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| Derris longiracemosa Boonprajan & Sirich., in Boonprajan, Oncham et Sirichamorn, 2025. |
Abstract
A new species of Derris Lour. (Fabaceae), Derris longiracemosa Boonprajan & Sirich., sp. nov., is described as the species bearing the longest inflorescences recorded in the genus to date, reaching up to 155 cm in length. Endemic to limestone areas in southwestern Thailand, it is possibly the third known limestone-adapted Derris species. Compared with its closest morphological relative and a partially sympatric species, D. solorioides, D. longiracemosa exhibits a longer and differently structured inflorescence, lower numbers of ovules per ovary (fewer than five ovules), and flowers that turn pinker with maturity. Leaf anatomical study reveals several differences, such as the shape of epidermal cells, the presence of secretory-like cavities, an atypical stomatal type that occasionally occurs, the presence of schizogenous cavities in the midrib cortex, and the distinct absence of lysigenous cavities in the pulvini cortex, as well as a thicker mesophyll compared to D. solorioides. Molecular phylogenetic analysis using nuclear ITS and plastid trnL-F and trnK-matK sequences confirms all four sampled populations as a single, well-supported species, distinct from other Derris taxa but showing a close relationship with D. rubrocalyx only in Bayesian inference. This combination of morphological, anatomical, and molecular evidence supports the recognition of Derris longiracemosa as a distinct species. A detailed description, distribution map, line drawing, photographs, and preliminary IUCN conservation status are provided.
Key words: Endangered, IUCN, leaf anatomy, legume, Leguminosae, Millettieae, Papilionoideae, Thailand’s southwestern forest
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| Derris 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). |
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| Derris 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). |
Derris longiracemosa Boonprajan & Sirich., sp. nov.
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).
Etymology. The specific epithet highlights the species’ distinction of possessing the longest inflorescence ever documented within the genus.
Thai name: (assigned here). “Priang prachim” (เปรียงประจิม) 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.
Punvarit Boonprajan, Saruta Oncham and Yotsawate Sirichamorn. 2025. Derris longiracemosa (Fabaceae), A New Species from Thailand with extraordinary limestone adaptations and the longest inflorescences ever recorded. PhytoKeys. 261: 13-32. DOI: 10.3897/phytokeys.261.156249
