Senna juchitanensis Saynes & R.Torres, 2019

Torres-Colín, Rafael & Saynes-Vásquez, Alfredo, 2019, A new species of Senna (Leguminosae) from the Isthmus of Tehuantepec, Oaxaca, Mexico, Phytotaxa 395 (3), pp. 227-234 : 228-232

Senna juchitanensis Saynes & R.Torres , sp. nov. ( Fig. 1 )

Senna juchitanensis is similar to S. velutina , but differs from it by its branches and rachis of leaves canescent-velutinous; leafllets elliptic to rarely obovate or oblong; stipules ovate to widely ovate or oblong-mucronate or apiculate; pedicel gland botuliform to pyriform; floral buds 1.1–1.6 × 0.8–1.2 cm, widely-elliptic; outer sepals triangular, moderately canescent-velutinous, inner sepals broadly ovate, glabrous; filaments of abaxial stamens 0.7–0.8 mm wide, central anthers ca. 5.5 mm long; legume 5.5–11.1 cm long, green-brownish-velutinous; seeds 2.4–3.2 mm long.

Type: — MEXICO. Oaxaca: Distr. Juchitan, Mpio. Unión Hidalgo, 12 km al SE de Unión Hidalgo, entrando por Rancho El Dorado , 16° 23ʼ 52.3ʼʼ N ; 94° 46ʼ 07.3ʼʼ W, 14 Noviembre 2017, Rafael Torres C. 18300 & Alfredo Saynes V. (holotype MEXU, isotypes BM, HUMO, K, MEXU, MO, NY, OAX, P, SERO, UAMIZ, XAL) .

Shrub 1–5 m high, erect to diffusely ascending, geniculate stem, young and old branches canescent-velutinous. Leaves pinnate (8–) 12–15(–22.4) cm × (4.5–) 6–7.5 (–13.5) cm; leaflets (4–) 5 pairs (–6), (2.4–) 3–5 (–7.5) cm × (1.2–) 2–3 (–4.4) cm, elliptic, rarely obovate or oblong, apex obtuse to rounded, emarginate, mucronate, mucro c. 1.5 (–4) mm long, base oblique, subcordate, upper surface strigulose, lower surface velutinous; rachis (3.5–) 5–8 (–12.7) cm long, canescent-velutinous; glands between all pairs of leaflets sessile, ca. 2.4 × c. 1.3 mm, botuliform to clavate, scarcely pubescent, reddish-brown; petiole (0.9) 1.5–2 (–2.9) cm long, velutinous-ferrugineous; stipules (1.2–) 2–3.5 (–4.9) cm × (0.7–) 1–1.7 (–3.3) cm; ovate to widely ovate or oblong-apiculate or mucronate, kidney-shaped base, abaxially tomentulose, tomentose in the central vein. Inflorescences racemose, (4–) 10–25 (–53) flowers, axillary, subterminal, rachis (3.7–) 6–10.5 (–28.5) cm long, velutinous with hairs white and ferruginous, floral buds, 1.1–1.6 × 0.8–1.2 cm, obliquely widely elliptic, glabrescent only canescent-velutinous in outer sepals; bracts ca. 5 × ca. 1 mm, linear-lanceolate, apex caudate, velutinous with hairs white and ferruginous; pedicels (1.2–) 1.5–2.5 (–3.3) cm long, subtended laterally at base by a gland, 1–2 × 0.4–1 mm, botuliform to pyriform; calyx with heteromorphic sepals, the smaller outer one 0.6–1 × 0.3–0.6 cm, triangular, cymbiform, green-brownish, moderately tomentulose adaxially, marginally pilose; the largest inner one 1.4–1.8 × 0.9–1.4 cm, broadly ovate, concave, yellow, all glabrous; corolla with subsymmetric petals, adaxial petals 2, 1.8–2.4 × 1.2–1.8 cm, the banner 1.9–2.2 × ca. 1.5 cm, abaxial petals 2, 2–2.8 × 1.6–2.1 cm, widely obovate, shortly unguiculate, yellow, brown-veined, pubescent only on the veins, margin crisped; fertile stamens 7, central stamens 4, 6.7–8 mm long, filaments 1.9–2.4 × 0.4–0.5 mm, 3 abaxial, 11–15 mm long, filaments dilated 8–11 × 0.7–0.8 mm, central anthers ca. 5.5 × 1.2–1.6 mm, straight or almost so, with a beak of 0.6 mm, abaxial anthers 12.2–13.2 × ca. 2 mm, falciform, with a beak of 1.2 mm, those of all anthers 2-porose; staminodes 3, 4.3–6 mm long, unguiculate, the claw 0.5–2.5 mm. the blade 2.7–4.3 x 1.5–2.4 mm, oblong, rarely obovate or ovate, glabrous; ovary 0.9–2.3 × ca. 0.2 cm, arcuate, sericeous; style 0.4–1.4 cm long, stipe 0.4–0.7 cm long, green-brownish, velutinous, stigma linear. Pod (5.5–) 7–9.9 (–11.1) × 0.2–0.4 cm, linear, arcuate, longitudinally ribbed, persistent style 0.4–1.4 cm, stipe 0.4–0.7 cm, green-brownish, velutinous; seeds 8–19, 2.4–3.2 × 1.5–2.5 mm, oblong, rarely square, adaxial face as a rectangular prism, light olive green, areole oval or elliptic.

Distribution and habitat: — Senna juchitanensis is endemic to Oaxaca. It is known from the Isthmus of Tehuantepec in the surroundings of Juchitán. It occurs in vegetation of savanna with Byrsonima crassifolia ( Linnaeus 1753:426) Kunth in Humbolt et al. (1821:149), Caesalpinia coriaria ( Jacquin 1763: 123–124) Willdenow (1799:532) , Calliandra grandifolia Allen P.H. (1956: 148) , Gliricidia sepium ( Jacquin 1760: 28) Kunth ex Walp. in Walpers (1842:679), Haematoxylon brassiletto H. Karsten (1862: 27–28) , and several species of the genera Bursera Jacquin ex L. in Linnaeus (1762: 471), Coccoloba P. Browne (1756: 2019–210) , Luehea Willdenow (1801: 410) , Lysiloma Benth. in Hooker (1844: 82–84), and Mimosa Linnaeus (1753: 516) , at an elevation of 24– 42 m. ( Fig. 3 )

Additional material examined: — MEXICO. Oaxaca: Distr. Juchitan, Mpio. Unión Hidalgo, 10.6 km, 134° L.R. del centro de Unión Hidalgo, SE, 16° 24ʼ 8.5ʼʼ N ; 94° 45ʼ 23.7ʼʼ W, 28 septiembre 2006, Alfredo Saynes V. 6711& Nicandro López ( MEXU). 12 km al SE de Unión Hidalgo, entrando por Rancho El Dorado , 16° 23ʼ 52.5ʼʼ N ; 94° 46ʼ 16.1ʼʼ W, 14 noviembre 2017, Alfredo Saynes V. 7052 & Rafael Torres C. ( MEXU) .

Phenology: —Flowering beginning in September; fruiting beginning in November.

Etymology:— The name of this species refers to Juchitán, Oaxaca.

Discussion: — Senna juchitanensis is morphologically related to several Mexican species belonging to several of the series proposed by the Irwin & Barneby (1982), such as S. ser. Brachycarpae Bentham (1871:529), S. ser. Confertae Bentham (1871:536), S. ser. Bacillares Bentham in Martius (1870: 96), and S. ser. Trigonelloideae Colladon (1816: 95) Irwin & Barneby (1982: 237). They are similar for presenting leaves with glands and more than two pairs of leaflets, seven fertile stamens arranged in two series, and anthers beaked and incurved. It differs from all these species in all these series for presenting glands in the pedicels and the legume with a nerve parallel and contiguous to the ventral suture. It is morphologically similar to S. velutina , but as can be seen in table 1 and figure 2, it differs in the type of indumentum of various structures; the shape of the stipules, leaflets, and gland of the pedicels; in the size and shape of the flower buds, shape and indumentum of the sepals, indumentum of the petals and width of the central anthers. There are also differences in the length of the central anthers, legume, and seeds, although the sizes of these structures overlap slightly. Senna velutina is assigned to S. ser. Laxiflorae ( Irwin & Barneby, 1982), whose species occur in distant areas such as Brazil, Paraguay, Bolivia, Guyana, and Venezuela. Similar patterns of distribution are presented in other taxa such as some sections of the genus Coursetia DC. (1825: 92) , which has two centers of diversification, one in Argentina and Peru and another in Mexico ( Lavin, 1988). Another similar example is Bauhinia L. (1753: 3749) sect. Bauhinia Wunderlin, Larsen & Larsen (1987: 11–12) , wich has its center of diversification in Mexico with 17 species, and in the northeastern of Brazil there is one species of this section ( Wunderlin et al., 1987). Senna juchitanensis is endemic to the region of the Isthmus of Tehuantepec, considered a region with high levels of endemism ( Pérez-García et al., 2001) and important biological diversification. The biodiversity of the coastal plain, where this species was discovered, had been little studied, nevertheless, there have been studies to evaluate the floristic richness and the loss of traditional ecological knowledge among the Zapotecs of that area ( Saynes-Vásquez et al. 2013).

Figures

FIGURE 1. Senna juchitanensis (Illustration by Mauricio Labastida-Astudillo). A. Branch with inflorescences, B. Pubescence of the branches. C. Gland in leaf rachis. D. Gland at the base of the pedicel. E. Calix. F.Androecium and gynoecium. G.Pubescence of petals. H. Flower. I. Legume. J. Abaxial view of the seed. K. Side view of the seed (A–D, G and H, from Alfredo Saynes V. 6711 & Nicandro López, MEXU; E, F, I, J and K from Rafael Torres C. 18300 & Alfredo Saynes V., MEXU).

FIGURE 3. Senna juchitanensis Saynes & R. Torres (Photograph by Rafael Torres C.).