Russelia tehuana Pérez-Calix & Guzmán-Díaz, 2021

Guzmán-Díaz, Salvador, Pérez-Calix, Emmanuel & Samain, Marie-Stéphanie, 2021, Russelia tehuana (Plantaginaceae), a new species from the tropical dry deciduous forest in Oaxaca, Mexico, Phytotaxa 491 (3), pp. 249-256 : 250-253

Russelia tehuana Pérez-Calix & Guzmán-Díaz , sp. nov. Fig. 1

Suffruticose or herbaceous perennial; stem 4–6-angulate, hirsute; leaves opposite or ternate, petiole 2–3 mm long, hirsute; lamina ovate, wide ovate or suborbicular, (10–) 12–30 mm long, (5–) 8–27 mm wide, hirsute, resinous lepidotes on both surfaces, margin serrate. Inflorescence an apical or axillary thyrsus; peduncles 5–7 mm long, quadrangular, hirsute; sepals lanceolate, 4–4.5 mm long, apex acute, hirsute; corolla scarlet, 10–12 mm long, externally glabrous, corolla tube narrowing in the central part, 8–9 mm long, interior of the corolla with a line of yellow dorsoventrally flattened hairs on the ventral part and surrounding the base of the tube. Morphologically, the closest species is R. obtusata , both species share the pubescent non-terete stems and similar leave shapes and sizes; however, R. tehuana can be distinguished from this species by its small flowers with a centrally constricted corolla tube and the presence of resinous lepidotes on both surface of the leaves.

Type: — MEXICO. Oaxaca: municipio San Pedro Pochutla, 3.1 km de San Juan Palotada por la carretera a Huatulco, 130 m, 15°44’33.4”N, 96°18’48.8”W, 20 January 2018, E . Pérez-Calix 6745 (Holotype IEB, isotypes MEXU, XAL) .

Suffruticose or herbaceous plant, perennial, 40–60 cm tall. Stem angulate, 4–6-sided, angles broadened into ridges, hirsute, internodes 3.9–6.6 (–8.7) cm long. Leaves deciduous, simple, opposite o ternate, petiole 2–3 mm long, hirsute; lamina ovate, wide ovate or suborbicular, (10–) 12–30 mm long, (5–) 8–27 mm wide, chartaceous, 4–6 veins on each side of the main vein, hirsute, more dense on the lower surface, resinous lepidotes present on both surfaces, abaxially more abundant, base cuneate or rounded, rarely truncate, apex acute, margin serrate, 5–9 teeth on each side of the lamina. Inflorescence a thyrsus, usually apical, sometimes axillary, 8–25 cm long, 4–6 pairs of complex cymes in each inflorescence; peduncles 5–7 mm long, quadrangular, angles broadened into ridges, hirsute; bracts lanceolate to triangular, 3.5 mm long, hirsute; pedicels 2.5–3 mm long, hirsute. Calyx 5-merous, sepals free, lanceolate, 4–4.5 mm long, apex acuminate, surface completely hirsute, resinous lepidotes absent, rarely present on the youngest floral buds; corolla bilabiate, fused along two thirds of its length, widening near the corolla mouth, scarlet, 10–12 mm long, externally glabrous, corolla tube narrowing in the central part, 8–9 mm long, upper lip emarginate, lobes 3 mm long, 2.7–3 mm wide, lower lip trilobulate, lobes 3–3.5 long, 2.5–3 mm wide, rounded at the apex; interior of the corolla with a line of yellow dorsoventrally flattened hairs on the ventral part and surrounding the base of the tube. Stamens 4, didynamous, anthers 1 mm wide, filament of the longest pair 4 mm long, those of the shorter pair 2.5 mm long, staminode 1 mm long. Stigma 5 mm long, style 3–3.5 mm long. Fruit a dehiscent capsule, globose or ovoid, 4–4.5 mm long, filled with numerous yellow hairs; seeds numerous, elliptic to suborbicular, 0.5 mm long.

Distribution and habitat: — Russelia tehuana is endemic to the southern part of Mexico in the state of Oaxaca, on the Pacific slope of the Sierra Madre del Sur and in the southern part of the Isthmus of Tehuantepec. This species is the only one of the genus endemic to Oaxaca. Russelia tehuana shares the same ecological affinities with most species of the genus since it inhabits rocky ridges within the tropical dry deciduous forest, as well as disturbed vegetation along roadsides with Bursera Jacq. ex L. (1762: 471), Mammillaria Haw. (1812: 177) , Bocconia L. (1762: 505) and Selaginella P. Beauv. (1805: 101) , at elevations between 15 and 1900 m.

It seems the species is restricted to metamorphic rocks, which are characterized by a physiological aridity, as a consequence of which it can thrive in extreme conditions. The tropical deciduous forest in the Isthmus of Tehuantepec is characterized by a particular high plant endemism with at least 50 species recorded in this region ( Sosa & De-Nova, 2012), e.g. Esenbeckia tehuana Ramos & E. Martínez (2018: 419) , Esenbeckia collina Brandegee (1914: 183) , Agave nizandensis Cutak (1951: 143) , Agave guiengola Gentry (1960: 98) , Mimosa torresiae R. Grether (1990: 435) , and Holographis leticiana T. F. Daniel (1988: 79) , all of these threatened with extinction.

Phenology: —This species flowers from July to January and fruits from February to March.

Etymology: —The name of this species refers to the region where it was originally discovered, the Isthmus of Tehuantepec in Oaxaca, Mexico.

Conservation status: —According to the criteria and categories of the IUCN Red List ( IUCN, 2012) and the available information, this species may be considered as vulnerable, as a consequence of its extent of occurrence (18,936 km 2), its area of occupancy (40 km 2) and its small number of known populations (nine). According to Rzedowski (1978), in Mexico the impact of human activities such as agriculture on the tropical deciduous forest had been considerably less than in other types of vegetation due to the shallow, rocky floors; however, some regions of tropical deciduous forest were exploited for livestock farming, selective woodcutting and affected by forest fires.

Additional specimens examined: — MEXICO. Oaxaca: Asunción Ixaltepec, alrededores de la vía del tren Transístmico, Nizanda , 110 m, 16°40’0”N, 95°1’17”W, 15 July 2000, E. A . Pérez-García & B . Reyes-Ríos 1872 ( MEXU); a 1 km al N de Nizanda, 15 m, 16°40’26”N, 95°0’49”W, 19 February 2001, J . Meave del Castillo 2426 ( MEXU); hacia el “agua tibia” a 500 m en línea recta al N de Nizanda, 115 m, 16°40’2”N, 95°0’35”W, 11 March 2000, E. A . Pérez-García & B . Reyes-Ríos 2031 ( MEXU). Juchitán de Zaragoza , 13.4 km de El Mezquite por la carretera a La Ventosa, 100 m, 16°36’34.1”N, 94°57’5.7”W, 19 January 2018, E . Pérez 6740 ( IEB). Santa María Jalapa del Marqués, Carretera a La Torre de Microondas , 26 February 1986, C . Martínez R . 455 ( MEXU). Santiago Astata, 36 km de Santiago Astata por la carretera a Salina Cruz , 90 m, 16°2’35.8”N, 95°24’26.9”W, 20 January 2018, E . Pérez 6743 ( IEB). Santiago Yosondúa, ladera sur del cerro Kava Ndivi, aprox. a la mitad del cerro, 1930 m, 16°50’39.4”N, 97°35’9.6”W, 7 August 2012, A . García-Mendoza et al. 9932 ( MEXU). Santo Domingo Tehuantepec, 6.6 mi W of junction of road to Concepción Bamba on Hwy 200, 31 mi W of Salina Cruz, 101 m, 9 March 1985, M . Luckow 2593 ( MEXU); entrando al arroyo por el Mármol, ladera Oriente, Cerro Guiengola , 175 m, 16°21’N, 95°19’W, 22 November 1986, M. L . Torres C . et al. 758 (MEXU).

Figures

FIGURE 1. Russelia tehuana Pérez-Calix & Guzmán-Díaz, sp. nov. A) Branch with leaves and inflorescence; B) stem segments showing sides and ridges at the angles; C) detail of the abaxial side of the lamina, resinous lepidotes are present across both surfaces of the lamina; D) lateral view of the flower, the centrally constricted corolla is a unique trait of this species; E) flower dissection; F) lateral view of the ovary; G) capsule; H) dehiscent capsule locule; I) seed. Drawing by Alfonso Barbosa García based on E. Pérez-Calix 6745 (IEB).