Axonopus fusiformis Valls & A.D. Silveira, 2016

Silveira, Andressa Dantas Da & Valls, José F. M., 2016, A new species of Axonopus sect. Axonopus ser. Axonopus (Poaceae; Panicoideae) from the Brazilian Pantanal, Phytotaxa 263 (3), pp. 291-296 : 292-295

Axonopus fusiformis Valls & A.D. Silveira sp. nov. ( Fig. 1.A–C , 2.A–B )

Ab Axonopus purpusii (Mez) Chase spiculis fusiformibus, horizontaliter supra pedicellos insertis, trichomatibus minimis secundum nervos differt.

Type: — BRAZIL. Mato Grosso: Santo Antônio do Leverger, 34.6 km em direção a Barão de Melgaço. Frequente em beira de estrada em área de campo de murundus, 15º59’33” S, 55º49’18” W, 135 m, 27 October 1985, J. F. M. Valls, A. Pott & L. B. Bianchetti 9415 (holotype CEN 70412!, isotypes CGMS!, CTES!, HUEFS!, K!, RB!, SP!, UFMT!, US!).

Herbaceous perennial, caespitose, with intravaginal innovations, without rhizomes or stolons. Flowering culms erect, 60–80 cm tall.Nodes glabrous.Sheaths 6–15 cm long, imbricate, the lowest eventually hiding a few elongated internodes. Ligule very short, membranous-ciliate, 0.2–0.3 mm long. Blades narrower than sheaths, linear, conduplicate, folded all along their length, tapering towards the boat-shaped apex, 6–43 cm long, 2–5 mm wide, surface glabrous, margins ciliate in the basal portion especially near the ligule. Terminal inflorescence generally well exserted from the upper sheath. Terminal and axillary inflorescences each with 2–3 racemose branches; branches conjugate or subdigitate, 8– 15 cm long. Rachis 0.5 mm wide, glabrous, sinuous. Pedicels 0.2–0.5 mm long, with expanded apex. Spikelets 2–3 mm long, 0.7–0.8 mm wide, fusiform, with acute apex, two evident nerves in both faces, each nerve accompanied by two parallel dark lines, these slightly pubescent, and two less evident submarginal nerves; midnerve normally suppressed in both faces. Straw colored upper lemma at least 0.3 mm shorter than the second glume, hairy at the tip. Anthers purple, stigmas whitish. Caryopsis not seen.

Paratypes:— BRAZIL. Mato Grosso: Nossa Senhora do Livramento, cerca de 45 km ao sul da Rodovia dos Imigrantes (BR-070), ao longo da MT-050, campo de murundus com Curatella , 16º04’09.44” S, 56º10’30.44” W, 142 m, 4 November 1986, J. F. M. Valls, A. Pott, L. Jank & G.Pereira-Silva 10484 ( BAA!, CEN!, CGMS!, COL!, ESA!, MO!, NY!, RB!, SI!, UB!, UFMT!), Fazenda Nossa Senhora Aparecida, Grade Pirizal PPBio, campo de murundus, 16º21’55.22” S, 56º20’46.35” W, October 2006, L. Rebellato s.n.-specimens sent for identification labeled with field numbers 83, 84, 95 ( CEN!). Poconé (Pantanal de Poconé), em área de campo de murundus, ao longo da rodovia Poconé-Porto Cercado, 7 km a sudeste do rio Bento Gomes, 16º20’47” S, 56º29’23” W, 125 m, 8–10 October 2011, L. Rebellato 236 ( CEN!).

Etymology:—The specific epithet fusiformis refers to the shape of the spikelet.

Distribution and habitat:— Axonopus fusiformis is only known from the Mato Grosso State, in West Central Brazil. It was initially found in October 1985, in the municipality of Santo Antônio do Leverger, in a “campo de murundus”, a flat grassy area with earth mounds of variable extension ( Ponce & Nunes da Cunha 1993), growing in clay soil with pebbles, on the roadside of a raised dirt road. A second collection was made in a similar environment, along a farm road, in early November 1986, in the adjacent municipality of Nossa Senhora do Livramento. Additional collections were made in October 2006, further south in Nossa Senhora do Livramento, in a farm area only with cattle trails, in the context of a detailed ecological survey ( Fernandes et al. 2010, Rebellato 2010, Rebellato et al. 2012). A more recent collection was made in October 2011, once again in a “campo de murundus”, in the neighbor municipality of Poconé, along the paved road to Porto Cercado. The maximum distance between known sites of occurrence (Valls et al. 9415 to Rebellato 236) barely exceeds 80 km.

Potential endemic status:—Based on the documented occurrence, Axonopus fusiformis seems to be an outstanding endemic grass species of the Brazilian Pantanal. The Pantanal is a geologically-recent Quaternary floodplain, formed in a slow alluvial filling process, which allowed the gradual migration of the surrounding flora, subject to annual adjustments determined by the flooding cycle. Thus, it is not recognized as a favorable environment for the emergence of endemic species ( Allem & Valls 1987, Pott et al. 2011). However, a few endemisms in the Pantanal are known from peculiar plant groups, such as the legume genus Arachis Linnaeus (1753: 741) , characterized by a highly restrictive dispersal mechanism. At least five species of Arachis are exclusive to local grassland/savanna stretches under periodic flooding ( Krapovickas & Gregory 1994, Valls & Simpson 2005, Pott et al. 2011). On the other hand, a compilation of over 230 grass species occurring in the Pantanal ( Allem & Valls 1987, Filgueiras et al. 2015) does not include a single endemic taxon. Every grass species originally described from collection sites within the geographic limits of the Pantanal has been subsequently found elsewhere. For example, the most recently described grass species from the Pantanal prior to the current one, Bothriochloa eurylemma Marchi & Longhi-Wagner (1995: 432) was soon recognized by Vega (1997, 2000) among plants collected in five Argentinian provinces. The case for the classification of A. fusiformis as one of the rare Pantanal endemics, and the only one so far in the grass family, would demand additional searches for this species in floodplains at higher elevations along the Cuiabá and São Lourenço rivers, and their tributaries, that, coming from Cerrado areas in the adjacent Plateau, drain into the northeastern sector of the Pantanal.

Phenology:—The known populations of Axonopus fusiformis were collected in reproductive state in October/ early November. Its flowering period is possibly short and centered in these months. Recurrent exploration in similar areas of the Pantanal in other months, including an intensive search in May 2015 at the precise type location by the same first collector, has not detected any flowering specimen of A. fusiformis , in spite of the ubiquitous presence of other species of Axonopus sect. Axonopus in reproductive phase, as A. purpusii , A. complanatus ( Nees 1829: 22) Dedecca (1956: 265) , and A. leptostachyus ( Flüggé 1810: 122) Hitchcock (1922: 471) .

Potential threats to the new species:—In addition to the cumulative effects of historical economic use of the natural grasslands of the Pantanal for extensive beef cattle raising ( Allem & Valls 1987, Pott et al. 2013), the site of the 1985 collection of Axonopus fusiformis (Valls et al. 9415) has since suffered some additional anthropogenic impact, as the nearby dirt road was further raised and paved in 2007–2008. Such impact is however not easily perceived on the vegetation of the local grassland. Another collection (Rebellato 236) was made in 2011, in a corresponding environment, near a similar road, but three years after its raising and paving. The potential impact of nearby road construction, generally blamed for influencing flood levels, is not seen as a relevant threat to this species. The other sites of collection are under much less anthropic influence. However, in spite of the difficulties for agricultural practices determined by the periodic flooding, there is a potential risk of establishment of paddocks of the African Urochloa humidicola ( Rendle 1899: 169) Morrone & Zuloaga (1992: 80) , now a popular farming strategy. Several areas of such cultivated pastures are already established in the three neighbor municipalities of Poconé, Nossa Senhora do Livramento and Santo Antônio do Leverger. Voluntary expansion of such exotic pasture species over the natural vegetation becomes, then, another potential threat, associated with the environmental disturbance. In each case, although this species is scarcely documented in herbaria, there is no noticeable evidence of population decline. So far, A. fusiformis would fit in the Data Deficient (DD) category, following the IUCN Red List criteria ( IUCN 2012).

Species relationships:—By the aspect of its tufts, including the flowering culms ( Fig. 1.A ), Axonopus fusiformis resembles the widespread, and locally sympatric, A. purpusii . However, its glabrescent spikelets show a distinct, acute apex, and the base attached horizontally to the pedicel ( Fig. 1.B , 2.A–B ), while in A. purpusii the pilosity along the nerves is dense, and the insertion of the spikelet on the pedicel is oblique ( Salariato et al. 2011, Giraldo-Cañas 2012). This oblique insertion of the A. purpusii spikelet allows for clear observation of the respective scar on its adaxial face ( Fig. 2.C ), an exclusive and distinctive feature, illustrated, apparently for the first time, by Morrone et al. (1994), in the treatment of Axonopus for the Flora of Paraguay.

The two other species of ser. Axonopus occurring in sympatry with A. fusiformis in the northern sector of the Pantanal are easily distinguished by their frankly stoloniferous habit. Furthermore, A. compressus has convolute leafblades, and A. fissifolius shows much smaller spikelets, with obtuse apices. As an atypical behaviour, rare plants of A. purpusii may produce some slender stolons, but the peculiar morphology of their spikelets, showing in adaxial view ( Fig. 2.C ) the scar corresponding to the insertion on the pedicel, will substantiate their correct identification.

Figures

FIGURE 1. Axonopus fusiformis Valls & A.D. Silveira (from Valls et al. 9415, CEN - holotype). A. Habit. B. Segment of rachis showing spikelet in abaxial view. C. Fertile floret in abaxial view, showing back of the upper lemma with hairs at the tip. Drawn by A.D. da Silveira.

FIGURE 2. Axonopus fusiformis Valls & A.D. Silveira (from Valls et al. 9415, CEN - holotype). A. Spikelet, abaxial view, showing upper glume. B. Spikelet, adaxial view, showing lower lemma. Axonopus purpusii (Mez) Chase (from Valls et al. 15932, CEN). C. Spikelet, adaxial view, showing lower lemma and the scar of oblique insertion on the pedicel. Photographs by A.D. da Silveira.