0.8 cm, dark red toward apex, white or pale green at the base, apex acute, pubescent. Fertile bracts 5–8 in number, 2.5–3 × 0.7–1.0 cm, lanceolate, basal bulbous, with a slightly curved downward at distal part, apex acute, pale green to white at the base, with dark red tinge toward apex, connate to other bracts about 1/4 to 1/5 of the length, pubescent, cincinnus with 3–4 flowers. Bracteoles narrowly ovate, 0.6–1.0 × 0.4–0.6 cm, whitish, apex acute, glabrous, one bracteole subtending a single flower, pubescent. Flowers 3–3.4 cm. Calyx tubular 0.7–0.8 cm long, apex 3-lobed, with unilateral incision up to 2–2.5 mm long, semi-translucent white, pubescent. Floral tube tubular, 1.7–1.9 cm long, yellow, sparsely hairs; dorsal corolla lobe 1, 10–13 × 5–6 mm, apex cucullate with mucronulate tip, pale yellow, sparsely hairs; lateral corolla lobes 2, c. 10 × 5 mm, apex narrowly obtuse, cucullate, pale yellow, sparsely hairs. Lateral staminodes 2, broadly obovate, 14–15 × 10–11 mm, apex rounded, yellow, sparse hairs. Labellum broadly obovate to almost orbicular, 16–18 × 15–17 mm, bilobed, with deep incision 3.5–4 mm long, yellow. Stamen 1; filament flattened, c. 2.5 × 3 mm, pale yellow; anther calcarate, yellow; anther spurs c. 2.5 mm long; anther thecae c. 4 mm long, with white pollen; crest not obvious. Stigma pale yellowish. Epigynous glands c. 3 mm long. Ovary subglobose, 2.3–2.5 × 2.0–2.5 mm, pubescent. Ovule axile placentation. Fruits and Seeds not seen.

Vernacular name––“Krachiao Sumon”, “Krachiao” is an Isan word used to call plants in the Curcuma genus, while “Sumon” is named honoured Professor Emeritus Dr. Sumon Sakolchai.

Etymology––The specific epithet ‘sumonii’ pays homage to Professor Emeritus Dr. Sumon Sakolchai, renowned for his extensive expertise in pharmacy. He has made notable advancements in pharmaceutical botany, both domestically and internationally. Furthermore, his substantial support extends to aiding researchers in botany and pharmaceutical sciences. Formerly occupying esteemed positions such as the presidency of Khon Kaen University and the Pharmacy Council of Thailand, he currently holds the position of president of the Burapha University Council in Thailand.

Distribution:––Thailand, Northern, Sukhothai Province, known only from the type locality.

Ecology:—In a deciduous forest, sandy soil.

Phenology—Flowering in late June to October; produced terminal inflorescence appearing between the innermost leaf sheaths, flowers fully open in the early morning, anthesis time in the morning, and last a single day. Leafy shoot emerges in late April and dormancy begins in late November.

Utilization:––Ornamental potted plants.

Conservation status:— This newly discovered taxon was found in Sukhothai Province, northern Thailand, in 2022. Based on available information, this species was found in only one location. The known extent of occurrence (EOO) is less than 50 km², and the area of occupancy (AOO) is also less than 5 km². Additionally, the limited number of mature individuals observed in its natural habitats is less than 50, and young individuals is less than 30. Therefore, we propose classifying this species according to the IUCN criteria¹¹ as Critically Endangered [CR B1b(i), B2b(ii), C2a(i, ii), D1] to prioritize conservation efforts until further research can provide more clarity on its distribution ranges and population. This conservation status is proposed according to IUCN criteria, but this species is not yet listed in the IUCN Red list.

2. Curcuma wanchaii Saensouk, P.Saensouk, Maknoi & Boonma sp. nov.—Figs. 4, 5, Table 2

Fig. 4.

Curcuma wanchaii Saensouk, P.Saensouk, Maknoi & Boonma sp. nov. (A) inflorescences, (B) flower, (C) side and front view of anther, (D) habits in the natural habitat, growing along with Globba substrigosa King ex Baker. (Photographs by Thawatphong Boonma).

Fig. 5.

Curcuma wanchaii Saensouk, P.Saensouk, Maknoi & Boonma sp. nov. (A) side view of the anther, (B) dorsal corolla lobe, (C) lateral corolla lobes, (D) lateral staminodes, (E) labellum, (F) bract, (G) ovary with epigynous glands, (H) calyx with ovary, (I) habit, (J) floral tube with anther, calyx and ovary (drawn by Thawatphong Boonma).

–Subgen. Curcuma.

Diagnosis: This new species is similar to Curcuma nepalensis M.Sabu, Kasaju & S.Subedi but differs. Curcuma wanchaii has rhizome pale yellow (vs. C. nepalensis having pale yellowish-brown); Ligule 6–8 mm long (vs. 10–13 mm long); Peduncle 14–16 cm long, sparsely hairy (vs. 11–13 cm long, glabrous); Coma bract whitish or tip with a pinkish tinge, pubescent (vs. light green or white with a purple tinge, glabrous); Fertile bracts 20–36 in number; Uniquely obovate, pubescent, white to creamy white with various levels of orange tinge (vs. 14–28 in number; Ovate–elliptical, glabrous, red with light greenish-red towards the tip); Bracteoles sparsely hairy (vs. glabrous); Flowers 5.2–5.5 cm long (vs. 4.5–5 cm long); Floral tube 3.7–3.9 cm long, sparse hairs, yellow (vs. 2.7–3 cm long, glabrous, white, sometimes tinged with pink or yellow); Dorsal corolla lobe 17–18 mm long, pale yellow (vs. 13–15 mm long, white with a pink tinge towards the tip); Lateral staminodes asymmetrical obovate, 13–14 × 6–7 mm (vs. oblong-lanceolate, 20–21 × 10–12 mm); Labellum 16–17 mm long, incision 3–3.5 mm long (vs. 20–23 mm long, incision 4–6 mm long); Filament c. 2 mm long, c. 2 mm broad at the connection point, c. 4 mm wide at the base, yellow (vs. 5–7 mm long, c. 3 mm broad at the connection point, c. 6 mm broad at the base, white); Anther vertical rectangle in the side view, yellow (vs. anther triangle in the side view, white); Ovary c. 2.0 × 1.8 mm (vs. 3–3.5 × 2.5–3 mm).

Type: Thailand, Southwestern, Kanchanaburi Province, elevation 200 m a.s.l., Boonma CK0035, 5 October 2022 (holotype KKU!, isotype FOF! ).

Description: Perennial herb, Primary rhizome ovoid, brownish externally, pale yellow internally, 2–2.5 × 3–3.5 cm, the new shoot grows from the side bud of the old rhizome and grows close to the old rhizomes. Root fibrous with tuberous roots. Leafy shoot 50–80 cm tall. Sheathing bracts 1–3 in number, 5–14 cm long, dark green with brownish red tinge, white underground part, apex mucronulate. Leaf-sheaths 3–5 in number, 14–20 cm long, distichous, green, glabrous; ligule membranous, bilobed, 6–8 mm long, pale green, apex acute, glabrous; petiole canaliculate, 13.0–22.0 cm long, green, glabrous, outer leaves with short petiole, innermost leaves with longer petioles. Lamina ovate, 33.0–56.0 × 16.0–24.0 cm, apex attenuate, base usually cordate, excepted for the innermost leaf may be rounded to attenuate base, margin entire, green margin, adaxially green, midrib green, prominent veins, abaxially pale green, glabrous on both surfaces. Inflorescence terminal; thyrse 18–24 cm long, 7–9 cm in diam. in the broadest part; peduncle 14.0–16.0 cm long, 0.9–1.2 cm in diameter, pale green, sparsely hairy. Coma bract 5–8 in numbers, elliptic to obovate, 2.5–4.0 × 1.0–2.0 cm, whitish or tip with a pinkish tinge, apex acute, pubescent. Fertile bracts 20–36 in number, 3.5–4.0 × 2.2–2.6 cm, uniquely obovate, bulbous with slightly curved outward distal part, apex rounded with slightly mucronate tip, white to creamy white with various levels of orange tinge, connate to other bracts about half of the length, pubescent, lowest bracts are the broadest and longest, cincinni with 4–5 flowers. Bracteoles triangular-ovate, 0.7–1.0 × c. 0.7 cm, whitish, apex acute, sparsely hairy, one bracteole subtending a single flower. Flowers 5.2–5.5 cm long. Calyx tubular, 10–11 mm long, apex 3-lobed, with unilateral incision up to c. 3 mm long, semi-translucent white, pubescent. Floral tube tubular, 3.7–3.9 cm long, yellow, sparse hairs at basal part; dorsal corolla lobe 1, 17–18 × 10 mm, apex cucullate with mucronulate tip, pale yellow; lateral corolla lobes 2 in number, 14–15 × 8–9 mm, apex obtuse, pale yellow. Lateral staminodes 2, asymmetrical obovate, 13–14 mm long, 6–7 mm at the broadest part, 5.0–5.5 mm at base, apex acute to narrowly obtuse, margin entire, yellowish. Labellum 16–17 mm long, 15–16 mm at the broadest part, 7–7.5 mm wide at the base, bilobed, with incision 3–3.5 mm long, yellowish. Stamen 1; filament flat, c. 2 mm long, c. 2 mm at the connection point, c. 4 mm wide at its base, yellow; anther calcarate, c. 6 mm long including spurs, pale yellow, c. 2.5 mm at the broadest part, pubescent; anther spurs c. 2.5 mm long; anther thecae c. 3.5 mm long, with white pollen; anther crest very short, c. 0.3 mm long, apex rounded. Stigma c. 1 × 1 mm, very pale yellowish white. Epigynous glands two, slender, c. 3 mm long, c. 0.7 mm diam., apex blunt. Ovary c. 2.0 × 1.8 mm, pale yellowish white, pubescent. Ovule axile placentation. Fruits and seeds not seen.

Vernacular name:––“Krachiao Wanchai” (Krachiao is an Isan word used to call plants in the Curcuma genus, while “Wanchai” is named in honour of Assist Prof. Wanchai Soomleg.

Etymology:––The specific epithet “wanchaii” is named in honour of Assist Prof. Wanchai Soomleg, the former dean of the Faculty of Science, Director of Nong Khai Campus, and Vice President of Nong Khai Campus, Khon Kaen University. He has been a constant supporter of researchers at Khon Kaen University’s botany, biology, and biodiversity research, leading to continued strength in research.

Distribution––Thailand, Southwestern, Kanchanaburi Province, known only from the type locality.

Ecology—The plant can be discovered in a dry evergreen forest, specifically on sandy soil with abundant humus. It grows at an elevation of 200–350 m above sea level and is often found with the Globba substrigosa King ex Baker plant.

Phenology—The plant blooms from late June to October, with flowers developing at the end of the stems between the innermost leaf sheaths. The flowers fully open in the early morning and remain open for only one day. The leafy branch appears in late April and enters hibernation in late November.

Utilization––Ornamental plant.

Conservation status—The newly discovered taxon was in Kanchanaburi Province, in the southwestern region of Thailand, in the year 2022. Based on available information and adhering to the IUCN criteria¹¹, this species was found in three locations. The known extent of occurrence (EOO) is less than 100 km², and the area of occupancy (AOO) is also less than 10 km², and the total numbers of mature observed in its natural habitats is less than 200. Therefore, we propose classifying this species as Critically Endangered [CR B1b(i), B2b(ii), C2a(i, ii)], to prioritize conservation efforts until further research can provide more clarity on its distribution ranges and population. This conservation status is proposed according to IUCN criteria, but this species is not yet listed in the IUCN Red list.

Key to 32 species of  Curcuma  subgen.  Curcuma  in Thailand.

1a. Rhizome without (or with short) branches	2
1b. Rhizome with well develop branches	5
2a. Leaves < 2.5 cm width; rhizome without branches; tuberous roots placed close to the rhizome (< 1 cm apart)	Curcuma sumonii
2b. Leaves > 2.5 cm width; rhizome with short branches; tuberous roots placed not close to the rhizome (> 3 cm apart)	3
3a. Produces both terminal and lateral inflorescence	Curcuma angustifolia
3b. Produces only terminal inflorescence	4
4a. Sheaths glabrous; leaf abaxial surface glabrous	Curcuma plicata
4b. Sheaths pubescent; leaf abaxial surface pubescent or glabrescent	Curcuma attenuata
5a. Inflorescence terminal	6
5b. Inflorescence lateral	20
6a. Anther ecalcarate	7
6b. Anther calcarate	8
7a. Staminodes, labellum and floral tube deep yellow to orange-yellow	Curcuma aurantiaca
7b. Staminodes, labellum and floral tube cream-white with yellow in the midrib of labellum	Curcuma roscoeana
8a. Inflorescence emerges at or near the base of the pseudostem through lateral slits	9
8b. Inflorescence between the innermost leaf sheaths above the pseudostems	10
9a. Coma bracts absent; fertile bracts bright red almost to the base; rhizome creeping	Curcuma rubrobracteata
9b. Coma bracts present; fertile bracts green with ruby pink at the tip; rhizome non-creeping	Curcuma suraponii
10a. Fertile bracts glabrous	11
10b. Fertile bracts pubescent	12
11a. Ovary glabrous	Curcuma antinaia
11b. Ovary pubescent	Curcuma sattayasaiorum
12a. Peduncle red	Curcuma phrayawan
12b. Peduncle green	13
13a. Midrib of leaf red or reddish-purple	Curcuma wanenlueanga
13b. Midrib of leaf green	14
14a. Rhizome deep orange-yellow	Curcuma longa
14b. Rhizome whitish to pale yellow	15
15a. Thyrse inverted conical	16
15b. Thyrse long cylindrical	18
16a. Coma bracts greenish white	Curcuma viridiflora
16b. Coma bracts pink	17
17a. Leaf abaxially puberulous; rhizome with the smell of green mango	Curcuma amada
17b. Leaf abaxially glabrous; rhizome non-aromatic	Curcuma rangjued
18a. Corolla lobes deep yellow; coma bracts violet or white with a violet tip	Curcuma cordata
18b. Corolla lobes white to very pale yellow; coma bracts pink or white with a pink tip	19
19a. Fertile bract pale green with distal part pink; leaf base usually cuneate	Curcuma petiolata
19b. Fertile bract white to creamy white with various levels of orange tinge; leaf base cordate	Curcuma wanchaii
20a. Lamina abaxial surface glabrous	21
20b. Lamina abaxial surface pubescent	29
21a. Petioles and leaf sheaths reddish brown	22
21b. Petioles and leaf sheaths green	23
22a. Rhizome white	Curcuma rubescens
22b. Rhizome bluish with a yellow core	Curcuma amarissima
23a. Midrib of leaf green	24
23b. Midrib of leaf red or reddish-purple	25
24a. Bracts glabrous	Curcuma leucorrhiza
24b. Bracts pubescent	Curcuma mangga
25a. Rhizome aeruginous green	Curcuma aeruginosa
25b. Rhizome pale ochraceous, or pale yellow or orange	26
26a. Leaves oblanceolate	Curcuma aromatica
26b. Leaves ovate or elliptic	27
27a. Fertile bract white with pink tip; rhizome pale ochraceous	Curcuma comosa
27b. Fertile bract green with pink tip; rhizome pale yellow or orange	28
28a. Corolla lobes white or with a very pale pink tinge; rhizome pale yellow	Curcuma picta
28b. Corolla lobes conspicuously pink to red; rhizome deep orange to yellowish orange	Curcuma zanthorrhiza
29a. Midrib of leaf green	Curcuma elata
29b. Midrib of leaf red or reddish-purple	30
30a. Bracts glabrous	Curcuma zedoaroides
30b. Bracts pubescent	31
31a. Spike almost globular (< 10 cm long); peduncle short, embedded in the ground	Curcuma globulifera
31b. Spike cylindrical (> 10 cm long); peduncle long, erect above the ground	Curcuma latifolia

Discussion

The first newly described taxon, Curcuma sumonii, bears a superficial resemblance to Curcuma angustifolia. However, upon meticulous examination, it exhibits several unique distinctions from the existing species. It features an ovoid rhizome devoid of branches, with a white internal coloration, measuring 1.8–2.2 × 1.2–1.8 cm, as opposed to C. angustifolia, which possesses an oblong rhizome with short branches, and a pale brown to white internal coloration, measuring 3–5 × 3–4 cm. Additionally, its tuberous roots are situated proximally to the rhizome, spaced approximately 0.05–0.3 cm apart, whereas in C. angustifolia, the tuberous roots are distally located, more than 3.0 cm apart. Additional differentiating features include shorter leaf sheaths, ligules, petioles, and narrower lamina compared to C. angustifolia. While the latter exhibits both lateral and terminal inflorescences, Curcuma sumonii solely produces terminal inflorescences. Furthermore, it displays lanceolate coma and fertile bracts, along with whitish bracteoles, setting it apart from its counterparts. Morphological differences are also evident in its flowers, with shorter floral tubes and pale yellow corolla lobes. Notably, the lateral staminodes and labellum are larger compared to those of C. angustifolia, while the anther spurs and epigynous glands exhibit notable length differences (Table 1).

The designated lectotype of Curcuma angustifolia (Roxburgh, Asiat. Res. (Calcutta) 11: 338 (1810)¹² indicates lateral inflorescences. However, other studies in India¹³ and Thai specimens show both lateral and terminal inflorescences. However, Curcuma sumonii solely produces terminal inflorescences and other different characters (Table 2) distinguishing it from C. angustifolia, which was previously considered the smallest species in the subgenus Curcuma. With the entire character of the newly described taxon C. sumonii, it now represents the smallest species within the subgenus Curcuma in Thailand. Additionally, C. sumonii differs from C. plicata Wall. ex Baker and C. attenuata Wall¹⁴. by its unbranched rhizome, closely placed tuberous roots, and narrow leaves (< 2 cm broad). In contrast, C. plicata and C. attenuata have broader leaves (> 2.5 cm) and taller leafy shoots.

If one examines superficially the second described taxon, one might mistakenly identify the new species, Curcuma wanchaii, as C. aurantiaca Zijp¹⁵ due to their shared characteristic of terminal inflorescence with long cylindrical thyrse, as well as salmon hues or orange tones of fertile bracts (a color variation observed in the bracts of.

C. aurantiaca)¹⁶, which are also found in the new taxon. However, C. wanchaii is differentiated by its calcarate anther, contrasting with the ecalcarate anther (lacking anther spurs) of C. aurantiaca. In comparison to other related taxa with terminal inflorescences and long cylindrical thyrses, such as C. cordata Wall^17,18. and C. petiolata Roxb¹⁹., C. wanchaii exhibits differences. It deviates from C. cordata by producing corolla lobes ranging from white to very pale yellow, and coma bracts that are white with pink tips, whereas C. cordata yields deep yellow corolla lobes and coma bracts that are violet or white with violet tips. Furthermore, C. wanchaii contrasts with C. petiolata by producing fertile bracts that are white to creamy white with varying levels of orange tinge, and possessing a cordate leaf base, unlike C. petiolata, which has fertile bracts that are pale green with pink at the distal part and typically cuneate leaf bases.

The closest resemblance to C. wanchaii is found in C. nepalensis, which was recently described in late 2023. However, numerous characteristics distinguish the new species from C. nepalensis. Notably, C. wanchaii exhibits a pale yellow rhizome measuring 2–2.5 × 3–3.5 cm, whereas C. nepalensis displays a pale yellowish-brown rhizome measuring 4–5 × 2.5–3 cm. Additionally, the ligule of C. wanchaii measures 6–8 mm in length, in contrast to C. nepalensis, which measures 10–13 mm. Other disparities include the length and texture of the peduncle, the color, and hairiness of the coma bracts, the number and shape of fertile bracts, the size and color of flowers and floral tube, and the characteristics of the dorsal corolla lobe, lateral staminodes, labellum, filament, and ovary. Particularly noteworthy is the difference in the anther morphology: the anther of C. wanchaii is a vertical rectangle in the side view, yellow (as opposed to C. nepalensis, which has a triangular anther in the side view, white) (Table 2).

The proposed conservation status for C. sumonii and C. wanchaii underscores their critical risk of extinction, which may primarily be due to habitat destruction intensified by climate change and potential threats from human activities. Immediate action is crucial to prevent these species from facing a significant risk of extinction. Further conservation measures must prioritize the protection of their natural habitats, address current threats, and ensure their long-term survival. Since both species are found in non-protected areas, strategies might include designating these regions as Other Effective Area-Based Conservation Measures (OECMs), involving local communities in conservation efforts, and increasing public awareness about biodiversity and sustainable plant propagation to prevent the removal of plants from their habitats. These actions are vital for preserving them and their natural habitats within their limited ranges of distribution.

Conclusion

The meticulous examination and characterization of the two newly described taxa, Curcuma sumonii and C. wanchaii, not only elucidate their distinct morphological features but also underscore the significance of taxonomic studies in biodiversity conservation and plant taxonomy. The detailed morphological analyses highlight their unique characteristics and distinguish them from closely related taxa. Despite superficial resemblances to known species such as C. angustifolia and C. aurantiaca, both C. sumonii and C. wanchaii exhibit distinct features in their rhizome structure, inflorescence types, bract morphology, and floral characteristics. By delineating these species and providing comprehensive descriptions, this work contributes to the broader understanding of plant diversity within the genus Curcuma and redefines taxonomic boundaries within the subgenus Curcuma in Thailand. Furthermore, the identification of unique traits in C. sumonii and C. wanchaii, such as rhizome structure, inflorescence types, and floral characteristics, has implications for both botanical research and further conservation efforts, particularly in Thailand where these species are endemic. This study underscores the importance of ongoing taxonomic research in documenting and preserving plant diversity amid environmental challenges and potential habitat loss due to human activities throughout Thailand.

Methods

Specimen collection

Botanical specimens have been collected from two specific areas in Thailand in 2022: Sukhothai Province in the north and Kanchanaburi Province in the southwest (Fig. 1). The collection consisted of both living and preserved samples obtained from their respective natural habitats. Twenty individuals were measured for each species. All experimental research and field studies involving plants, whether cultivated or wild, including the collection of plant material, were conducted in strict accordance with relevant institutional, national, and international guidelines and legislation, and all plant samples used were collected from forests in non-protected areas. All holotype specimens (Surapon SKT-001 and Boonma CK0035) were deposited at the Khon Kaen University Herbarium (KKU). Furthermore, additional specimens were carefully preserved in 70% ethyl alcohol (isotype) deposited at the Herbarium of Faculty of Forestry (FOF) and the Mahasarakham University Herbarium (MSU), while the remainder of the living samples were cultivated at designated locations: the garden of Diversity of Family Zingiberaceae and Vascular Plant for Its Applications Research Unit in Maha Sarakham Province, and at Brio Botanical Research Garden in Nakhon Nayok Province.

Morphological analysis

In order to guarantee accurate documentation of morphological features, precise measurements were carried out using tools such as rulers and vernier calipers, in conjunction with thorough inspections using a stereoscopic microscope (Stemi 2000-C, ZEISS, Oberkochen, Germany).

Comparison with existing species

For accurate identification, a thorough comparison of our specimens’ morphological attributes was made with documented characteristics in the protologues of morphologically similar species. Extensive comparison of digital imagery and data from reputable herbarium collections, including Aarhus University Herbarium (AAU), The Forest Herbarium (BKF), Royal Botanic Garden Edinburgh Herbarium (E), The Herbarium of Faculty of Forestry (FOF), Royal Botanic Gardens Herbarium (K), National Museum of Natural History (P), Queen Sirikit Botanical Garden Herbarium (QBG), and Singapore Botanic Gardens Herbarium (SING), was undertaken, primarily performed through visual inspection. Additionally, online resources from the Zingiberaceae Resource Center (ZRC) were accessed via the web address https://padme.rbge.org.uk/ZRC/data/specimens. We considered over 300 herbarium specimens of Curcuma plants from reputable collections for the extensive comparison of digital imagery and data. These specimens were carefully selected to ensure thorough representation and accurate comparison for the study, especially on species belonging to the subgenus Curcuma, to ensure comprehensive and valuable analysis. Furthermore, an exhaustive review of published literature on Curcuma was conducted, focusing on morphological descriptions of all accepted species, with particular attention to those within the subgenus Curcuma. This review involved several major databases, including Scopus, Web of Science, and Google Scholar, as well as our library resources. Specific keywords such as “Curcuma,” “new species,” “new record,” and “nomenclature change” were used to ensure a comprehensive search. Additionally, descriptions of Curcuma species from global collections were examined. This thorough approach allowed us to verify and compare relevant information effectively.

Author contributions

Conceptualization P.S.(Piyaporn Saensouk), S.S., and T.B.; Data curation S.S. and T.B.; Formal identification of the plant material P.S.(Piyaporn Saensouk), S.S., and T.B.; Formal analysis P.S.(Piyaporn Saensouk), S.S., T.B., A.R., C.M., C.T., P.S.(Pathomthat Srisuk), and P.S.(Phetlasy Souladeth); Funding acquisition S.S.; Investigation S.S., T.B., and C.T.; Methodology P.S.(Piyaporn Saensouk), S.S., and T.B.; Project administration P.S.(Piyaporn Saensouk), S.S. and T.B.; Resources S.S., T.B., and C.T.; Software T.B.; Supervision P.S.(Piyaporn Saensouk), and S.S.; Validation P.S.(Piyaporn Saensouk), S.S. and T.B.; Visualization T.B.; Writing - original draft T.B.; Writing - review and editing P.S.(Piyaporn Saensouk), S.S., T.B., A.R., C.M., C.T., P.S.(Pathomthat Srisuk), and P.S.(Phetlasy Souladeth). All authors reviewed the manuscript.

Funding

This research project was financially supported by Mahasarakham University.

Data availability

Data is contained within the article.

Declarations

Competing interests

The authors declare no competing interests.