Colocasia sookchaloemiae Chatan & Promprom sp. nov. Figs 1, 2, 3, 4, 5 Type. Thailand • Mukdahan Provinces. 16°43'08.4"N, 104°25'24.1"E, alt. ca. 400 m, 30 August 2023, W. Chatan 2472 (holotype, BKF!; isotype: BK!). Diagnosis. Colocasia sookchaloemiae differs from C. fallax in having seasonal dormancy (vs. evergreen); broader (11–25 cm) and non-glaucous lamina [vs. narrower (3.5–15 cm) and slightly glaucous beneath]; larger (2.3–3.1 mm in diameter), speckled green ovary and a convex stigma with a depressed center [vs. smaller ovary (ca. 1.1 mm in diameter), stigma 3 - lobed]; pistillate zone with basal rows of pistillodes (vs. rows of nail- / peg-like staminodes); sterile interstice and staminate zone sparsely hairy (vs. glabrous); sterile interstice shape cylindrical and slightly constricted (vs. tapering); appendix thicker, ivory, and clearly stipitate (vs. slender, often sessile / shortly stipitate, creamy-yellow to purple). Details of the morphological comparison between the new species and C. fallax are shown in Table 1. Description. Terrestrial, perennial herbs with a seasonal dormancy period, forming colonies or sometimes solitary among rocks, 30–45 cm tall. Rhizomes depressed-globose (1.2–1.8 × 1.8–2.2 cm) or elongate (1–5 cm long and 1–1.5 cm in diameter), with numerous stolons up to 40 cm long and 3–4 mm in diameter. Leaves always 2; petiole cylindrical, greenish, glossy, 23–63 cm long and 0.5–1.3 cm in diameter; sheath 5–15 cm long and 0.8–2.0 cm in diameter; leaf blade broadly ovate to slightly circular, peltate, 15–35 cm long and 11–25 cm wide; apex acuminate; base cordate. Upper surface slightly dull green; lower surface dull green, not glaucous. Primary veins palmate at the lamina base, with 2–3 pairs of primary lateral veins pinnately arranged toward the lamina apex, raised on the lower surface; midrib, lateral veins, and other veins level with the epidermal cells; primary lateral veins well demarcated; interprimary veins forming a weak interprimary collecting vein. Inflorescences 1–3 together, 20–30 cm long; peduncle cylindrical, pale green, dull, 18–22 cm long. Spathe constricted at about the lower third; the lower part (below the constriction) convolute, forming an elliptic or ovate organ, pale green, 1.8–2.2 cm long, and 1.0– 1.2 cm in diameter. Limb broadly ovate, erect during the early blooming period, later fully spreading and folding downward at anthesis, ca. 5.5 × 2.5 cm, apex acuminate, dull greenish yellow or white with a faint tinge of yellow; adaxial surface mainly yellowish green, becoming clearly dirty yellow to pale orange-yellow near the apex; coloration faint or absent on the abaxial surface. Spadix 6–9 cm long, shorter than the spathe. Pistillate flower zone 1.0– 1.2 cm long and 0.5–0.6 cm in diameter, with 3–4 rows of white to ivory, globose to broadly ovate pistillodes at the base. Ovaries broadly to very broadly ovate, 2.3–3.1 mm in diameter and 2.0– 2.2 mm high, speckled green, not interspersed with staminodes, unilocular; ovules numerous, fusiform, translucent; placentae 3–4, parietal; stylar region short to absent; stigma apex convex with a depressed center, rod to broadly ovate pistillodes present at the end of this zone. Sterile interstice cylindrical, constricted at the middle with both ends slightly expanded; sparsely hairy, with soft white hairs becoming more distinct at maturity; 10–14 mm long and 1.7–2.6 mm in diameter. Lower part of sterile interstice with weakly clavate to ovate staminodes; middle and upper parts with rhombohexagonal synandrodes; entire interstice white to ivory. Flowers unisexual; perigone absent. Staminate flower zone white to ivory, 18–23 mm long and 6–8 mm in diameter, sparsely hairy with soft white hairs, becoming more distinct at maturity; synandria 3.0– 5.2 mm in diameter, white to ivory. Appendix 3.0– 3.8 mm long and 7–8 mm in diameter (ca. ¹ / 3 the length of the spadix), tapering cylindrical, with a rough surface. Distinctly stipitate, with a zone of 2–3 rows of staminodes at the base; sparsely hairy, with soft white hairs becoming more distinct at maturity. Male flowers 3–4 - androus; stamens connate at the slightly grooved apex of the synandrium; thecae lateral, cylindrical, dehiscing by irregular rupture. Berry depressed-globose to irregular in shape, 5 mm in diameter, yellow. Seed solitary, ovoid to ellipsoid, ca. 2 mm in diameter, enclosed in a pale brown, translucent aril. Fruiting peduncle reflexed; fruiting spathe ovoid, ca. 2.5 × 1 cm, emerging as the leaves unfold. Etymology. The specific epithet honors Assoc. Prof. Dr. Duangchai Sookchaloem, an expert in aroid taxonomy in Thailand. She has devoted her career to the study and taxonomy of the family Araceae. Phenology. Flowering time June to July; fruiting time (mature fruits) October to December. Distribution and habitat. Colocasia sookchaloemiae is currently known from two populations occurring in Mukdahan and Sakon Nakhon Provinces, at elevations of 350– 400 m. Nakhon Phanom Province is also expected to be part of its distribution range, as it is situated near the type locality and lies within the same protected area. The species grows on rocks and cliffs in dry dipterocarp and mixed deciduous forests at elevations of 350–400 m (Fig. 6). Preliminary conservation status. The preliminary conservation assessment, applying the IUCN criteria (IUCN Standards and Petitions Committee 2024), suggests that the species should be assigned to the Data Deficient (DD) category. In our opinion, the current data on population size, distribution, and habitat trends are insufficient to accurately evaluate its risk category. Further field surveys and long-term monitoring are necessary to assess population dynamics and potential threats, such as forest disturbance, tourism development, and climate variability in northeastern Thailand. Documenting and protecting narrowly distributed species remain crucial not only for biodiversity conservation but also for safeguarding valuable genetic resources within this culturally and agriculturally important genus. Discussion. The discovery of Colocasia sookchaloemiae highlights the underestimated diversity of the genus in Thailand. Until recently, the Thai flora recognized only a limited number of Colocasia species (three, i. e., C. esculenta, C. fallax, and C. menglaensis; Boyce and Sookchaloem 2012). The addition of C. sookchaloemiae demonstrates that northeastern Thailand, particularly protected areas such as Phu Pha Yol National Park, harbors cryptic taxa awaiting discovery. This finding is consistent with recent studies of Araceae from the region, including Alocasia sakonakhonensis and Amorphophallus sakonnakhonensis (Promprom et al. 2023; Promprom et al. 2024), which collectively reinforce the importance of this area as a regional center of aroid diversity. The recognition of Colocasia sookchaloemiae also contributes to broader discussions of species delimitation within Colocasia. As noted by Sangnin and Sookchaloem (2008), morphological variability in Thai populations often masks cryptic diversity, and the new species exemplifies how traits such as dormancy, stigma form, and interstice indumentum can provide diagnostic value. These findings support the view that the genus in mainland Southeast Asia remains incompletely understood, with further field and herbarium studies likely to yield additional taxa. From an ecological perspective, Colocasia sookchaloemiae occupies rocky habitats within dry dipterocarp and mixed deciduous forests at mid elevations (350–400 m a. s. l.). This contrasts with the typically moist habitats of many congeners, such as C. esculenta, C. fallax, and C. menglaensis, which commonly occur in moist to wet environments (Mayo et al. 1997; Boyce and Sookchaloem 2012), suggesting ecological specialization. Such adaptation may have driven morphological divergence, as proposed for other narrowly distributed Araceae in seasonal habitats. Its occurrence in at least two populations across provincial boundaries implies a potentially wider, though still restricted, distribution within the protected landscape of Phu Pha Yol National Park. However, its confinement to a limited area and apparent habitat specificity warrant conservation attention. In summary, Colocasia sookchaloemiae enriches the taxonomy of Colocasia in Thailand and underlines the floristic uniqueness of Phu Pha Yol National Park. Its clear morphological distinction from C. fallax and related taxa supports its recognition as a new species, while its restricted distribution underscores the importance of continued exploration and conservation in the understudied ecosystems of northeastern Thailand. Additional examined specimens (Paratype). Thailand • Sakon Nakhon, 31 August 2023, alt. ca. 400 m, W. Chatan 2473 (paratype: BKF).

Published as part of Promprom, Wilawan, Munglue, Phukphon, Pasorn, Pattana, Lanorsavanh, Soulivanh & Chatan, Wannachai, 2026, Colocasia sookchaloemiae (Araceae), a new species from northeastern Thailand, pp. 131-140 in PhytoKeys 269 on pages 131-140, DOI: 10.3897/phytokeys.269.175203