Stylosanthes Sw. comprises about 50 species and is distributed mainly in the Neotropics, with only two species in the Paleotropics (‘t Mannetje, 1984; Costa, 2006; Ferreira et al., 2020). It is characterized by subshrubby habit, trifoliolate leaves, amplexicaul fused stipules with two teeth, spiciform inflorescences, and papilionaceous, generally yellow flowers. Fruits are loments with one or two fertile articles and with a rostrum formed by the residual style (‘t Mannetje, 1984; Costa, 2006; Gissi, 2020). The genus was previously divided into two sections, sect. Stylosanthes, diagnosed by the presence of only one inner bracteole and the absence of a rudimentary axis in the inflorescence, and sect. Styposanthes Vogel, with two inner bracteoles and the presence of a rudimentary axis (i.e., an undeveloped, stiff structure that occurs beneath the flowers adjacent to the inner bracteoles) in the inflorescence (Vogel, 1838). This system was followed by other authors and is useful for distinguishing species (Mohlenbrock, 1958; Kirkbride & Kirkbride, 1987; Costa, 2006). The sections, however, were found to be non-monophyletic in molecular phylogenetic studies (Vander Stappen et al., 2002, 2003; Oliveira et al., 2021).

Brazil harbors more species of Stylosanthes than any other country, with 32 species, 14 of which are endemic (Gissi, 2020). Within Brazil, 25 species of Stylosanthes are distributed mainly in the Cerrado domain, in open vegetation physiognomies (following Ribeiro & Walter, 2008), such as cerrado típico, campo sujo, campo limpo, campo rupestre, and veredas (Costa, 2006; Vanni, 2017; Gissi, 2020). Several species also occur in pastures, lawns, and roadsides (Gissi, 2020). They are sometimes considered to be agricultural weeds due to their invasive behavior in cultivated fields (Carvalho, 2013). Also, cultivars of S. capitata Vogel and S. macrocephala M.B. Ferreira & Sousa Costa are sold commercially under the name of Estilosantes ‘Campo Grande’ and are planted for livestock forage (Embrapa Gado de Corte, 2007; Karia et al., 2010; Machado et al., 2010).

While examining herbarium specimens at SLUI, we encountered a specimen of Stylosanthes collected in Chapada das Mesas National Park in the state of Maranhão. Although previously identified as S. angustifolia Vogel, the specimen not only did not belong to that species but was new to science. Here, we provide a description and illustration of this new species.

Material and Methods

For this study, specimens of all South American Stylosanthes species deposited in the following herbaria were analyzed: ALCB, ASE, BOTU, CEN, EAC, ESA, HBRA, HUEFS, HVASF, IAN, INPA, L, MFS, NY, P, R, RB, SLUI, SP, UB, US, WAG (acronyms according to Thiers, 2021). The relevant taxonomic literature on Stylosanthes was also consulted (Vogel, 1838; Mohlenbrock, 1958; ‘t Mannetje, 1984; Costa, 2006; Vanni, 2017; Gissi, 2020). The use of morphological terms mainly follows Harris and Harris (2001) and Radford (1974). For a more complete terminology of leaves, Ellis et al. (2009) and Hickey (1973) were also consulted, especially regarding venation terms. Herborized leaflets were radiographed using a digital X-ray machine (Faxitron X-Ray, model MX-20 DC12), with an exposure time of 8 sec at a voltage of 26 kV (Schneider et al., 2018). Photographs of minute morphological traits were taken under a Leica M205C stereomicroscope (Leica, Germany) with an attached Leica DFC 425 camera. The radiographs were used to better characterize venation patterns. A distribution map was generated using QGis 3.22, and a conservation status assessment was made employing the IUCN Red List categories and criteria (IUCN Standards and Petitions Committee, 2019).

Taxonomic Treatment

Stylosanthes acicularis Gissi & Fort.-Perez, sp. nov.—Type: Brazil. Maranhão, Carolina, Parque Nacional Chapada das Mesas, 337 m, 07°18′53.2”S, 47°02′28.0”W, 06 Apr 2017 (fl, fr), R. V. C. Saraiva s.n. (holotype: SLUI herb. no. 4987 [!]). (Figs. 1–3.)

Fig. 1
figure 1

Stylosanthes acicularis. A. Habit with inflorescences. B. Leaflet C. Outer bracteole. D. Inflorescence E. Loment. (Drawn from the holotype by Maíra G. Mezzacappa).

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Fig. 2
figure 2

Stylosanthes acicularis (A–E). A. Stem with bristles with secretion drops (arrowheads) and non-glandular trichomes. B. Outer bracteole. C. Inflorescence highlighting the bract leaflets (arrowheads). D. Loment. E. Seed. F. Stylosanthes angustifolia, inflorescence bearing loments with exserted rostrums (arrowhead). Stylosanthes hippocampoides (G–I). G. Stem with bristles H. Outer bracteole I. Loment. [Scale bars: A–B; D–E, G–I = 1 mm; C = 5 mm; F = 1 mm.]

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Fig. 3
figure 3

Comparison of the leaflets of three species of Stylosanthes by X-ray method emphasizing venation. A. S. acicularis. B. S. hippocampoides C. S. angustifolia. [Scale bars = 5 mm.]

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Diagnosis.—Stylosanthes acicularis is similar to S. hippocampoides Mohlenbr. but differs by having leaflets linear, less than 1 mm wide, with the primary vein imperceptible, the secondary vein angles 5°–8° and uniform, the bristles on the stem up to 1 mm long and pilose-scabrous, and the outer bracteole bilobed (vs. leaflets elliptical, primary vein easily discernible, secondary vein angles 10°–15° and irregular, stem bristles 2–4 mm long and hispid-pubescent, and the outer bracteole unlobed in S. hippocampoides).

Perennial subshrub, 0.6 m tall, branched from the base. Stems erect, ligneous near the base, upper branches green, striate, indumentum pilose-scabrous, viscid, with scattered bristles up to 1 mm long and more numerous non-glandular trichomes, the latter mostly restricted to one side of each internode (the side above the adjacent leaf, i.e., alternating between internodes). Stipules amplexicaul with a pilose-scabrous indumentum, sheath 3–6 mm long, the apex with two slightly spreading teeth on each side, the teeth 3–4 mm long. Leaves trifoliolate; pulvinus 0.5–0.75 mm long, petiole 5–7 mm long, adaxially channeled, sericeous with scattered bristles; rachis up to 1 mm long, pilose, rarely with bristles; pulvinules 0.5 mm long, pilose; leaflet blades 10–18 × 0.4–1 mm, linear, with the L:W ratio 12:1 to 25:1, mucronate at apex, narrowly acute-cuneate at base, chartaceous, V-shaped in cross-section; adaxial surface sparsely scabrous with scattered short bristles and appressed non-glandular trichomes; abaxial surface scabrous, with bristles concentrated mainly on the primary vein, primary vein imperceptible, secondary veins in 3–4 pairs, the angles with the primary vein 5°–8° and uniform, eucamptodromous with regular spacing and excurrent attachment to the midvein, immersed and inconspicuous above, prominent below, marginal vein lacking. Inflorescences terminal and axillary, formed by 1–2 congested spikes, more or less obovoid, 6–10 mm long, 2–4-flowered; outermost bract stipule-like, sheathed at base, with a 1–3-foliolate reduced leaf distally, the sheath 4–5 × 2–3 mm, with 3–5 pairs of veins, pilose-scabrous, rounded at base and more or less oblong, the apex with two slightly spreading teeth on each side, the teeth cuspidate-triangular, 2.8–4.2 mm long; other bracts similar to the outermost bract, but always 1-foliolate; bracteoles 2, hyaline, pilose externally, and margin ciliate; outer bracteole adaxially convex, truncate at base, somewhat dilated distally, apically bilobed, 2.6–3.6 × 1.1 mm, the lobes unequal, triangular-apiculate, the larger one 2 mm long, the smaller one ca. 1.5 mm long, inner bracteole entire, ovate, abaxially convex, truncate at base, caudate at apex, ca. 3 mm long. Flowers exserted; calyx 3–4 mm long, 5-lobed, with the sepals united for ca. 2 mm, ciliate on the lobe margins, otherwise glabrous, dorsal lobes 1.5–1.6 mm long, obtuse, lateral lobes 1.4–1.6 mm, acute-triangular, ventral lobe 2–2.5 mm, acute-triangular, keeled; corolla yellow, glabrous, standard petal suborbicular, unguiculate, apically retuse, ca. 3.4 × 7.6 mm; wings elliptic, spurred at base, ca. 5.9 × 1.6 mm, keel petals apically fused, falcate, spurred at base, ca. 7.7 mm long; androecium monadelphous; anthers dimorphic, the shorter 5 basifixed, the longer 5 dorsifixed; ovary biovulated. Loment with one fertile article, orbicular, 3.2–3.5 × 2.3–2.4 mm, papillose at apex, reticulate; rostrum ca. 1 mm long, coiled, covered with long papillae. Seeds ca. 1.9 × 1.5 mm, reniform, brown with a lustrous testa and dark brown rim aril.

Etymology.—The specific epithet refers to the linear leaflets of the species.

Distribution and habitat.––Known from a single collection from Chapada das Mesas National Park (CMNP) in the state of Maranhão, Brazil, where it was found in typical cerrado vegetation (Fig. 4) at 353 m. According to Saraiva et al. (2020), the relief in the CMNP ranges from 250 m in the valleys to 524 m in the hills, the local soil is mostly quartz sands, the local climate is classified as Tropical Aw in the Köppen (1884) classification, the average annual temperature is 26.1°C, and annual precipitation is 1250–1500 mm (IBAMA, 2013). The rainy season starts in November and extends through April. During the dry season (May to October) the local vegetation is especially vulnerable to fire outbreaks. (Saraiva et al., 2020).

Fig. 4
figure 4

Geographical distribution of Stylosanthes acicularis in the Chapada das Mesas National Park (CMNP). A. Map of Brazil highlighting the state of Maranhão. B. Location of the CMNP (black-shaded area) in Maranhão. The colors represent the phytogeographic domains present in the state. C. Cerrado vegetation in the CMNP; the black dot represents the type locality of S. acicularis.

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Phenology.—The species was collected in April with flowers and mature seed-bearing fruits.

Conservation status.—Since the species is known from only a single collection, we assess it as Data Deficient (DD; see IUCN Standards and Petitions Committee, 2019). The species has been collected inside a protected area, but it may occur in other nearby Cerrado areas since other species of Stylosanthes are generally widely distributed. However, Cerrado areas are often threatened by the expansion of agriculture and fire in the region (Saraiva et al., 2020).

The combination of a single fertile article and very short rostrum in the loment, the absence of a rudimentary axis beneath the flowers in the inflorescence, and the presence of only a single inner bracteole suggest a close relationship between Stylosanthes acicularis and the ca. eight species of the S. guianensis complex (sensu ‘t Mannetje, 1977; Gissi et al., 2022). Species with these characteristics that have been sampled in molecular phylogenies form a well-supported clade (Vander Stappen et al., 2002; Vander Stappen et al., 2003; Oliveira et al., 2021).

Among species of the Stylosanthes guianensis complex, S. acicularis is most similar to S. hippocampoides, a wide-ranging species distributed from northeastern Brazil to central Bolivia, Paraguay, and northern Argentina. The two species can be distinguished from other species of the S. guianensis complex by the combination of small capitate inflorescences and the leaflets having up to five secondary veins and lacking a marginal vein. Although the new species is known from just a single specimen, and the type locality may lie within the geographical distribution of S. hippocampoides, we are confident of its distinction from S. hippocampoides. The latter species is relatively well characterized, being known from more than 50 analyzed collections, including type specimens, over its broad geographical range (Appendix 1). As enumerated in the diagnosis and Table 1, the type of S. acicularis differs by multiple morphological characters, including indument, leaflet shape and venation, and lobing of the outer bracteole, from all collections of S. hippocampoides, including those from nearby localities in Northeast Brazil (Table 1).

Table 1 Morphological comparison of three Brazilian species of Stylosanthes
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The new species is also similar in some features to Stylosanthes angustifolia; indeed, the type specimen of S. acicularis was previously misidentified as S. angustifolia. Both species lack a rudimentary axis beneath the flower, and have only one inner bracteole, loments with only one fertile article, and linear leaflets lacking a marginal vein. Stylosanthes angustifolia is not considered a member of the S. guianensis complex because of the shape of the inflorescence and the rostrum length. With respect to its phylogenetic position, it is nested in the clade with other species with long rostrums (Vander Stappen et al., 2002). The geographical range of S. acicularis is also contained within that of S. angustifolia, which ranges from the Northeast Brazil to the Guianas. However, the new species consistently differs from S. angustifolia in multiple morphological characters, most notably in its shorter rostrum and much less elongate inflorescence (Table 1).

Key to the species of Stylosanthes in Maranhão

  • 1. Flowers and fruits subtended by a rudimentary axis; inner bracteoles 2.

  • 2. Bracts wider than long, more than 10 mm broad ………………………………………………………………… S. capitata

  • 2. Bracts longer than wide, up to 10 mm broad.

  • 3. Plants hispid; with long bristles, ca. 5 mm long; rostrum coiled…………………………………………………S. pilosa

  • 3. Plants scabrous; with short bristles, ca. 2 mm long; rostrum uncinate……………………………………………S. scabra

  • 1. Flowers and fruits not subtended by a rudimentary axis; inner bracteole 1.

  • 4. Leaflets linear, less than 2 mm broad.

  • 5. Inflorescences very narrow and elongated; rostrum longer than the upper article ……………….………. S. angustifolia

  • 5. Inflorescences short and capitate; rostrum shorter than the upper article ……...……………………….….… S. acicularis

  • 4. Leaflets elliptical or obovate, more than 2 mm broad.

  • 6. Rostrum longer than the upper article…………………………………………………………………………. S. humilis

  • 6. Rostrum shorter than the upper article.

  • 7. Loment with conspicuous oil glands at the base ………………………………………………………… S. guianensis

  • 7. Loment without conspicuous oil glands at the base.

  • 8. Leaflets with marginal vein …………………………………………………………….........……………S. gracilis

  • 8. Leaflets without marginal vein.

  • 9. Loment with 1 article.

  • 10. Leaflets with more than 5 secondary veins …....…....…....…....…...…....…....…...…………….. S. grandifolia

  • 10. Leaflets with less than 5 secondary veins …....…....…....…....…....…....…....…………… S. hippocampoides

  • 9. Loment with 2 articles.

  • 11. Rostrum coiled, loment straw-colored…....…....…......…....…....…....…....…....…....…………….... S. viscosa

  • 11. Rostrum uncinate, loment brown.......................................................……………...………................... S. nunoi