A new species of Rhynchospora section Pluriflorae (Cyperaceae) from the southwestern edge of the Brazilian Amazon

ABSTRACT A new species of Rhynchospora (Cyperaceae) only known from the Parecis plateau, on the southwestern edge of the Brazilian Amazon, is described here. Rhynchospora parecisensis was found growing only in open environments of deciduous seasonal forest and savanna enclaves in the midst of seasonal evergreen forest, in the state of Mato Grosso, western Brazil. Rhynchospora parecisensis differs from the other species of Rhynchospora sect. Pluriflorae subsect. Subebracteatae by the distally puberulent to pubescent glumes. The new species resembles Rhynchospora acanthoma, but differs from it by the subtriangular to quadrangular scapes in cross-section and the distally puberulent to pubescent glumes, with ciliate to ciliolate margins. Taxonomic comments, illustrations and a distribution map are also provided.


INTRODUCTION
Rhynchospora Vahl comprises about 400 species worldwide (Buddenhagen et al. 2017;Larridon et al. 2021;Silva Filho et al. 2021) and is most diverse in the Americas and especially in the Neotropics (Strong 2006).It is the most species-rich genus of Cyperaceae in Brazil, with 168 species distributed across all states and habitats (Schneider et al. 2020).Kükenthal (1949) revised Rhynchospora and described the sections Pauciflorae Kük., with 1-3 (rarely 4) flowers per spikelet (currently comprising 24 species), and Pluriflorae Kük., with 4-9 flowers per spikelet (22 species), both characterized by capituliform inflorescences.During a taxonomic revision involving these two sections (Alves et al., unpublished data), a new species of Rhynchospora sect.Pluriflorae was found and is described here.The new species is morphologically circumscribed in Rhynchospora sect.Pluriflorae subsect.Subebracteatae Kük. for presenting scale-like bracts, five plumose perianth bristles, and biconvex nutlets (Kükenthal 1949).

MATERIAL AND METHODS
Rhynchospora specimens available in the herbaria BHCB, IAN, INPA, IPA, JPB, MBM, MFS, MG, PEUFR, R, RB, SP, SPF, and UFP were analyzed (acronyms according to Thiers 2020 -continuously updated).Morphological descriptions were based on dried materials, using a stereomicroscope.Morphological terminology of the specific features of Cyperaceae are based on terms used by Strong (2006), and Simpson (2006), the inflorescence morphology followed Reutemann et al. (2012), and the trichome descriptions followed Harris and Harris (2001).A taxonomic identification key for Rhynchospora sect.Pluriflorae subsect.Subebracteatae is provided and was based on published descriptions (i.e., Humboldt and Bonpland 1816;Boeckeler 1880;Araújo et al. 2003Araújo et al. , 2008)), and on the unpublished revision by Araújo (2001).Digital images were taken using a stereomicroscope equipped with a digital camera, and the LAS package.The plates were composed using the software Gimp 2.10 (GNU Image Manipulation Program).
Images of the nutlets were taken in a scanning electron microscopy (SEM) with a FEG-type electron gun (field emission weapon).The samples were coated with Au for 2 minutes and 30 seconds, with a thickness of 10 to 15 nm.The images were generated by secondary electron detection, using a voltage acceleration of 5 kV and a working distance between 15 and 25 mm.
Geographic distribution and habitat data were obtained from the labels of the herbarium specimens examined, and field observations by the authors.Conservation status assessment was performed following IUCN Red List Categories and Criteria v.14 (IUCN 2022).The area of occupancy (AOO) of the species (grids of 10 km) was estimated using the GeoCAT -Geospatial Conservation Assessment Tool (Bachman et al. 2011).

Distribution and habitat
Rhynchospora parecisensis occurs in open environment areas of deciduous seasonal forest (mata seca) and open savanna (Cerrado) within seasonal evergreen forest (floresta estacional sempre verde or floresta do Planalto do Parecís; IBGE 2012), along the banks of the Teles Pires River on the southwestern edge of the Brazilian Amazon (Figure 6).This region is covered by a mosaic of forests and savannas due to its proximity to the Cerrado domain.The specific habitats where the new species occurs are open environments with shallow soils and rocky outcrops.Because of poor drainage during the rainy season, the soils are wet, while in the dry season, the soils dry out completely, resulting in an herbaceous-shrubby vegetation (Figure 5a,b).Some newly described species, Anemia paripinnata Labiak & J.T. Mickel (Anemiaceae) and Zamia brasiliensis Segalla and Calonje (Zamiaceae) occur in sympatry with R. parecisensis (Labiak et al. 2018;Segalla and Calonje 2019), showing the high diversity of dry forests in the Amazon domain.

Etymology
The epithet of this species refers to the Parecis plateau, the only known location for this species at this time.

Taxonomic relationships
Rhynchospora parecisensis is characterized by a blackened to vinaceous rigid base, the (4-) 5 perianth bristles which are proximally plumose and scabridulous distally, and nutlets with a slightly reticulate surface basally.It is closer morphologically to the species in section Pluriflorae (sensu Kükenthal 1949) due to the solitary and rigid stems, and the spikelets with 4-9 flowers.It is also similar to species in subsection Subebracteatae Kük., which is characterized by stramineous to brown scale-like bracts, hemispheric, turbinate, oblong or digitate capituliform inflorescences, five plumose perianth bristles, and biconvex nutlets (Kükenthal 1949;Araújo et al. 2003).Araújo & Longhi-Wagner, which are similar and probably closely related (Araújo et al. 2008), and which were placed in Rhynchospora sect.Pluriflorae subsect.Subebracteatae by both Kükenthal (1949) and Araújo (2001).These species are similar in having turbinate to hemispherical inflorescences with scalelike involucral bracts, spikelets with only one bisexual flower, 4-5 perianth bristles which are plumose and scabridulous distally, and biconvex nutlets.Rhynchospora parecisensis differs from all these species by having involucral bracts with an apex that is apiculate to long aristate (in immature inflorescences, glumes which are pubescent distally to pubescent, and nutlets with a distally papillose surface).It differs from R. acanthoma due to the triangular to quadrangular stems in cross section (vs.cylindrical to subcylindrical in R. acanthoma), and its distally puberulent to pubescent glumes (vs.glumes glabrous in R. acanthoma) with ciliate to ciliolate margins (vs.margins glabrous in R. acanthoma).It differs from R. bracteovillosa by glabrous scapes, involucral bracts with margins ciliate, never lanuginose, and glumes with non-glabrous to pubescent keels (vs.villous culms, involucral bracts with lanuginose margins, and glumes with villous keels in R. bracteovillosa); differs from R. capitata by the involucral ciliated to ciliolated-margin bracts (vs.glabrous margins in R. capitata); differs from R. exilis by the 1.8-2.2mm long perigonial bristles, exceeding the fruit body (vs.1.7 mm long perigonial bristles, smaller than the fruit body in R. exilis); differs from R. globosa in the absence of bulbiform structures at the base of the scape (vs.8-17 mm wide bulbiform structures at the base of the scapes in R. globosa); and finally differs from R. leucoloma by the involucral bracts 4-5 (vs.involucral bracts 2 in R. leucoloma).

Conservation status
According to the IUCN (2022) criteria, Rhynchospora parecisensis can be categorized as Data Deficient (DD) because only two specimens are known and appropriate data to better evaluate its abundance and/or distribution are presently unknown.Nevertheless, it is possible that, after a more thorough survey, R. parecisensis may be considered Critically Endangered (CR), due to restricted distribution, fragmentation, degradation and decrease of the natural habitat in the region where the new species occurs.The species is known from only two populations in the municipality of Itaúba, in an ecotone between Amazon Forest and Cerrado biomes.As known today, the species has an Area of Occupancy (AOO) of 8000 km 2 , and the Extent of Occurrence (EOO) cannot be calculated from only two points.Specimens of the new species were collected during the plant rescue because of the installation of the Colíder hydroelectric dam power plant.After the creation of the reservoir, a residential condominium was built at the edge of the reservoir, adversely affecting one of the two known populations.Additionally, after an extensive review of herbaria and literature, no other records of R. parecisensis were found.

CONCLUSIONS
We describe a new species of Rhynchospora (Cyperaceae), sect.Pluriflorae, subsect.Subebracteatae Kük., from the Parecis Plateau, on the southwestern edge of the Brazilian Amazon.The species is positioned in subsect.Subebracteatae Kük.due to the scale-like bracts, five plumose perianth bristles, and biconvex nutlets.It can be recognized by the subtriangular to quadrangular scapes in cross-section, and the distally puberulent to pubescent glumes, with ciliate to ciliolate margins.

Figure 1 .
Figure 1.Holotype of Rhynchospora parecisensis [M.E. Engels et al. 5367 (MBM)].Courtesy of the Herbarium MBM curators.This figure is in color in the electronic version.

Figure 3 .
Figure 3. Rhynchospora parecisensis digital images in stereomicroscope.A -Inflorescence with spikelets apices and involucral bracts; B -Immature inflorescence showing aristate apex of involucral bract; C -Spikelet; D -Side view of nutlet, with bristles; E -Side view of nutlet, without bristles [Illustrations from M.E.Engels et al,.5367(MBM)].This figure is in color in the electronic version.

Figure 5 .
Figure 5. A-B -Habitat of Rhynchospora parecisensis in the rainy season and in the dry season, respectively; C -Detail of inflorescences; D -Plant habit.Photos by M.E.Engels.This figure is in color in the electronic version.

Figure 2 .
Figure 2. Rhynchospora parecisensis.A -Habit; B -Inflorescence with spikelets apices and three involucral bracts; C -Spikelet; D -Side view of nutlet, with five surrounding bristles.E -Side view of nutlet, without bristles; F -Detail of nutlet surface.Line drawing by Elielson Rocha, from the isotype [M.E.Engels et al. 5367 (RB)].This figure is in color in the electronic version.