Derris longiracemosa Boonprajan & Sirich., 2025

Boonprajan, Punvarit, Oncham, Saruta & Sirichamorn, Yotsawate, 2025, Derris longiracemosa (Fabaceae), a new species from Thailand with extraordinary limestone adaptations and the longest inflorescences ever recorded, PhytoKeys 261, pp. 13-32 : 13-32

Derris longiracemosa Boonprajan & Sirich. sp. nov.

Figs 4 , 5

Type.

Thailand • Ratchaburi Province, Photharam District, Tao Pun Sub-district, Wat Khao Chong Phran , ca. 80 m elevation, 13°43'08.8"N, 99°46'21.4"E, 23 December 2021, Y. Sirichamorn & S. Oncham, YSM 2021-36 (holotype BKF!; isotypes K!, L!) .

Diagnosis.

The species exhibits several morphological traits that distinguish it from the coexisting species Derris solorioides . The texture of the leaflets is subcoriaceous to coriaceous (compared to chartaceous in D. solorioides ). The leaflet apices are more emarginate. The inflorescences are clearly pseudoracemose or pseudopaniculate with brachyblasts (vs. a true panicle without brachyblasts in D. solorioides ). These inflorescences can reach up to 155 cm in length, making them the longest recorded in the genus. The peduncle, rachis, and lateral branches of the inflorescence – including brachyblasts, pedicels, and calyces – are pubescent (as opposed to almost glabrous in D. solorioides ). The number of flowers per brachyblast is also highest, with up to 16 flowers. Petal color changes during maturation, ranging from pure white to pink. The ovary contains fewer ovules, with 1 to 4 ovules (vs. ca. 8 in D. solorioides ). Additionally, the flowering time is slightly earlier, occurring from November to December (vs. January to February in D. solorioides ).

Description.

Woody climber, bark thin, smooth, pale greyish-brown; twigs glabrous or thinly hairy, lenticellate. Stipules caducous, triangular, 1.7–2 by 1.6–2 mm, outside glabrescent, margin fimbriate, inside glabrous. Leaves subcoriaceous to coriaceous; petiole 4.7–13 cm long, grooved above, glabrous or with some scattered hairs, rachis 4–24.5 cm long; pulvinus 6–9 mm long. Leaflets 5–7; petiolules 4–10 mm long; stipels absent; terminal one elliptic to obovate, 7.5–17.5 by 5–11 cm, base cuneate to round, apex usually emarginate, upper surface and lower surface glabrous, midrib flat or slightly raised above, distinctly raised below, veins flat above, raised below, lateral veins 7–12 pairs, 10–30 mm apart, not reaching the margin but curving towards the apex, sometimes anastomosing near the margin, venation reticulate; lateral ones elliptic, ovate, or obovate, 9–15 by 4.5–7 cm, base broadly cuneate to obtuse, apex shortly emarginate or round, rarely shortly acuminate, upper and lower surface glabrous. Inflorescences axillary or terminal, pseudoracemes / pseudopanicles, 44–155 cm long, pubescent; peduncle 4–11 cm long, pubescent; lateral branches 23–45 cm long; bracts subtending brachyblasts ovate to triangular, 0.8–1 by 1.2–1.5 mm, outside thinly pubescent and densely pubescent at base, inside glabrous, margin fimbriate. Brachyblasts knob-like to cylindrical, 3–12 mm long, pubescent, with 9–16 flowers throughout; floral bracts ovate or triangular, ca. 1 by ca. 0.6 mm, outside thinly pubescent and densely pubescent at base, inside glabrous, margin fimbriate; pedicels 4–9 mm long, pubescent; bracteoles at calyx base, ovate or triangular, 0.9 by 0.55–0.65 mm, outside pubescent, inside glabrous. Calyx reddish or with greenish tinge, cup-shaped, 3.5–4 mm high, outside pubescent, inside glabrous; tube ca. 2 mm long, upper lip with 2 short triangular lobes, 1.5 by 1.5 mm; lateral lobes triangular, 1.5–2 by 1.4–1.8 mm; lower lobe triangular, 1–1.3 by ca. 2 mm. Flower fragrant. Corolla white, gradually turning pink during maturation; standard white or pale pinkish with light green central patch, broadly obovate or orbicular, 7–10 by 7–7.5 mm, apex emarginate, basal callosities absent, outside and inside glabrous, claw 2–2.5 mm long; wings white or pale pinkish, oblong, 5.5–6.3 by ca. 3 mm, apex rounded, outside and inside glabrous, upper auricle 0.5–0.75 mm long, lower auricle indistinct, claw 3.2–4 mm long; keel white or pale pinkish, boat-shaped, 5.5–6.5 by 2–3.5 mm, apex rounded, outside glabrous or thinly ciliate at apex, inside glabrous, upper auricle 0.75–1 mm long, lateral pocket 1–2 mm long, claw 2.5–3.5 mm long. Stamens 8–11 mm long, free part of filaments 3.5–5 mm long, glabrous; anthers 0.3–0.5 by 0.3–0.4 mm, glabrous. Disc indistinct or annular, up to 0.3 mm long. Ovary up to 10 mm long, 1–4 - ovuled, pubescent; style ca. 4.5 mm long, glabrous but thinly pubescent at base. Pods elliptic, oblong, or sometimes strap-like, 5–9.5 by 2–3.5 cm, thin, with a wing along both sutures, upper wing 1–5 mm wide, lower wing 0.5–2 mm wide, young pod reddish and gradually turning to light green during maturation, dry pod light brown, glabrescent, seed chamber indistinct but usually slightly darker around the seed. Seeds 1–3, discoid or bean-shaped, 9–15 by 6–10 by 2–3 mm; hilum central ca. 2 mm long.

Phenology.

Flowering from November to December; fruiting from January to February.

Etymology.

The specific epithet highlights the species’ distinction of possessing the longest inflorescence ever documented within the genus.

Thai name

(assigned here). “ Priang prachim ” (เปร ี ยงประจ ิ ม) consists of two components: Priang is an archaic and rarely used noun with an unclear etymology. It has three distinct meanings, one of which refers to a vine or climbing plant; and Prachim, a Thai term influenced by the Sanskrit word “ paschimaam ”, meaning “ the west. ” Thus, Priang Prachim translates to “ vines of the west. ” The name reflects the plant’s habit as a liana and its occurrence in Thailand’s western forests.

Distribution.

Endemic to Southwestern Thailand: Ratchaburi (type, Wat Khao Chong Phran, Photharam District), Phetchaburi (Kaeng Krachan National Park: Kaeng Krachan Dam Rope Bridge, Khao Pakarang, and vicinity of Ban Krang Substation) (Fig. 6 ).

Ecology.

limestone hill (usually found on or near the summit) or occasionally in non-limestone, edge of mixed deciduous forest, 50– 450 m.

Proposed IUCN conservation assessment.

This new species is known only from a single small limestone hill in Ratchaburi Province and three localities in Kaeng Krachan National Park, Phetchaburi Province. The total estimated number of mature individuals is likely fewer than 2,500, with fewer than 250 mature individuals in each subpopulation. The EOO and AOO are approximately 900 km 2 and 24 km 2, respectively. Its type locality, Wat Khao Chong Phran, is also a well-known tourist attraction in Ratchaburi Province, famous for the spectacular evening emergence of over a million bats from a cave. The species is thus threatened by ongoing human disturbance. Based on these factors, we provisionally assess its conservation status as Endangered (EN) under the IUCN Red List criteria B 1 ab (iii) + B 2 ab (iii), following the guidelines of the IUCN Standards and Petitions Committee (2022, v. 15.1).

Additional specimens examined.

Thailand • Ratchaburi Province: Photharam District, Tao Pun Sub-district, Wat Khao Chong Phran , c. 80 m elevation, 5 February 2022, Y. Sirichamorn & S. Oncham, YSM 2022-1 ( BKF, pod specimens) .

Note.

When this species was first discovered on the limestone hill of Wat Khao Chong Phran, it was initially believed to occur on other nearby limestone formations in Ratchaburi Province. However, despite our extensive field surveys conducted from 2021 until the present, we have not found it on any other limestone hills in this province. Instead, only Derris solorioides has been recorded. Subsequently, the species was rediscovered in two non-limestone habitats within Kaeng Krachan National Park, Phetchaburi Province, raising questions about whether the species is strictly restricted to limestone substrates or may exhibit some degree of ecological plasticity. This uncertainty persisted until it was later found on the summit of Khao Pakarang, a limestone hill in Kaeng Krachan National Park. We hypothesize that the non-limestone populations may have originated from seeds dispersed from limestone-dwelling mother plants. Given that this species produces numerous lightweight pods that are easily carried by the wind, it is possible that some seeds “ escaped ” and successfully established themselves in non-limestone habitats. Such a pattern raises intriguing questions about the dispersal capacity and ecological flexibility of this taxon, warranting further investigation.

Although Derris longiracemosa and D. solorioides are partially sympatric, as they grow in similar limestone environments and exhibit overlapping distributions, notably, they have never been observed coexisting in the same habitat; i. e., in a given locality, only one species is present at a time.

Special note to IUCN status of Derris solorioides

This limestone species of Derris , previously classified as Critically Endangered (CR B 1 a + 2 a; D) by Sirichamorn et al. (2014), has since been discovered in additional populations across western and central Thailand. As a result, its conservation status warrants reassessment. With the inclusion of these newly documented populations, the extent of occurrence (EOO) is now estimated at 28,200 km 2, and the area of occupancy (AOO) at 28 km 2. At least nine localities have been recorded, and the total number of mature individuals is estimated to range between 250 and 1,000. Although the AOO falls within the threshold for Endangered (EN), and the number of locations meets the Vulnerable (VU) threshold under Criterion B, the species does not currently show evidence of continuing decline or extreme fluctuations required to satisfy all subcriteria under Criterion B. Considering the relatively small estimated population size and following the IUCN Red List criteria (IUCN 2024, v. 15.1), we propose reclassifying D. solorioides as Vulnerable (VU D 1).

Figures

Figure 4. D. longiracemosa sp. nov. A. Inflorescence with leaf; B. Close-up of the inflorescence; C. Close-up of the inflorescence (close bud with brachyblast); D – F. Flowers (front and side view); G. Standard petal; H. Keel petal; I. Wing petal; J. Stamens; K. Pistil; L. Adaxial; M. Abaxial leaves; N. Pods; O. Seeds. Scale bars: 5 cm (A); 1 mm (B – K, O); 1 cm (L – N). Drawn by Yotsawate Sirichamorn (A, C, L – O) and Punvarit Boonprajan (B, D – K).

Figure 5. D. longiracemosa. A, B. Habit and habitat; C, D. Inflorescence; E. Close-up of inflorescence, showing flowers on brachyblasts; F. Flowers on a late blooming day, turning to a more pink hue; G. Flowers on an early blooming day, displaying white coloration; H. Pods; (I – III) flowers (front and side view); (IV) standard petal; (V) wing petals; (VI) keel petals; (VII) stamens and pistil; (VIII) pistil; (IX) stamens; (X) inflorescence with leaves. Scale bars: 1 mm (IV – IX); 2 mm (I – III); 10 cm (X). Photos by Punvarit Boonprajan (A – C, G, X), Yotsawate Sirichamorn (D – F, H), and Saruta Oncham (I – IX).

Figure 6. Natural distribution of Derris longiracemosa (white symbols) and D. solorioides (black symbols) in southwestern Thailand. The bright orange areas on the map indicate the distribution of limestone karsts in Thailand. The right panel shows a magnified view of the southwestern region, highlighting the localities where the specimens were collected.