Psychotria honbaensis Bao, Vuong, Tagane & V.S.Dang, 2025

Nguyen, Quoc Bao, Truong, Ba Vuong, Nguyen, Quoc Cuong, Tagane, Shuichiro, Pham, Van Ngot, Quach, Van Toan Em, Huynh, Huu Duc, Pham, Quoc Trong & Dang, Van- Son, 2025, Psychotria honbaensis, a new species of Psychotria (Rubiaceae) with bacterial nodules from Hon Ba Nature Reserve, southern Vietnam, Phytotaxa 698 (4), pp. 269-276 : 270-274

Psychotria honbaensis Bao, Vuong, Tagane & V.S.Dang , sp. nov. ( Figures 1 & 2 )

Diagnosis: Psychotria honbaensis is clearly distinguished from the other species of P sychtoria in the Indochina region and surrounding countries by its bacterial nodules on abaxial surface of lamina, ovate–triangular stipules, and inflorescence bilateral winged peduncle, and yellowish flowers.

TYPE:— VIETNAM. Khanh Hoa Province, Hon Ba Nature Reserve, in evergreen forest, 12°06′55.668″N, 108°56′36.826″E, 1558 m elev., 30 June 2023, Quoc Bao Nguyen, Ba Vuong Truong, Van Lam Mang QB060 (holotype VNM!, isotype VNM!).

Shrubs, 1–1.3 m tall. Twigs glabrous, dark green in vivo, dark brown in sicco, cylindrical with two opposite longitudinal ridges when young; internodes 1.5 – 5.5 cm long. Stipules ovate-triangular, 3 – 4 mm long, green to yellowish-green in vivo, dark brown in sicco, caducous, outer surface puberulous, with one long costa from the base to the apex of stipule, inner surface glabrous, apex acute to attenuate, margin entire, ciliate. Leaves simple, oppositedecussate, petiolate; lamina elliptic-oblong to ovate-oblong, 7.5 – 10.5 × 3.5 – 4.5 cm, adaxial surface dark green in vivo, blackish-brown in sicco, glabrous, abaxial surface light green in vivo, blackish-brown in sicco, glabrous to sparsely minutely puberulent, apex attenuate to acuminate, base cuneate, margin entire, venation mixed brochidodromous, midrib prominent on both surfaces, lateral veins 7 – 10 pairs, without domatia; bacteria nodules present on the abaxial side, secondary veins, randomly; petioles 1.8 – 2.2 cm long, glabrous. Inflorescence terminal in new branches, cymose, loose paniculate; peduncle 2 – 4 cm long, terete with two broad longitudinal wings (ca. 1.5 mm wide), sparsely puberulent to glabrous; primary axes 1.2 – 1.9 cm long, secondary axes 2–5 mm long, green to dark green in vivo, puberulent; bracts caducous, linear, 2.2 – 2.5 mm long, green in vivo, puberulent, apex acute, margin sparsely ciliolate. Flower buds ellipsoid, green to yellowish when young. Flowers 5-merous, pedicellate; pedicel 2 – 3.5 mm long, puberulent; bracteoles triangular, 1.4 – 1.8 mm long, glabrous on both surfaces, apex acute, margin entire. Calyx greenish brown, cupuliform, puberulent outside, tube 1.4 – 1.9 mm long, lobes 5, triangular, 0.5 – 0.8 mm long, apex acute, margin ciliolate. Corolla yellowish to yellow-orange, lighter color in tube, darker color on lobes, tube 2.1 – 3 mm long, puberulent on the outer surface, glabrous on the inner surface lobes 5, ovate-triangular, 2 – 3 mm long, outer surface puberulent, inner surface glabrous, densely villous at throat inside, apex acute, thick and recurved. Stamens 5, filaments ca. 0.5 mm long, glabrous, erect; anthers oblong, 1.2 – 1.4 mm long, apex rounded, dorsifixed. Ovary 2- locular, with 1 ovule in each locule; style 1.8–2.2 mm long, glabrous; stigma 2-lobed, lobes lanceolate, flat, glabrous. Fruits drupaceous, ellipsoid, 1.3 – 1.5 cm long, 8 – 10 mm in diam. when young, shiny, glabrous, apex with persistent calyx lobes; pyrene 2, plan ventral with one longitudinal groove, convex dorsal with 4–6 longitudinal ridges and 3–5 longitudinal grooves. Seeds ellipsoid to hemi-ellipsoid, 7 – 9 × 4 – 5 mm, irregularly sulcate on the abaxial face, blackish brown when dry, albumin ruminate, with a T-shaped fissure on the ventral.

Distribution and habitat: — Psychotria honbaensis is so far found only in Hon Ba Nature Reserve, Khanh Hoa Province, southern Vietnam. It grows in evergreen broad-leaved forests at an elevation of 1250–1600 m.

Phenology: —Flowering and fruiting from late June to September.

Etymology: —The species epithet is derived from the type locality of the new species, Hon Ba Mountains, Hon Ba Nature Reserve.

Vernacular name (Vietnamese name): —Lấu Hòn Bà

Preliminary conservation assessment: — Psychotria honbaensis was known only from the high elevations at 1250–1600 m in Hon Ba Mountains, Hon Ba Nature Reserve, Khanh Hoa Province. The Area of Occupancy (AOO) and the Extent of Occurrence (EOO) are estimated 4 km 2 and 12 km 2, respectively. The environment of Psychotria honbaensis grows in the national park’s protected areas, which are stable and well-preserved, and the estimated number of mature individuals is approximately 200. Under these circumstances, we propose its status as Endangered (EN) according to the IUCN Red List criteria D ( IUCN 2022).

Taxonomic notes:— Psychotria honbaensis is unique among all species in the Psychotria genus in the Indochina in its having bacterial-nodules on leaf blades, yellowish or yellow-orange corolla, terminal inflorescence, broadly winged peduncle, and the presence of albumin on seeds. The morphological characters of the new species maybe resembles P. pteropus Lachenaud & Harris (2010: 229) which is distributed in Central Africa in leaf shape and winged peduncle. However, P. honbaensis clearly differs from P. pteropus in having entire triangular stipules (vs. bifid with linear lobes in P. pteropus ), longer corollas (tube 2.1 – 3 mm long and lobes 2–3 mm long vs. tube 0.5 – 0.8 mm long and lobes 1–1.5 mm long), broadly oblong anthers (vs. narrowly elliptic in P. pteropus ), larger ellipsoid fruits (8 – 10 mm in diam. vs. 4–5 mm in diam.), and seeds shape. A detailed morphological comparison between the two species is presented in Table 1.

Based on the morphological characters, the new species can be placed into leaf-nodulated Psychotria clade, which is only distributed in Africa and the first occurrence record for this clade in Vietnam. The discovery of a nodulated species in this region is unexpected and contradicts current hypotheses regarding Psychotria . This represents the first documented occurrence of bacterial nodules in the genus outside Africa-Madagascar. Although a close evolutionary relationship between this new Asian species and African taxa cannot be ruled out, it remains unlikely. Therefore, this finding introduces a novel biogeographic perspective on Psychotria , warranting further investigation into the species’ distribution. The disjunct population of f leaf-nodulated Psychotria clade (subg. Tetramarae) provide another example of the taxa previously known only from Africa and Vietnam, such as the genus Karomia ( Lamiaceae ), which might be reflected by the connection of these regions as parts of Sundaland in glacial periods. In the case of Psychotria honbaensis , the primary nodules appear and develop into the lateral nodules (spot) of the mature leaves, midrib, or secondary vein, and most of them are between 0.3 mm to 0.7 mm in diam.. In micromorphology, the nodule is under mesophyll tissue, and the three spongy mesophyll cell layers near the nodule are compressed, without the conspicuous intercellular spaces ( Fig. 3D & E ). It functions spread on both sides horizontally of the lamina which is the reason for the epidermis layer, the mesophyll layer is broken through, and compressed ( Fig. 3G & H ). Moreover, the cells outside the spot have lignified cell walls, while the cells inside do not.

Figures

FIGURE 1. Psychotria honbaensis Bao, Vuong, Tagane & V.S.Dang A. Flowering branch, B. Lamina (Adaxial surface on the left, abaxial surface on the right), C. Stipule, D. Inflorescence, E. Flower buds, F. Flower (front view) G. Young flower bud. H. Flower (side view) I. corolla opened out showing anthers and villous hairs at the throat. J. Calyx, style, and stigma. K. Infructescence, L. Fruit (side view), M. Transverse section of fruits showing pyrenes, and seeds, N. Seeds. All photos taken by Quoc Bao Nguyen based on the holotype Quoc Bao Nguyen et al. QB060 (VNM)).

FIGURE 2. Psychotria honbaensis in natural habitat. A. Habitat. B. habit.

FIGURE 3. The bacterial nodules on the Psychotria honbaensis leaves. A. Abaxial leaf surface (arrows indicate the bacteria-nodules) B. Primary bacterial-nodule. C, D. Transverse section of nodule showing the spongy-mesophyll compressed (C. Not dyed, D. Dyed with Carmin-Methylene Blue), E. Laterial bacterial-nodule. F, G. Transverse section of nodule showing the epidermis, the mesophyll broken through and compressed (F. Not dyed, G. Dyed with Carmin-Methylene Blue). Abbreviations: U-Ep: Upper Epidermis, Me: Mesophyll, S-Me: Spongy Mesophyll, B-N: Bacterial-nodule, L-Ep: Lower epidermis, Ch: chloroplast. Scale bars: A: 3 cm, B 0.2 mm, C, D: 50 µm, E, 0.3 mm, F, G. 100 µm.