A new saxicolous species of Acisanthera (Melastomataceae: Marcetieae) from Chapada dos Guimarães National Park, Mato Grosso,

Acisanthera saxatilis is described, illustrated, and compared with putative relatives. The first specimen of this new species was collected 119 years ago by Oskar Andersson Gustaf Malme during his “second expedition Regnelliana”. Acisanthera saxatilis is a little collected species apparently endemic to rocky sandstone formations in Chapada dos Guimarães, Mato Grosso state, Brazil. It differs from similar species by its perennial habit, lignotubers, isomorphic to slightly subisomorphic androecium, stamens positioned in a circle around the style during anthesis, antesepalous stamens with short connectives, small inconspicuous staminal appendages, and purple anthers. Photos of living specimens, a distribution map, scanning electron photographs of seeds and leaves, a preliminary conservation status assessment, as well as comments on vascular plant endemism at the Chapada dos Guimarães are provided.


Introduction
Acisanthera (Melastomataceae) is a neotropical genus comprising seven species of annual or perennial herbs to subshrubs (Kriebel 2008;Rocha et al. 2018).This genus has a complex taxonomic history and remarkable morphological variation (Kriebel 2008;Guimarães et al. 2017).About 18 species were traditionally recognized in a broadly circumscribed Acisanthera.Recent analyses based on both morphological and molecular data sets support the re-circumscription of Acisanthera s.l.into four genera, i.e.Rostranthera, Dicrananthera, Noterophila, and Acisanthera s.s.(Rocha et al. 2016(Rocha et al. , 2018)).In this context, Acisanthera s.s. was re-defined to include only species with 5-merous flowers and 3-locular ovaries (Rocha et al. 2018), matching Acisanthera sect.Acisanthera as previously delimited by Cogniaux (1885).Species of Acisanthera s.s.occur from Cuba south to Argentina.In the southern portion of their distributional ranges they are best represented in the mountainous regions of southern Brazil, where A. variabilis, A. alsinaefolia and A. uniflora are found (Kriebel 2008;Flora do Brasil 2020).
The Chapada dos Guimarães is a region of flat-topped mountains located in the State of Mato Grosso, Midwestern Brazil (Ross 2014).The summits of these flattened formations reach elevations between 600-800 m, consisting predominantly of Devonian and Cretaceous sandstones (Ross 2014).These mountains, which are located in the Cerrado Domain, are home to a wide variety of habitats, such as savannas, gallery forests, grasslands and palm swamps (IBAMA 2009).Remarkable plant communities grow directly on the rocky sandstone outcrops, with a predominance of Vellozia, lichens and bromeliads (IBAMA 2009).
The Chapada dos Guimarães National Park was established in 1989.It comprises 32,630.70ha and protects a significant part of the local biota (IBAMA 2009).The region currently delimited as the National Park was initially visited by several naturalists in the 19th century who gathered relevant ethnological and scientific information about the people, fauna, flora and famous landscapes (Barman 1971;Santos 2016).Initial botanical explorations were undertaken by Georg Heinrich Langsdorff (1774-1852) and Ludwig Riedel (1790-1861) in 1827, during the Russian Imperial Scientific Expedition to Brazil (Barman 1971), and by Oskar Andersson Gustaf Malme (1864-1937) during his expeditions to Mato Grosso in 1892-1894and 1901-1904(Santos 2016)).Despite these early efforts, biological studies in the Chapada dos Guimarães are still scarce.Several endemic taxa have been described from the Chapada in recent years; these include invertebrates (Vilela et al. 2018;Garcia & Urso-Guimarães 2018), fish (Tagliacollo et al. 2011), a burrowing snake (Santos et al. 2018), lichens (Ferraro & Lücking 2003), lycophytes (Valdespino et al. 2015) and flowering plants (Rapini et al. 2010;Leme et al. 2019).Here, we describe a new species of Acisanthera that is apparently endemic to rocky sandstone walls of the Chapada dos Guimarães.The first specimen of this new species was collected 119 years ago by Oskar Andersson Gustaf Malme during his "second expedition Regnelliana" (Plantae Itineris Regnelliani II.di), a productive exploration to the interior of Brazil, Argentina and Paraguay, which resulted in about 2,600 botanical collections (Santos 2016).
Scanning Electron Microscopy (SEM) studies used dried herbarium material.Leaf fragments and seeds were placed on aluminum stubs with carbon tape and covered with a 30 to 40 nm layer of gold; SEM photos were taken using a FEI Quanta 200 scanning electron microscope at the Universidade Estadual de Maringá, Brazil (COMCAP/UEM).
We used the GeoCAT online tool (Bachman et al. 2011) to estimate the Extent of Occurrence (EOO) and Area of Occupancy (AOO), based on coordinates obtained directly at the type locality and from specimen labels; these estimations and the recommendation for the conservation status follow IUCN guidelines and criteria (IUCN 2012; IUCN Standards and Petitions Committee 2019).The distribution map was prepared using QGIS 3.10 (QGIS Development Team 2021).Acisanthera saxatilis may be recognized by its perennial habit, lignotubers (xylopodia) membranaceous leaves with sparse glandular trichomes on the adaxial surface, isomorphic to slightly subisomorphic androecium, stamens erect and positioned in a circle around the style (during anthesis), connectives of both antesepalous and antepetalous stamens measuring 0.2-0.3mm long, inconspicuous ventral appendages divided in two lobes ca.0.1 mm long, and purple anthers.
Recognition-Acisanthera saxatilis is somewhat similar to A. erecta and A. uniflora in leaf shape, indumentum, and in having solitary flowers.Acisanthera saxatilis consistently differs from both species by its perennial habit, production of lignotubers, isomorphic to slightly subisomorphic androecium (vs.conspicuously dimorphic in the compared species), stamens positioned in a circle around the style during anthesis (vs.forming a cluster at one side of the flower), antesepalous stamens with shorter connectives 0.2-0.3mm long.(vs.1-2.5 mm long in the compared species), inconspicuous appendages ca.0.1 mm long.(vs.conspicuous, usually 0.5-1 mm long) and all anthers purple (vs.usually bicolored, purple and cream).Comparative morphological characters among the species of Acisanthera reported for Brazil are provided in Tab. 1.  Etymology-The specific epithet, saxatilis, which means living among rocks, highlights the habitat preference of this species on exposed rocky outcrops of the Chapada dos Guimarães.
Distribution, habitat and phenology-Acisanthera saxatilis is probably endemic to the Chapada dos Guimarães, Mato Grosso, Brazil (Fig. 6).It grows on shaded rocky sandstone outcrops, crevices and in round holes in these rocks, which accumulate organic material and may be externally covered with lichens and bryophytes (Fig. 3B-D June and July.According to specimen labels, its habitat is shared with the endemic bromeliad Fosterella lilliputiana, two rupicolous Gesneriaceae, i.e.Mandirola sp. and Goyazea petraea, and at least two species of ferns, i.e.Cheilanthes pohliana, Anemia sp., and one Lycophyte, Selaginella sp.; all of these taxa were collected on 13 February 1975 at the same sandstone rocky outcrops, during an expedition led by Gerdt Guenter Hatschbach (G. Hatschbach et al. 36107,36109,36110,36111,36112,36113,36114).
Conservation-All collections of A. saxatilis with geographic coordinates came from the same locality (Fig. 6D).The type series (R. Pacifico 416), was obtained from a population of about 15 individuals.Using GeoCAT we estimated the EOO and AOO of A. saxatilis to be 0.001 km 2 and 4 km 2 , respectively.If criterion B of IUCN ( 2012) and IUCN Standards and petitions subcommittee ( 2019) is applied, we suspect that a conservation status of Critically Endangered (CR) would be assigned to A. saxatilis: B2ab(iii).Additional specimens examined-Brazil.Mato Grosso: Chapada dos Guimarães, 13 February 1975, fl., fr., G. Hatschbach 36107 et

Discussion
Morphological variation is complex in Acisanthera s.s. and presents challenges for species delimitations (Kriebel 2008).Here, we treat A. saxatilis as a distinct species largely based on stamen shape and size, together with its perennial habit, production of lignotubers, peculiar habitat on rocky sandstone outcrops, and limited distribution on the Chapada dos Guimarães, Brazil.We know of no other species of Acisanthera with developed lignotubers but they are known in species of Microlicia (Microlicieae), especially those in the Lavoisiera clade (Martins & Almeda 2017), and in Chaetogastra (Melastomateae) (Meyer et al. 2021).These woody subterranean structures are common in species of the Cerrado and other neotropical savannas (Sarmiento 1985).They serve as an organ that buffers plants against extremes of water loss, temporal mineral deficiency, and also provide protection against fires (Gottsberger & Silberbauer-Gottsberger 2006).The positioning of the stamens in a circle around the style (during anthesis), the reduced ventral connectives and staminal appendages, and the anthers of both cycles of stamens similar in color are other apparent synapomorphies of A. saxatilis.
Acisanthera saxatilis seems to occur only on the Chapada dos Guimarães.Currently, it is the only species of the genus endemic to Brazil.The compared species of Acisanthera have never been collected at the Chapada dos Guimarães, making sympatric occurrence improbable.Acisanthera uniflora is widely distributed in South and Central America (Kriebel 2008) and was cited in relevant treatments of Neotropical Melastomataceae such as the floras of Mesoamerica (Almeda 2009), Panama (Gleason 1958), Venezuela (Wurdack 1973), Venezuelan Guayana (Berry et al. 2001), andBrazil (Cogniaux 1885;Rocha et al. 2020).Acisanthera erecta is largely restricted to northern South America and Central America (Kriebel 2008), and was cited as A. quadrata in the floras for Peru (Macbride 1941), Mesoamerica (Almeda 2009), Panama (Gleason 1958), Venezuelan Guayana (Berry et al. 2001) and British Guiana (Gleason 1932); its occurrence in Brazil was not confirmed in a recent treatment of Acisanthera for this country (Rocha et al. 2020).A recent report of A. erecta (as A. quadrata) from Mato Grosso do Sul state by Romero et al. (2018) needs confirmation.
The narrow endemism of A. saxatilis is unusual in a genus with predominantly widespread taxa.Its apparent restriction to sandstone substrates suggests that it may be an edaphic endemic.To our knowledge, there is no catalogue of edaphic endemics for Brazil's vascular flora.Among Melastomataceae, many species of Microlicia have narrow distributions and are endemic to quartzitic soils on Cadeia do Espinhaço (Pacifico et al. 2020); both Microlicia macedoi and Pterolepis haplostemona are endemic to serpentine in Goiás state (Reeves et al. 2007;Almeda & Martins 2015), whereas Brasilianthus carajensis and Pleroma carajasense are endemic to ironstone outcrops (canga) in Pará state (Almeda et al. 2016;Rocha et al. 2017).Further investigations using molecular data and chromosome number determinations may provide useful insights into the origin and interspecific relationships of A. saxatilis.

Figure 3 .
Figure 3. Habitat of Acisanthera saxatilis.A. Landscape at the Chapada dos Guimarães, Mato Grosso, Brazil, showing the Véu da Noiva Waterfall and a large sandstone rocky outcrop.B-C.Sandstone rocky outcrops on the trail to the Morro São Jerônimo, the type locality of A. saxatilis.The black arrow indicates one individual of A. saxatilis.D. Detail of a hole in a rocky sandsone outcrop on the trail to the Morro São Jerônimo, covered by lichens and accumulating organic matter, where individuals of A. saxatilis grow.

Figure 4 .
Figure 4. Photos of living individuals of Acisanthera saxatilis.A-B.Habit.C. Leaf.D. Floral bud with reflexed calyx lobes.E. Capsule enveloped by hypanthium and attached calyx lobes.F. Flower.G. Close-up photo of a flower.All photos taken from the type series, R. Pacifico 416 (CAS, CEPEC, HUEM, K, RB).

Figure 5 .
Figure 5. Scanning Electron Microscopy photos of Acisanthera saxatilis.A. Seed in lateral symmetrical plane view.B. Seed in raphal view showing raphal zone and hilum.C. Seed in anti-raphal view.D. Detail of the seed testa cells showing anticlinal boundaries and periclinal walls.E. A spherical gland on the leaf abaxial surface.F. Foveolate appearance of the leaf adaxial surface.G. Petiole with glandular trichomes.All photos taken from the holotype, R. Pacifico 416 (HUEM).

Figure 6 .
Figure 6.Distribution of Acisanthera saxatilis.A. South America with Mato Grosso State highlighted in black.B. Mato Grosso state with Chapada dos Guimarães municipality highlighted in black.C. Chapada dos Guimarães region showing the boundaries of the municipality (outlined in pink) and the National Park (outlined in blue), and the distribution of A. saxatilis.D. Trail to the Morro São Jerônimo in 3D view with the collection localities (white circles) of A. saxatilis.

Forming
(2009), the main threats to the vegetation of the Chapada dos Guimarães are: humancaused fires, human occupation, illegal logging, extraction of plants by locals, habitat degradation by cattle, unregulated tourism, and invasive alien species.

Table 1 .
Comparative morphological features among the four species of Acisanthera reported for Brazil.