Senna acatlanensis C. Rojas-Martínez & A. Delgado, 2019

Rojas-Martínez, Cecilia, Flores-Olvera, María Hilda, Ochoterena, Helga & Delgado-Salinas, Alfonso, 2019, An outstanding new species of Senna (Fabaceae: Caesalpinioideae) from southern Puebla, Mexico, Phytotaxa 405 (3), pp. 113-120 : 114-117

Senna acatlanensis C. Rojas-Martínez & A. Delgado , sp. nov. ( Figures 1 , 2 , and 3)

Type:— MEXICO. Puebla: Municipio Acatlán, Cerro El Pelado, ejido de Tetelcingo, 18° 13.477’ N, 98° 0.174’ W ; 1591 m elev., 22 October 2017 (fl.), C. Rojas-Martínez 574, H. Ochoterena, H. Flores-Olvera, A. Delgado & A. Inés (holotype MEXU! # 1502125) .

Diagnosis:— Senna acatlanensis differs from other species of Series Bacillares by its combination of 5–6 cm flowers in diameter, with yellow petals (not fading to rust reddish brown, while drying), four staminodes and 7–10 fertile stamens, displaced gynoecium, fruits with pubescent valves and sutures, and brown or black seeds without an areole.

Description:— Shrubs up to 1.70 m tall, with several virgate branches 4–5 cm in diameter; young stems with longitudinal ribs, densely strigillose to villous, with whitish hairs. Stipules narrowly subulate or setiform, 0.8–1.3 × 0.5–1 cm, erect, with a conspicuous point of attachment and colleters inside their bases, densely hirtellous on both surfaces, caducous. Leaves alternate, paripinnate with two pair of leaflets, 2.5–7.5 cm; petiole 0.4–1.5 cm, with undifferentiated pulvinus, ca. 2 mm; rachis 0.2–1 cm; glands (extrafloral nectaries) present between the two pairs of leaflets, with colleters at their bases; the gland between the proximal pair of leaflets enlarged, with sessile, fusiform secretory body, 2.5–3 mm, green, strigillose, persistent, distal gland setiform, at tip of rachis, caducous; pulvinuli ca. 1.5 mm, densely strigillose; leaflets ovate-elliptic, chartaceous, fibrous internally, bicolored, adaxial surface dull green, sparsely strigillose, denser when young, abaxial surface lighter green, sparsely to densely strigillose, proximal leaflets 2–3.5 × 1–1.5 cm, the distal pair larger, 3.5–5 × 1.3–2 cm, obliquely elliptic-acute, apex mucronulate, margins slightly revolute, midrib slightly eccentric at the base, becoming median towards apex, secondary veins 9 pairs, pinnate broquidodromous. Inflorescences racemose, axillary, when terminal panicle-like, 3–7 cm, 6 to 12-flowered, axillary; peduncles 1–3 cm; bracts 1.5–4 mm, covering younger buds (hood-like), green to brownish tan before falling, triangular to obovate, with colleters inside their bases, densely strigillose dorsally; pedicels in flower 2.3–3.8 cm, in fruit ca. 4 cm; bracteoles absent; flower buds light brown, globose, densely strigillose. Flowers fully-expanded 5–6 cm in diameter; calyx with five sepals yellowish green, ovate, obtuse at apex, two short ones 6–8 × 5–6 mm, concave, others 1.2 cm × 8.5 mm, densely strigillose on the outer surfaces; petals bright yellow not fading while drying, with three main veins, claws 2–3 mm, dorsally strigillose from the base to the median portion and sparsely hairy ventrally, sometimes along the veins, abaxial petal 2.8–3.5 × 1.7–2.2 mm, symmetrically obovate, emarginate; the lateral ones 2.3–2.7 × 1.7 mm; adaxial ones 2.8–3 × 1.7–2 mm, slightly concave; androecium heteromorphic and variable in number, staminodes 4, ca. 5 mm, yellow, flattened with reduced distal beaks; fertile stamens 7–10, orange-yellow, filaments sparsely pilose, and anthers slightly curved, with marked pilose sutures (not functional): 4–5 central stamens, 11–13 mm (filaments 2–3 mm, anthers 8–9 mm, with distal divaricate beaks, 1 mm); sometimes also with 1–2 medium-sized stamens, 13.5–18.5 mm (filaments 3–4 mm, anthers 10–13 mm with prorrect beaks, 1.5 mm) and 2–3 long abaxial stamens, 19.5–23 mm (filaments 8 mm; anthers sub-basifixed, 10–13 mm, sparsely pilose in the grooves, apex extending in 2- porose porrect beaks); pollen tricolporate, prolate, with micro rugulate exine ornamentation, long colpi, endoapertures circular covered by operculum; gynoecium centrally placed or slightly off the centre, ovary green ca. 2.5 cm, densely sericeous, with ca. 120 ovules, style ca. 6 mm, incurved, distally glabrous, stigma truncate, crateriform, with fringing hairs. Fruit pendulous, straight, subcylindric, with base and apex obtuse, stipe 3–5 mm, and beak ca. 5 mm, valves chartaceous, slightly undulate, brownish-black, strigillose, sutures wide, yellowish, 7.5–10 × 1–1.2 cm, inter-seminal septa well-developed, with seeds biseriate, transverse to long axis of the pod and a black pulpy endocarp enveloping seeds when fresh, septa ca. 3 mm apart, dehiscence unknown. Seeds ca. 35, 5–6 × 3.5–4 mm, shape flattened-pyriform, with testa glossy black or brown, exareolate and hilum 0.5 mm, ovate.

Additional specimens examined (paratypes):— MEXICO. Puebla: Municipio Acatlán, Cerro El Pelado, ejido Tetelcingo, 18°13.311’N, 98°00.0507’W, 1631 m elev., 15 December 2012 (fr.), C. Rojas-Martínez 68 & M. Martínez ( MEXU) ; ídem, 1591 m elev., 11 October 2013 (fl.), C. Rojas-Martínez 299 & L. A. Castillo-Hernández ( MEXU) .

Distribution, habitat and phenology:—The new species is, at present, known only from an ecotone between seasonal dry tropical forest and oak forest of Quercus glaucoides M. Martens & Galeotti (1843: 209) ( Guízar-Nolazco et al. 2010); in grassland slopes on outcrops of metamorphic rocks, at an elevation of 1500–1700 m. Senna acatlanensis blooms during the summer (June to December), when bees ( Anthophoridae ) were observed travelling between flowers of different plants, buzzing the anthers of each visited flower, suggesting that it is a buzz-pollinated species ( Amorim et al. 2017). Fruiting in December.

Etymology:— The epithet refers to the municipality of Acatlán, in the state of Puebla.

Conservation status:— The species is geographically limited (<10 km 2), and rare. An actual estimate of the population size is less than 50 mature individuals. For this reason and following IUCN Red List Threatened Criteria (2012), the species is evaluated to be critically endangered at this time. However, further explorations should be undertaken to understand its threat status.

Figures

FIGURE 1. Habitat, plant habit, leaves and inflorescence with flowers of Senna acatlanensis.A—Location of S. acatlanensis populations, view of the city of Acatlán de Osorio, Puebla. B—Habitat, ecotone of seasonal dry tropical forest and oak land, with cactus Escontria chiotilla (F.A.C. Weber) Rose. C—CRM holding an inflorescence of this new species. D—Leaf with two pair of leaflets. E—Stipule. F—Leaf with gland on proximal pair of leaflets. G—Inflorescence. H—Flower buds. I—Flower starting anthesis. J—Flower with style on the right side. K—Flower with style on the left side. L—Flower stamens and staminodes. Photos by Helga Ochoterena and Cecilia Rojas-Martínez.

FIGURE 2. Illustration of Senna acatlanensis. A—Flowering branch, showing ribs, filiform stipules, 4-leaflets’ leaves and axillary racemose inflorescence, with floral buds and flowers. B—Detail of leaf gland on proximal pair of leaflets. C—Standard petal, clawed, and three-veined. D—Androecium heteranthery, staminodes (4), central stamens (6); medium central stamen (1), and large abaxial stamen (2) and gynoecium. E—Fruit, cylindrical and with a wide suture. F—Seed, oblong, without areoles.A–F: C. Rojas-Martínez 68 (MEXU). Scale bars at structures. Illustration by Elvia Esparza.